- When diagnosing a clutch that will not disengage fully, which of the following is LEAST likely to be the cause?
- Air in the hydraulic system
- Worn clutch disc facing
- Faulty release bearing
- Misadjusted clutch linkage
Correct answer: Worn clutch disc facing
Correct answer: Worn clutch disc facing. Explanation: A worn clutch disc facing would typically cause a clutch to slip, not fail to disengage. Air in the hydraulic system, a faulty release bearing, or misadjusted clutch linkage would prevent the clutch from disengaging fully.
- A heavy-duty truck's clutch is slipping under load. The MOST likely cause is:
- A broken clutch disc torsion spring
- An over-tightened clutch cable
- Contaminated clutch disc facings
- An improperly adjusted clutch pedal free play
Correct answer: Contaminated clutch disc facings
Correct answer: Contaminated clutch disc facings. Explanation: Contaminated clutch disc facings, such as oil or grease, would prevent the clutch from gripping properly, leading to slippage, especially under load.
- During clutch engagement, a technician notices a chattering noise that diminishes as the vehicle accelerates. This symptom is MOST likely caused by:
- A defective pilot bearing
- Loose flywheel bolts
- Warped pressure plate
- Misaligned transmission input shaft
Correct answer: Warped pressure plate
Correct answer: Warped pressure plate. Explanation: A warped pressure plate can cause the clutch disc to engage unevenly, leading to a chattering noise during engagement that diminishes as the vehicle accelerates and the clutch fully engages.
- Which of the following conditions would most likely cause a clutch to engage near the floor?
- Excessive pedal free play
- A stretched clutch cable
- Worn clutch linkage bushings
- Over-extended clutch actuator piston
Correct answer: Excessive pedal free play
Correct answer: Excessive pedal free play. Explanation: Excessive pedal free play would delay the point of engagement of the clutch, causing it to engage closer to the floor.
- A clutch that disengages too quickly with minimal pedal movement may indicate:
- A worn clutch disc
- An over-adjusted clutch pedal linkage
- A weak pressure plate spring
- Air in the hydraulic release system
Correct answer: An over-adjusted clutch pedal linkage
Correct answer: An over-adjusted clutch pedal linkage. Explanation: An over-adjusted clutch pedal linkage would reduce the travel needed to disengage the clutch, causing it to disengage too quickly.
- After replacing a clutch assembly, a technician finds that the clutch drags when attempting to shift gears. The FIRST step should be to:
- Check the clutch pedal free play
- Inspect the clutch disc for proper installation
- Verify the presence of pilot bearing lubrication
- Re-bleed the hydraulic clutch system
Correct answer: Re-bleed the hydraulic clutch system
Correct answer: Re-bleed the hydraulic clutch system. Explanation: Air trapped in the hydraulic system can prevent full clutch disengagement, causing drag. It should be addressed before investigating other potential mechanical issues.
- A vehicle with a hydraulic clutch system has a pedal that feels spongy and engages the clutch almost immediately. This could be an indication of:
- A leak in the hydraulic system
- Worn clutch pedal pivot points
- A failed pressure plate
- An overfilled clutch fluid reservoir
Correct answer: A leak in the hydraulic system
Correct answer: A leak in the hydraulic system. Explanation: A spongy pedal and immediate clutch engagement typically indicate air in the hydraulic system, which can be caused by a leak.
- What could cause a newly installed clutch to have a hard pedal feel and difficult disengagement?
- Using the wrong type of hydraulic fluid
- A misaligned clutch disc
- Over-torqued pressure plate bolts
- Incorrect throw-out bearing installation
Correct answer: Over-torqued pressure plate bolts
Correct answer: Over-torqued pressure plate bolts. Explanation: Over-torquing the pressure plate bolts can distort the pressure plate, leading to a hard pedal feel and difficulty in disengaging the clutch.
- A technician finds that a truck's clutch disengagement is inconsistent. What should be inspected FIRST?
- Clutch master cylinder
- Flywheel surface condition
- Release fork for bending or wear
- Clutch pedal return spring
Correct answer: Clutch master cylinder
Correct answer: Clutch master cylinder. Explanation: The clutch master cylinder is crucial for consistent hydraulic pressure in the clutch system. Any inconsistency in disengagement would suggest inspecting it first.
- The pilot bearing supports the:
- Clutch disc on the input shaft
- Input shaft at the flywheel
- Output shaft in the transmission
- Pressure plate assembly
Correct answer: Input shaft at the flywheel
Correct answer: Input shaft at the flywheel. Explanation: The pilot bearing is located in the flywheel and supports the transmission input shaft.
- Excessive transmission gear noise that diminishes when the clutch pedal is depressed is MOST likely due to a:
- Defective transmission input shaft bearing
- Worn gear synchronizer
- Damaged clutch pressure plate
- Faulty pilot bearing
Correct answer: Defective transmission input shaft bearing
Correct answer: Faulty pilot bearing. Explanation: A faulty pilot bearing can cause gear noise that is present when the clutch is engaged and diminishes when the pedal is depressed, as the bearing is under load when the transmission is engaged.
- A manual transmission is difficult to shift into first gear and reverse, but shifts into higher gears without issue. What is the MOST likely cause?
- Worn synchronizers in higher gears
- Defective first gear and reverse idler gear
- Faulty shift lever assembly
- Misaligned clutch disc
Correct answer: Misaligned clutch disc
Correct answer: Misaligned clutch disc. Explanation: A misaligned clutch disc can cause difficulty in engaging gears that require full disengagement of the clutch, such as first gear and reverse, while not affecting higher gears as much.
- A technician finds that a transmission pops out of second gear intermittently. Which of the following is the LEAST likely cause?
- Worn shift fork for second gear
- Worn detent mechanism for second gear
- Loose transmission mount
- Worn second gear teeth
Correct answer: Loose transmission mount
Correct answer: Loose transmission mount. Explanation: A loose transmission mount could cause a misalignment issue but is less likely to cause the transmission to pop out of a specific gear like second gear. Wear in the gear itself, the detent mechanism, or the shift fork are more direct causes of this problem.
- When diagnosing a transmission that exhibits noise in all gears except fourth, which component is MOST likely to be the cause?
- Input shaft bearing
- Output shaft bearing
- Counter shaft bearing
- Main shaft bearing
Correct answer: Counter shaft bearing
Correct answer: Input shaft bearing. Explanation: Noise in all gears except for direct drive (often fourth gear in a five-speed transmission) is a typical symptom of a faulty input shaft bearing, as the load is reduced on this bearing in direct drive.
- A driver reports that a transmission is noisy in neutral with the clutch engaged, but the noise stops when the clutch pedal is depressed. What is the MOST likely cause?
- Faulty pilot bearing
- Worn input shaft bearing
- Defective clutch release bearing
- Damaged output shaft bearing
Correct answer: Worn input shaft bearing
Correct answer: Worn input shaft bearing. Explanation: A worn input shaft bearing can cause noise in neutral with the clutch engaged because the input shaft is spinning. When the clutch is disengaged (pedal depressed), the input shaft stops spinning, thus stopping the noise.
- Technician A says that grinding during shifting into all gears may be due to an issue with the clutch system. Technician B says it could be due to worn synchronizers. Who is correct?
- Technician A only
- Technician B only
- Both Technician A and B
- Neither Technician A nor B
Correct answer: Both Technician A and B
Correct answer: Both Technician A and B. Explanation: Grinding during shifting into all gears could be due to incomplete disengagement of the clutch (Technician A) or worn synchronizers (Technician B), as both would prevent smooth gear engagement.
- When a transmission exhibits a whining noise that increases with engine speed regardless of gear selection, this is MOST likely due to:
- Worn gear teeth
- Defective main shaft bearings
- Low transmission fluid level
- Misaligned transmission housing
Correct answer: Low transmission fluid level
Correct answer: Low transmission fluid level. Explanation: A whining noise that increases with engine speed in all gears can be caused by a low transmission fluid level, which results in poor lubrication and cooling.
- A manual transmission does not maintain engagement in any gear. The shifter feels loose and does not have the normal resistance when moving it between gears. The MOST likely cause is:
- Worn shifter bushings
- Damaged shift forks
- Broken shift linkage
- Defective shift rail
Correct answer: Worn shifter bushings
Correct answer: Worn shifter bushings. Explanation: Worn shifter bushings can cause a loose-feeling shifter with insufficient resistance, leading to a failure to maintain gear engagement.
- Excessive play in a manual transmission's input shaft can cause:
- Hard shifting into reverse only
- Difficulty in shifting into all gears
- Noise in one specific gear
- Vehicle to move while in neutral
Correct answer: Difficulty in shifting into all gears
Correct answer: Difficulty in shifting into all gears. Explanation: Excessive play in the input shaft can lead to misalignment of internal components, causing difficulty in engaging all gears.
- After overhauling a transmission, a technician notes that there is a new noise when the vehicle is coasting in gear, but it is not present under acceleration. The noise is MOST likely from:
- Incorrect backlash between the gears
- Improperly torqued fasteners
- Bearing pre-load being too loose
- A misaligned transmission case
Correct answer: Incorrect backlash between the gears
Correct answer: Incorrect backlash between the gears. Explanation: Incorrect backlash can cause a noise during coasting when the gears are not under load, but the noise can disappear when the gears are loaded during acceleration.
- A technician finds metal shavings in the transmission fluid during a service. What is the FIRST step that should be taken?
- Replace the fluid and recheck after a short drive
- Inspect the magnetic drain plug for excessive metal
- Disassemble the transmission for internal inspection
- Flush the transmission cooler lines and radiator
Correct answer: Disassemble the transmission for internal inspection
Correct answer: Disassemble the transmission for internal inspection. Explanation: Metal shavings in the transmission fluid are indicative of internal wear or failure, and the transmission should be disassembled for a thorough inspection to determine the source of the metal shavings.
- A technician is diagnosing a truck with a manual transmission that is difficult to shift out of gear while running. The MOST likely cause is:
- Defective clutch master cylinder
- Worn transmission mounts
- Binding gear shift linkage
- Misadjusted shift cables
Correct answer: Defective clutch master cylinder
Correct answer: Defective clutch master cylinder. Explanation: A defective clutch master cylinder can prevent complete disengagement of the clutch, making it difficult to shift out of gear when the engine is running.
- During the test drive of a vehicle with a newly rebuilt transmission, the technician notices a vibration at high speeds that was not present before the rebuild. What is the MOST likely cause?
- Unbalanced drive shaft
- Improperly installed transmission
- Incorrectly torqued flywheel
- New transmission mounts
Correct answer: Unbalanced drive shaft
Correct answer: Unbalanced drive shaft. Explanation: If the drive shaft was removed during the rebuild and not reinstalled correctly, it could be out of balance, causing vibration at high speeds.
- A transmission grinds when shifting into third gear, but only when downshifting. This problem is MOST likely due to:
- A worn third gear synchronizer
- A damaged shift fork for third gear
- An issue with the clutch not fully disengaging
- Worn or damaged third gear teeth
Correct answer: A worn third gear synchronizer
Correct answer: A worn third gear synchronizer. Explanation: A worn synchronizer for third gear would specifically affect the ability to mesh gears smoothly when downshifting into third, as the synchronizer's role is to match the speed of the gear and the shaft.
- A driveshaft is vibrating at high speeds. The MOST likely cause of this issue is:
- Worn universal joints
- Incorrect driveshaft angle
- Driveshaft tubing is out of round
- Improperly torqued driveshaft flange bolts
Correct answer: Driveshaft tubing is out of round
Correct answer: Driveshaft tubing is out of round. Explanation: A driveshaft that is out of round can create an imbalance, leading to vibrations at high speeds.
- Technician A says that a clunking noise when accelerating or decelerating may indicate a problem with the universal joints. Technician B says that it can be due to excessive play in the differential. Who is correct?
- Technician A only
- Technician B only
- Both Technician A and B
- Neither Technician A nor B
Correct answer: Both Technician A and B
Correct answer: Both Technician A and B. Explanation: A clunking noise on acceleration or deceleration can be due to worn universal joints, which cause play in the driveshaft, or excessive play in the differential gears.
- When inspecting a driveshaft, you notice that the slip yoke is scored and has a rough surface. What is the MOST likely cause?
- Lack of lubrication
- Excessive u-joint angle
- Faulty transmission output shaft bearing
- Driveshaft imbalance
Correct answer: Lack of lubrication
Correct answer: Lack of lubrication. Explanation: Lack of lubrication can cause the slip yoke to wear prematurely, leading to scoring and a rough surface.
- A technician is diagnosing a vehicle with a suspected driveshaft issue. Which of the following is a proper method to check for driveshaft straightness?
- Placing the driveshaft on a flat surface and checking for gaps
- Using a dial indicator to measure runout while the driveshaft is rotating
- Inspecting for visual bends or dents
- Measuring the length from end to end and comparing it to factory specifications
Correct answer: Using a dial indicator to measure runout while the driveshaft is rotating
Correct answer: Using a dial indicator to measure runout while the driveshaft is rotating. Explanation: A dial indicator is used to measure driveshaft runout accurately while the driveshaft is rotating, which can identify even slight bends or warping not visible to the naked eye.
- If a vehicle with rear-wheel drive shudders during initial acceleration, the cause could be:
- Over-inflated tires
- Out-of-balance wheels
- Worn center support bearing
- Defective torque converter
Correct answer: Worn center support bearing
Correct answer: Worn center support bearing. Explanation: A worn center support bearing can cause shuddering during initial acceleration due to movement or misalignment of the driveshaft.
- During a road test, a technician notices a rhythmic thumping noise that changes with vehicle speed. This noise is MOST likely due to:
- Worn differential gears
- Damaged or improperly seated u-joint caps
- Faulty wheel bearings
- A loose pinion gear nut
Correct answer: Damaged or improperly seated u-joint caps
Correct answer: Damaged or improperly seated u-joint caps. Explanation: A rhythmic thumping noise that changes with vehicle speed can indicate damaged or improperly seated u-joint caps as they create a regular pattern of noise with each rotation of the driveshaft.
- A rear-wheel-drive vehicle has a vibration that increases with speed. The MOST likely cause of this vibration is:
- An imbalanced rear wheel
- A faulty rear axle bearing
- An imbalanced driveshaft
- Loose lug nuts on the rear wheel
Correct answer: An imbalanced driveshaft
Correct answer: An imbalanced driveshaft. Explanation: An imbalanced driveshaft can cause a vibration that increases with the speed of the vehicle because the rotational speed of the driveshaft is directly related to the vehicle speed.
- A driveshaft with missing balance weights would MOST likely cause:
- A clunking noise when shifting from drive to reverse
- A high-pitched squeal at all speeds
- Vibration at certain speeds
- Difficulty turning the vehicle
Correct answer: Vibration at certain speeds
Correct answer: Vibration at certain speeds. Explanation: Missing balance weights on a driveshaft can cause imbalance, leading to vibration at speeds where the rotational speed of the driveshaft resonates with the natural frequency of the vehicle's chassis or other components.
- Which of the following conditions can cause a universal joint to fail prematurely?
- Over-greasing the u-joint
- Using non-matching universal joints on the same driveshaft
- High ambient temperatures
- Excessive engine power
Correct answer: Using non-matching universal joints on the same driveshaft
Correct answer: Using non-matching universal joints on the same driveshaft. Explanation: Using non-matching universal joints on the same driveshaft can cause uneven stress and wear, leading to premature failure.
- The presence of rust around the seals of a universal joint indicates:
- The joint is over-greased
- Inadequate sealing and possible entry of contaminants
- The joint is new and has not been broken in yet
- Proper lubrication maintenance
Correct answer: Inadequate sealing and possible entry of contaminants
Correct answer: Inadequate sealing and possible entry of contaminants. Explanation: Rust around the seals of a universal joint indicates that the seals may be failing, allowing contaminants to enter and causing the joint to wear prematurely.
- After replacing a universal joint, a technician notices a squeaking noise at low speeds. The MOST likely cause is:
- The universal joint is not the correct size
- The bearings in the universal joint were not filled with grease
- The drive shaft has been incorrectly balanced
- The snap rings were not installed properly
Correct answer: The bearings in the universal joint were not filled with grease
Correct answer: The bearings in the universal joint were not filled with grease. Explanation: If the bearings in the universal joint are not properly greased, they can cause a squeaking noise, particularly noticeable at low speeds when less road noise is present.
- During the disassembly of a drive axle, the technician notices that the pinion depth shim is damaged. Which of the following could be a direct result of a damaged pinion depth shim?
- Premature seal wear
- Incorrect wheel alignment
- Gear tooth contact pattern misalignment
- Brake caliper malfunction
Correct answer: Gear tooth contact pattern misalignment
Correct answer: Gear tooth contact pattern misalignment. Explanation: The pinion depth shim determines the position of the pinion gear in relation to the ring gear. Damage to this shim can lead to incorrect gear tooth contact pattern, causing noise and premature wear.
- A technician finds metal flakes in the axle fluid during a routine service. What is the MOST likely source of these flakes?
- Normal wear of the gears and bearings
- Deterioration of the axle seals
- Breakdown of the axle lubricant
- Failing differential carrier bearings
Correct answer: Failing differential carrier bearings
Correct answer: Failing differential carrier bearings. Explanation: Metal flakes in the axle fluid typically indicate excessive wear or failure of metal components such as the differential carrier bearings.
- When diagnosing a rear drive axle, the technician notices a whining noise that changes pitch with vehicle speed. The MOST likely cause is:
- Worn wheel bearings
- Defective tires
- Worn ring and pinion gears
- A loose differential case
Correct answer: Worn ring and pinion gears
Correct answer: Worn ring and pinion gears. Explanation: A whining noise that changes pitch with vehicle speed is often caused by wear or improper adjustment of the ring and pinion gears in the differential.
- Which of the following symptoms would most likely indicate a problem with the limited-slip differential?
- A clicking noise when turning
- Vibration at highway speeds
- Chatter or shudder during cornering
- Fluid leakage from the rear cover seal
Correct answer: Chatter or shudder during cornering
Correct answer: Chatter or shudder during cornering. Explanation: Chatter or shudder during cornering is a common symptom of issues with the limited-slip differential, typically due to worn clutches or insufficient friction-modifier additive in the differential fluid.
- What is the proper procedure when adjusting the bearing preload on a differential?
- Tighten the bearing caps to the manufacturer's torque specifications
- Use a dial indicator to measure ring gear backlash
- Adjust the carrier shims to obtain the specified rolling torque
- Set the pinion depth using a micrometer
Correct answer: Adjust the carrier shims to obtain the specified rolling torque
Correct answer: Adjust the carrier shims to obtain the specified rolling torque. Explanation: Bearing preload is typically set by adjusting the carrier shims to achieve the manufacturer's specified rolling torque on the bearings.
- A vehicle with a solid rear axle has excessive tire wear and a slight pull to one side. This condition could be caused by:
- A misaligned rear axle housing
- Worn front suspension components
- Overinflated tires
- A stuck brake caliper on one wheel
Correct answer: A misaligned rear axle housing
Correct answer: A misaligned rear axle housing. Explanation: Excessive tire wear and a pull to one side in a vehicle with a solid rear axle can be due to the rear axle housing being out of alignment, causing uneven tire wear and affecting vehicle tracking.
- A differential is noisy under load but quiet when coasting. The MOST likely cause of this condition is:
- Worn differential side gears
- A loose pinion bearing preload
- Insufficient backlash between the ring and pinion gears
- Worn pinion bearings
Correct answer: Worn pinion bearings
Correct answer: Worn pinion bearings. Explanation: Noise under load that goes away during coasting is typically indicative of worn pinion bearings. Load puts stress on these bearings, causing noise.
- If a rear axle assembly is overheating, what could be the cause?
- Overfilled axle fluid
- Too little axle fluid or incorrect fluid type
- Faulty axle vent
- Worn brake pads
Correct answer: Too little axle fluid or incorrect fluid type
Correct answer: Too little axle fluid or incorrect fluid type. Explanation: Insufficient axle fluid can lead to inadequate lubrication and overheating, as can using the incorrect type of fluid for the vehicle's specifications.
- When performing a final drive ratio check, the technician must rotate the drive wheel and count the number of pinion shaft revolutions. This procedure is done to:
- Determine the gear ratio of the differential
- Check the operation of the wheel bearings
- Evaluate the condition of the axle shafts
- Assess the need for axle fluid replacement
Correct answer: Determine the gear ratio of the differential
Correct answer: Determine the gear ratio of the differential. Explanation: Rotating the drive wheel and counting the pinion shaft revolutions helps to determine the gear ratio by comparing the number of turns of the wheel to one turn of the pinion.
- A technician is unable to remove the axle shaft from the hub assembly. The BEST course of action is to:
- Apply heat to the hub assembly
- Use a slide hammer with an axle puller attachment
- Strike the axle shaft with a hammer
- Pry the axle shaft out with a large screwdriver
Correct answer: Use a slide hammer with an axle puller attachment
Correct answer: Use a slide hammer with an axle puller attachment. Explanation: A slide hammer with an axle puller attachment is the correct tool to use when an axle shaft is stuck in the hub assembly, as it applies even force without damaging the shaft or hub.
- The discovery of fine metallic particles attached to the magnetic drain plug of an axle housing suggests:
- Normal wear of the axle components
- Excessive backlash in the differential gears
- An impending bearing failure
- The need for an immediate fluid change
Correct answer: An impending bearing failure
Correct answer: An impending bearing failure. Explanation: While some metallic particles are normal, fine metallic particles on the magnetic drain plug can indicate abnormal wear and the possibility of an impending bearing failure within the axle.
- On a medium/heavy truck manual transmission, what is the primary function of the pilot bearing (or pilot bushing)?
- It locks the input shaft when the clutch pedal is fully depressed
- It rides on the transmission input shaft and applies pressure to the release fingers
- It supports and aligns the front (pilot) end of the transmission input shaft inside the crankshaft or flywheel bore
- It absorbs torsional vibration between the engine and the clutch disc
Correct answer: It supports and aligns the front (pilot) end of the transmission input shaft inside the crankshaft or flywheel bore
The pilot bearing supports and centers the forward (pilot) end of the transmission input shaft inside the recess of the crankshaft or flywheel. This keeps the input shaft concentric with the crankshaft so the clutch disc engages smoothly and the splines line up. The release bearing, not the pilot bearing, acts on the release fingers, and the clutch brake is what slows the input shaft.
- A worn pilot bearing on a heavy truck most characteristically produces noise under which condition?
- Continuously regardless of clutch pedal position
- Only during initial clutch engagement from a stop
- Only at highway cruise speed with the clutch engaged
- Only with the clutch pedal fully depressed and the transmission in neutral with the engine running
Correct answer: Only with the clutch pedal fully depressed and the transmission in neutral with the engine running
A failing pilot bearing makes its loudest whine or growl with the clutch pedal pushed fully to the floor while the engine runs. With the clutch disengaged, the crankshaft and the input shaft turn at different speeds, forcing the pilot bearing to rotate and revealing its wear. A release bearing, by contrast, is loaded the moment the pedal begins to move, so distinguishing the two depends on when the noise appears.
- A driver reports a growling or rumbling noise that appears only while the clutch pedal is being held down and goes away when the pedal is released. Which component should the technician suspect FIRST?
- The clutch release (throwout) bearing
- The clutch disc torsional springs
- The transmission countershaft bearing
- The flywheel ring gear
Correct answer: The clutch release (throwout) bearing
The clutch release (throwout) bearing is the prime suspect when noise occurs only while the pedal is depressed and stops when the pedal is released. The release bearing is loaded and spinning against the release fingers only during disengagement, so its failure shows up exactly during that interval. Countershaft and input-shaft noises typically appear with the clutch engaged and the transmission in neutral instead.
- Which set of symptoms is MOST consistent with clutch release bearing failure on a heavy-duty truck?
- Slipping under load with a burnt odor and high engine rpm
- Squealing or rattling and pedal roughness that occur when the pedal is depressed, sometimes with harder disengagement
- A whine that rises and falls with road speed in all gears
- Vibration felt in the steering wheel above 50 mph
Correct answer: Squealing or rattling and pedal roughness that occur when the pedal is depressed, sometimes with harder disengagement
Squealing or rattling noise and a rough or stiff feel when the pedal is pressed, along with harder shifting, point to a worn clutch release (throwout) bearing because that bearing is engaged only during pedal travel. Slipping with a burnt odor indicates worn or contaminated facings, and a whine tied to road speed points to driveline or axle bearings, not the release bearing.
- A heavy-duty clutch slips under heavy load even though pedal free travel is correct. Which condition is LEAST likely to be responsible?
- Worn clutch disc friction facings near the rivets
- Oil-contaminated clutch disc facings from a leaking rear main seal
- A worn or damaged pilot bearing
- Weak or broken pressure plate springs
Correct answer: A worn or damaged pilot bearing
A worn pilot bearing causes noise and possible disengagement or alignment problems, not slippage, so it is the least likely cause of slip. Slip happens when clamping force or friction is lost, which results from oil-contaminated facings, weak or broken pressure plate springs, or facings worn down toward the rivet heads. Loss of clamp load or friction surface, not the pilot bearing, is what allows the disc to slip.
- On a non-synchronized heavy truck transmission, what is the purpose of the clutch brake?
- To slow or stop the rotating input shaft and gearing so the truck can be shifted into a low or reverse gear from a standstill
- To prevent the clutch disc from slipping when starting on a grade
- To dampen torsional vibration during clutch engagement
- To hold the clutch disengaged after the pedal is released
Correct answer: To slow or stop the rotating input shaft and gearing so the truck can be shifted into a low or reverse gear from a standstill
A clutch brake slows or stops the spinning transmission input shaft and gears so the driver can engage a starting gear (low or reverse) from a complete stop without gear clash. It is squeezed only at the bottom of pedal travel when starting from a standstill. It is not used for upshifts or downshifts while rolling, and it does nothing to control disc slip or torsional damping.
- A truck with a non-synchronized transmission grinds badly every time the driver tries to put it in gear from a complete stop, but shifts cleanly while rolling. What is the MOST likely cause?
- A worn-out or broken clutch brake
- Excessive flywheel runout
- A failed pilot bearing
- A slipping clutch disc
Correct answer: A worn-out or broken clutch brake
A worn out or broken clutch brake is the most likely cause when a truck clashes going into gear from a standstill but shifts fine while rolling. The clutch brake's only job is to stop the input shaft for engagement from a dead stop; once moving, gears are matched by speed instead. Slip, flywheel runout, and a bad pilot bearing produce other symptoms and would not selectively affect only standstill engagement.
- When adjusting a heavy-duty pull-type clutch, in what order should the technician perform the adjustments?
- External linkage first, then internal adjustment last
- Clutch brake squeeze first, then internal adjustment, then linkage
- Pedal free travel first, then internal adjustment
- Internal adjustment (release bearing travel/clutch brake clearance) first, then external linkage (pedal free travel) last
Correct answer: Internal adjustment (release bearing travel/clutch brake clearance) first, then external linkage (pedal free travel) last
The internal adjustment is performed first to set release bearing travel and the clutch-brake clearance, and the external linkage (in-cab pedal free travel) is set last as the final check. The internal adjusting ring controls the bearing position relative to the clutch brake; the external linkage only sets free travel. Doing the linkage first would mask an out-of-spec internal setting.
- On a heavy-duty pull-type clutch, which measurement is controlled by the INTERNAL clutch adjustment rather than the external linkage?
- The length of the master cylinder pushrod
- The height of the clutch pedal off the floor
- The in-cab clutch pedal free travel
- The clearance between the release bearing and the clutch brake (release bearing travel)
Correct answer: The clearance between the release bearing and the clutch brake (release bearing travel)
The internal adjustment sets the gap between the release bearing and the clutch brake, which establishes release bearing travel. Turning the internal adjusting ring repositions the bearing toward or away from the transmission. The external linkage adjusts in-cab pedal free travel and pedal height, so those belong to the external adjustment, not the internal one.
- A technician needs to verify release bearing travel on a 15.5-inch Eaton-style heavy-duty pull-type clutch. Which clearance specification represents the typical acceptable range between the release bearing and the clutch brake?
- About 1 inch (roughly 1.000 inch)
- About 1/16 inch (roughly 0.060 inch)
- About 1/8 inch (roughly 0.125 inch)
- About 1/2 inch (roughly 0.490 to 0.560 inch)
Correct answer: About 1/2 inch (roughly 0.490 to 0.560 inch)
Acceptable release bearing travel on a typical 15.5-inch heavy-duty pull-type clutch is about 1/2 inch; the official Eaton specification is 0.490 inch to 0.560 inch (approximately 1/2 to 9/16 inch) between the release bearing and the clutch brake. The 1/8-inch figure refers instead to the clearance between the release yoke fingers and the bearing wear pads, and 1 inch or more is the in-cab pedal free travel. Mixing these specs up is a common adjustment error.
- What does the term clutch pedal free play (free travel) describe on a heavy truck?
- The clearance between the clutch disc and the flywheel when engaged
- The amount the pedal rises after the clutch is fully released
- The distance the pedal moves before the release mechanism begins to load the release bearing against the clutch
- The total pedal travel from rest to the floor
Correct answer: The distance the pedal moves before the release mechanism begins to load the release bearing against the clutch
Clutch pedal free play is the small amount of pedal movement at the top of the stroke before the linkage takes up slack and the release bearing starts to act on the clutch. It confirms the bearing is not riding the clutch under static conditions. It is not total travel, not pedal rise, and not disc-to-flywheel clearance, which is normally zero when the clutch is engaged.
- A heavy truck clutch shows almost no measurable pedal free play, and the release bearing runs hot and noisy. What is the MOST likely consequence if this is left uncorrected?
- The pilot bearing will be over-lubricated
- The flywheel ring gear teeth will wear rapidly
- Premature release bearing failure and gradual clutch slippage as the bearing partially loads the pressure plate
- The clutch brake will fail to release the input shaft
Correct answer: Premature release bearing failure and gradual clutch slippage as the bearing partially loads the pressure plate
Too little free play lets the release bearing ride against the release fingers constantly, which overheats and wears out the bearing and slightly relieves clamping force, leading to slip. Proper free travel keeps the bearing off the fingers until the pedal is pressed. Insufficient free play does not over-lubricate the pilot bearing or attack the ring gear, and it tends to keep the clutch slightly released rather than affecting the clutch brake's release function.
- A technician needs to verify clutch brake squeeze on a heavy-duty truck after adjustment. Which procedure correctly checks it?
- Measure pedal free play at the top of the stroke
- Place a feeler gauge (about 0.010 inch) between the release bearing and clutch brake, then press the pedal fully to the floor and confirm the gauge is pinched
- Hold the pedal halfway and measure the gap with a depth micrometer
- Spin the input shaft by hand with the pedal released and feel for drag
Correct answer: Place a feeler gauge (about 0.010 inch) between the release bearing and clutch brake, then press the pedal fully to the floor and confirm the gauge is pinched
Clutch brake squeeze is checked by inserting a thin feeler gauge (around 0.010 inch) between the release bearing and the clutch brake, then pushing the pedal fully to the floor; the gauge should be lightly clamped, confirming the brake is being squeezed at the bottom of travel. The squeeze occurs only at the very end of pedal travel, so a half-pedal check is wrong, and free play at the top of the stroke is a separate measurement.
- What is the fundamental operating difference between a pull-type and a push-type clutch used on commercial trucks?
- A pull-type uses a diaphragm spring while a push-type can only use coil springs
- A pull-type has no release bearing, relying on linkage alone
- In a pull-type, the release bearing pulls the release fingers/diaphragm toward the transmission to disengage; in a push-type, the bearing pushes them toward the engine
- A push-type is used only on tandem-axle tractors
Correct answer: In a pull-type, the release bearing pulls the release fingers/diaphragm toward the transmission to disengage; in a push-type, the bearing pushes them toward the engine
The defining difference is the direction the release bearing acts: a pull-type clutch disengages when the bearing pulls the release fingers or diaphragm toward the transmission, while a push-type disengages when the bearing pushes them toward the engine. Most modern heavy trucks use pull-type clutches for mechanical advantage at high torque. Both designs still use a release bearing, and spring type and axle configuration are unrelated to the pull/push distinction.
- How does a self-adjusting heavy-duty truck clutch compensate for friction facing wear?
- The driver must turn an external linkage nut at every service interval
- Centrifugal weights gradually tighten the pressure plate as engine speed rises
- A wear sensor detects increased release bearing travel and advances an internal adjusting (worm and gear) ring to reposition the bearing and maintain free travel
- A hydraulic accumulator automatically lengthens the master cylinder pushrod
Correct answer: A wear sensor detects increased release bearing travel and advances an internal adjusting (worm and gear) ring to reposition the bearing and maintain free travel
A self-adjusting clutch uses a wear sensor that detects the extra release bearing travel created as facings wear, then drives an internal worm-and-gear arrangement to turn the adjusting ring and reposition the bearing, keeping pedal free travel and release travel in spec. This reduces, but does not eliminate, manual service. It is mechanical wear sensing, not a hydraulic accumulator, centrifugal weights, or routine manual linkage turning.
- Before installing a new clutch, a technician measures flywheel face runout with a dial indicator. Which statement BEST describes correct practice?
- Mount the dial indicator on the flywheel housing, sweep the friction face while rotating the crankshaft, and compare total indicator reading to the manufacturer's maximum runout spec
- Ignore flywheel runout because it has no effect on a new clutch
- Measure runout only at a single point without rotating the crankshaft
- Check runout by feeling the face for high spots with a fingernail
Correct answer: Mount the dial indicator on the flywheel housing, sweep the friction face while rotating the crankshaft, and compare total indicator reading to the manufacturer's maximum runout spec
Flywheel face runout is measured by mounting a dial indicator to a fixed point such as the flywheel housing, placing the plunger on the friction face, rotating the crankshaft a full turn, and comparing the total indicator reading to the manufacturer's maximum spec. Excessive face runout causes clutch chatter and uneven engagement, so it cannot be ignored, and a single static point or a fingernail check cannot reveal true runout around the face.
- A heavy truck clutch grabs and shudders (chatters) during slow, careful engagement from a stop. Which of the following is the LEAST likely cause?
- Excessive flywheel or pressure plate face runout
- Oil or grease contamination on the clutch disc facings
- A slipping clutch disc caused by low pressure plate spring tension
- Loose or worn engine or transmission mounts
Correct answer: A slipping clutch disc caused by low pressure plate spring tension
A slipping clutch disc from weak springs causes loss of drive and a burnt odor, not the grab-and-shudder pattern of chatter, so it is the least likely chatter cause. Clutch chatter typically results from contaminated facings, flywheel or pressure plate runout, or loose mounts that let the driveline jump during engagement. Chatter is a release-quality problem, whereas slip is a clamping-force problem.
- A heavy truck clutch will not fully release: the driver cannot get it into gear from a stop and gears clash, even though pedal travel is full and free play is in spec. After ruling out the clutch brake, which clutch-side condition should the technician inspect?
- A worn, distorted, or oil-soaked clutch disc, or bent/uneven pressure plate release fingers preventing the disc from freeing from the flywheel
- A scored flywheel friction surface causing slip
- Weak pressure plate springs reducing clamp load
- An over-lubricated release bearing
Correct answer: A worn, distorted, or oil-soaked clutch disc, or bent/uneven pressure plate release fingers preventing the disc from freeing from the flywheel
When a clutch will not release with correct pedal travel and free play and a good clutch brake, the fault is usually on the clutch itself: a warped, swollen, or oil-soaked disc that drags, or bent and uneven pressure plate release fingers that fail to pull the pressure plate evenly off the disc. Weak springs and a scored flywheel cause slip (a clamping problem), which is the opposite symptom from a clutch that will not release.
- In a twin countershaft heavy-duty transmission, the two countershafts are positioned on opposite sides of the mainshaft primarily to:
- Permit the input shaft to turn in the opposite direction of the output shaft
- Eliminate the need for an auxiliary section
- Split the load between two gear sets so torque capacity increases without larger gears
- Allow the mainshaft to use synchronizers in every gear
Correct answer: Split the load between two gear sets so torque capacity increases without larger gears
Splitting the load between two gear sets so torque capacity increases without larger gears is the reason for the twin countershaft layout. With identical gear sets meshing on both sides of the mainshaft, each tooth carries roughly half the load, so the transmission handles high torque while staying compact. It does not eliminate the auxiliary section and does not reverse output shaft direction.
- A defining characteristic of the mainshaft in an Eaton-style twin countershaft transmission is that it:
- Rotates faster than the input shaft in all gears
- Floats radially and is supported only by the meshing countershaft gears under load
- Carries the parking pawl for the driveline brake
- Is rigidly splined to every mainshaft gear
Correct answer: Floats radially and is supported only by the meshing countershaft gears under load
Floating radially and being supported only by the meshing countershaft gears under load describes mainshaft float. The mainshaft is not held by its own bearings in the middle; the constantly meshed countershaft gear pairs center and support it, which equalizes tooth loading between the two countershafts. The mainshaft gears are clutched to the shaft individually rather than rigidly splined to all gears at once.
- Why must the two countershafts in a twin countershaft transmission be timed (clocked) correctly during assembly?
- So the inertia brake will release at the proper engine speed
- So both countershafts share the load equally on the same mainshaft gear teeth
- So the deep reduction gear can engage without the clutch
- So the speedometer signal stays calibrated
Correct answer: So both countershafts share the load equally on the same mainshaft gear teeth
So both countershafts share the load equally on the same mainshaft gear teeth is why timing matters. Timing marks on the gears must align so both countershafts contact each mainshaft gear simultaneously; if one is off a tooth, that side carries the full load and fails early. Timing has nothing to do with the inertia brake, deep reduction, or speedometer.
- What is the function of a synchronizer in a manual transmission?
- It matches the speed of the gear and the shaft so they can mesh without clashing
- It applies the inertia brake during upshifts
- It meters lubricant to the mainshaft gears
- It locks the differential during hard acceleration
Correct answer: It matches the speed of the gear and the shaft so they can mesh without clashing
Matching the speed of the gear and the shaft so they can mesh without clashing is the function of a synchronizer. Its friction cone brings the rotating gear and the shaft to the same speed before the dog teeth engage, allowing smooth, clash-free shifting. Note that most heavy-duty truck transmissions are non-synchronized and rely on the driver matching speeds with double-clutching or float shifting.
- A driver of a non-synchronized heavy-duty transmission reports gear clash every time he shifts even though the clutch fully releases. The MOST likely cause is:
- Low differential lube level
- A leaking inertia brake air line
- Engine and transmission speeds are not being matched (improper double-clutching technique or high idle)
- A worn auxiliary range cylinder
Correct answer: Engine and transmission speeds are not being matched (improper double-clutching technique or high idle)
Engine and transmission speeds not being matched is the most likely cause of clash in a non-synchronized box. These transmissions have no synchronizers, so the driver must match shaft speeds by double-clutching; clashing on every shift with a properly releasing clutch points to technique or a high idle, not a hardware failure. A range cylinder or inertia brake fault would not clash every single shift.
- The auxiliary section of a typical Eaton-style heavy-duty transmission provides:
- Range and/or splitter ratios that multiply the number of main-section ratios
- Only forward and reverse selection
- Power flow to the PTO only
- Clutch brake actuation
Correct answer: Range and/or splitter ratios that multiply the number of main-section ratios
Range and/or splitter ratios that multiply the number of main-section ratios is what the auxiliary section provides. By adding a high/low range and sometimes a splitter, the auxiliary section multiplies the front (main) section's gears, so a 5-speed main section becomes a 10-speed, 13-speed, or 18-speed. It does not handle reverse selection or clutch brake actuation by itself.
- In a 10-speed range-shift transmission, how is the range (high/low) shift normally actuated?
- By an air-operated range cylinder triggered by the range selector
- By the inertia brake solenoid
- By engine vacuum
- By a mechanical cable from the shift lever
Correct answer: By an air-operated range cylinder triggered by the range selector
An air-operated range cylinder triggered by the range selector actuates the range shift. The driver preselects high or low with the range valve, and the shift completes with shop air moving the range cylinder as the lever passes through neutral. Range shifting is not done by cable, vacuum, or the inertia brake.
- A truck with an air range-shift transmission will shift normally within low range but will not complete the shift into high range. Air pressure is normal. The MOST likely cause is:
- Low transmission lube level
- A faulty range cylinder, range valve, or sticking range yoke
- A defective inertia brake
- Worn main-section synchronizers
Correct answer: A faulty range cylinder, range valve, or sticking range yoke
A faulty range cylinder, range valve, or sticking range yoke is the most likely cause when low range works but high range will not engage. The range shift relies on air moving the range piston and yoke; a stuck piston, bad slave/range valve, or binding yoke prevents the auxiliary section from completing the range change. Synchronizer wear and lube level affect main-section shifting, not range completion.
- On a transmission equipped with a splitter, the splitter valve (button) operates the:
- Reverse idler gear
- Power divider lockout
- Clutch brake to stop input shaft rotation
- Auxiliary splitter clutch to provide an intermediate ratio between main gears
Correct answer: Auxiliary splitter clutch to provide an intermediate ratio between main gears
The auxiliary splitter clutch that provides an intermediate ratio between main gears is what the splitter valve controls. Pressing the splitter button shifts the splitter from direct to overdrive (or vice versa) within the auxiliary section, effectively halving the step between main-section ratios. It does not operate the clutch brake, reverse idler, or power divider.
- What is the purpose of a deep reduction (low-low) gear in a heavy-duty manual transmission?
- To provide the highest road speed on the highway
- To provide an extra-low starting ratio for heavy loads or steep grades
- To disengage the auxiliary countershafts
- To drive the PTO at idle
Correct answer: To provide an extra-low starting ratio for heavy loads or steep grades
Providing an extra-low starting ratio for heavy loads or steep grades is the purpose of the deep reduction gear. Selected by a separate lever or button, deep reduction gives the lowest available ratio for maximum torque multiplication when starting a fully loaded truck or creeping on a grade. It does not provide top speed and is not used to drive the PTO.
- An automated manual transmission (AMT) differs from a traditional automatic mainly in that it uses:
- A torque converter and planetary gearsets
- A hydraulic valve body with friction packs
- A mechanical clutch and gearset shifted by electronically controlled actuators
- A continuously variable belt
Correct answer: A mechanical clutch and gearset shifted by electronically controlled actuators
A mechanical clutch and gearset shifted by electronically controlled actuators is what defines an AMT. It is essentially a manual transmission with computer-controlled clutch and shift actuators rather than a torque converter and planetary gears like a true automatic. Unlike a traditional automatic, an AMT does not use a hydraulic valve body with friction packs.
- When diagnosing an automated manual transmission that fails to complete shifts, the technician should FIRST:
- Adjust the shift lever detents
- Replace the clutch as a complete unit
- Retrieve fault codes and check actuator air pressure and electrical signals with the proper diagnostic software
- Drain and refill the transmission lube
Correct answer: Retrieve fault codes and check actuator air pressure and electrical signals with the proper diagnostic software
Retrieving fault codes and checking actuator air pressure and electrical signals with diagnostic software is the correct first step on an AMT. Because the shifts are commanded electronically, the controller logs faults that point to the failed actuator, sensor, or supply pressure before any parts are replaced. Replacing the clutch or changing lube without diagnosis wastes time and may not address the actual fault.
- On an Eaton-style automated transmission that shifts without disengaging the master clutch, the inertia brake (input shaft brake) is applied during an upshift to:
- Hold the truck on a grade
- Engage the PTO
- Speed up the input shaft to match the lower gear
- Slow the input shaft and gearing so the next higher gear can engage quickly
Correct answer: Slow the input shaft and gearing so the next higher gear can engage quickly
Slowing the input shaft and gearing so the next higher gear can engage quickly is the inertia brake's job during an upshift. Because input speed must drop for a higher gear, the brake decelerates the input shaft and associated gearing rapidly to synchronize and shorten shift time. It does not speed up the input shaft, hold the truck, or run the PTO.
- What is an inertia brake on a heavy-duty transmission?
- A friction device that stops the output shaft for parking
- A clutch brake that lets the truck be started in gear
- A retarder mounted on the driveshaft
- A device that decelerates the input shaft and gearing for faster upshifting
Correct answer: A device that decelerates the input shaft and gearing for faster upshifting
A device that decelerates the input shaft and gearing for faster upshifting is what an inertia brake (also called an input shaft brake or upshift brake) is. By braking the spinning input-side mass, it lets the transmission reach the lower input speed needed for a higher gear more quickly. It is not a parking device, a clutch brake, or a driveline retarder.
- A heavy-duty transmission repeatedly jumps out of high range under load. After confirming proper range air operation, the MOST likely internal cause is:
- Worn or damaged clutching teeth on the auxiliary high-range gear or worn range synchronizer
- An overfilled differential
- Low engine oil pressure
- A glazed clutch disc
Correct answer: Worn or damaged clutching teeth on the auxiliary high-range gear or worn range synchronizer
Worn or damaged clutching teeth on the auxiliary high-range gear, or a worn range synchronizer, is the most likely internal cause of jumping out of high range. Rounded or chipped engagement teeth cannot stay fully meshed under torque, so the gear walks out. Engine oil pressure, clutch glazing, and differential fill level have nothing to do with the auxiliary section staying in range.
- A manual transmission jumps out of a particular main-section gear under acceleration. Which condition is LEAST likely to be responsible?
- Worn shift fork or worn yoke pads
- Worn clutching teeth on that gear
- Worn detent spring or detent notch
- A clogged transmission breather
Correct answer: A clogged transmission breather
A clogged transmission breather is least likely to cause a gear to jump out. Jumping out of gear comes from anything that lets the engagement hold release: worn shift forks, weak or worn detents, or rounded clutching teeth. A plugged breather causes pressure buildup and seal leaks, not gear disengagement.
- A driver complains of hard shifting on a non-synchronized manual transmission. The clutch fully releases and lube is correct. Which is the MOST likely remaining cause?
- Bent or binding shift bars/yokes or worn shift tower
- Excessive driveshaft U-joint angle
- A leaking wheel seal
- A worn pinion bearing
Correct answer: Bent or binding shift bars/yokes or worn shift tower
Bent or binding shift bars/yokes or a worn shift tower is the most likely remaining cause of hard shifting once the clutch and lube are confirmed good. Binding in the shift rails, yokes, or tower mechanism makes the lever stiff and gears hard to engage. Pinion bearings, U-joint angle, and wheel seals are unrelated to shift effort.
- Before road-testing a heavy-duty transmission for a shifting complaint, how should the lube level be checked?
- By reading a dipstick at operating temperature only
- With the truck on a steep grade, engine running
- With the vehicle level and lube at the bottom of the fill plug hole
- By weighing the drained fluid
Correct answer: With the vehicle level and lube at the bottom of the fill plug hole
With the vehicle level and the lube at the bottom of the fill plug hole is the correct way to check a heavy-duty transmission lube level. These transmissions use a fill plug, not a dipstick; the oil should be even with the bottom of the fill opening with the truck sitting level. Checking on a grade or by weight gives a false reading.
- A heavy-duty transmission runs noticeably hot and shows accelerated gear wear. The lube was even with the fill plug. The next thing to verify is:
- That the PTO is disengaged
- That the clutch brake is adjusted
- That the correct lubricant type and viscosity were used and the cooler (if equipped) is working
- That the inertia brake solenoid is grounded
Correct answer: That the correct lubricant type and viscosity were used and the cooler (if equipped) is working
Verifying the correct lubricant type and viscosity and a working cooler is the right next step for overheating with a proper fill level. The wrong oil or a non-functioning lube cooler causes high temperatures and wear even when the level is correct; many heavy transmissions specify a particular synthetic and may use a cooler. Clutch brake and inertia brake adjustments do not cause transmission overheating.
- How is a Power Take-Off (PTO) on a heavy-duty transmission normally driven?
- From the inertia brake hub
- From the output shaft after the auxiliary section
- From the differential carrier
- From a PTO aperture gear meshed with a countershaft gear
Correct answer: From a PTO aperture gear meshed with a countershaft gear
From a PTO aperture gear meshed with a countershaft gear is how a transmission-mounted PTO is normally driven. The PTO bolts to an opening on the transmission case and its gear meshes directly with a countershaft gear, so it turns whenever the input shaft turns (clutch engaged). It is not driven off the output shaft, differential, or inertia brake.
- When selecting and mounting a transmission PTO, the technician must verify the PTO code on the transmission tag mainly to confirm:
- The clutch brake thickness
- The engine horsepower rating
- The rear axle ratio
- The correct gear ratio and aperture/bolt pattern for that transmission
Correct answer: The correct gear ratio and aperture/bolt pattern for that transmission
Confirming the correct gear ratio and aperture/bolt pattern for that transmission is why the PTO code on the tag matters. The code identifies the PTO opening, drive gear, and mounting pattern so the chosen PTO meshes properly and turns at the intended speed. The tag PTO code does not specify engine horsepower, axle ratio, or clutch brake thickness.
- A transmission's identification tag is needed during diagnosis primarily to determine the:
- Last service date
- Tire size and wheelbase
- Brand of installed clutch
- Model, ratio set, torque rating, and option content of the transmission
Correct answer: Model, ratio set, torque rating, and option content of the transmission
Determining the model, ratio set, torque rating, and option content of the transmission is the main reason to read the ID tag. The model nomenclature encodes torque capacity, number of speeds, ratio set, and options, which the technician needs to order correct parts and look up specs. The tag does not list tire size, clutch brand, or service history.
- A fleet's overall drive ratio uses a 0.73 overdrive top gear and a 3.55 rear axle ratio. What is the approximate combined final ratio in that gear?
- About 0.21 to 1
- About 4.28 to 1
- About 2.59 to 1
- About 3.55 to 1
Correct answer: About 2.59 to 1
About 2.59 to 1 is the combined final ratio. The transmission ratio is multiplied by the axle ratio: 0.73 times 3.55 equals roughly 2.59 to 1. Adding the ratios (4.28) or ignoring the transmission ratio (3.55) both give incorrect results; gear ratios in series are multiplied, not added.
- To calculate a single gear ratio inside a transmission, the technician divides the:
- Input torque by the output torque
- Number of teeth on the drive gear by the engine RPM
- Output shaft speed by the tire diameter
- Number of teeth on the driven gear by the number of teeth on the drive gear
Correct answer: Number of teeth on the driven gear by the number of teeth on the drive gear
Dividing the number of teeth on the driven gear by the number of teeth on the drive gear gives the gear ratio. For example, a 36-tooth driven gear and an 18-tooth drive gear yield a 2.0 to 1 reduction. Mixing teeth with RPM, shaft speed with tire size, or torque values does not produce a gear ratio.
- A technician is checking the U-joint operating (working) angles on a medium-duty truck driveline. For long, vibration-free U-joint life, what is the generally accepted MAXIMUM continuous operating angle at each U-joint?
- About 15 degrees
- About 1/2 degree
- About 8 degrees
- About 3 degrees
Correct answer: About 3 degrees
About 3 degrees is the accepted maximum continuous operating angle for each U-joint. Most driveline manufacturers recommend operating angles of 3 degrees or less for maximum U-joint life; angles between roughly 3 and 6 degrees shorten life, and anything beyond about 6 degrees causes excessive vibration and rapid failure. A 1/2-degree value is the recommended MINIMUM needed to keep the needle bearings rotating and lubricated, not the maximum.
- When measuring driveline working angles on a truck, a technician places a magnetic-base inclinometer on the transmission output, the driveshaft tube, and the rear pinion yoke flat. What is the technician actually trying to determine from these readings?
- The difference in slope between connected components at each U-joint
- The final drive gear ratio
- The runout of the driveshaft tube
- The total length the slip yoke can travel
Correct answer: The difference in slope between connected components at each U-joint
The difference in slope between connected components at each U-joint is what these inclinometer readings determine. The working (operating) angle at a joint equals the difference between the slope of the driving member and the slope of the driven member, so the technician measures transmission output slope, driveshaft slope, and pinion slope and compares them. Reading length, tube runout, or gear ratio requires different procedures and tools.
- A driveshaft produces a torsional vibration that worsens with load. Inspection shows the two yokes on a single-piece shaft are NOT aligned in the same plane. This out-of-phase condition causes vibration because:
- The slip yoke cannot extend and retract
- The two U-joints speed up and slow down out of sync instead of canceling each other
- The companion flange runout exceeds specification
- The center support bearing carries no load
Correct answer: The two U-joints speed up and slow down out of sync instead of canceling each other
The two U-joints speed up and slow down out of sync instead of canceling each other when a shaft is out of phase. A single U-joint runs at a fluctuating (non-constant) velocity through each revolution; correct phasing places both end yokes in the same plane so the second joint's fluctuation cancels the first's, smoothing output. When the yokes are misaligned the fluctuations add instead of cancel, producing a torsional vibration. Slip-yoke travel, flange runout, and support-bearing load are separate issues.
- A technician greases a truck driveline U-joint with an NLGI Grade 2 EP grease but new grease purges from only three of the four bearing-cap seals. What is the correct next step?
- Consider the joint fully lubricated since most caps purged
- Continue applying grease, tapping or prying to relieve the dry seal until it purges, or loosen that cap's capscrews
- Replace the slip yoke
- Reduce the U-joint operating angle
Correct answer: Continue applying grease, tapping or prying to relieve the dry seal until it purges, or loosen that cap's capscrews
Continue applying grease while relieving the dry seal until all four caps purge is correct. Proper U-joint lubrication requires fresh grease to purge from all four bearing-cap seals, which confirms each bearing is filled and flushes out moisture and contaminants; a cap that will not purge can sometimes be freed by tapping or prying the trunnion away from that seal, or by loosening that cap's capscrews to relieve seal tension. If it still will not purge, the joint is replaced. Three of four purging does not confirm the dry bearing is greased.
- During driveshaft service a technician notices deep scoring and galling on the splines of the slip (slip-joint) yoke. Besides allowing power transfer, what is the PRIMARY function the slip yoke performs that makes spline condition important?
- It sets the U-joint operating angle
- It supports the center of a two-piece driveshaft
- It balances the driveshaft at high speed
- It allows the driveshaft to change length as the axle moves through suspension travel
Correct answer: It allows the driveshaft to change length as the axle moves through suspension travel
Allowing the driveshaft to change length as the axle moves through suspension travel is the slip yoke's primary function. The splined slip joint lets the shaft lengthen and shorten as the rear axle moves up and down and the distance between transmission and axle changes; worn or galled splines bind, which can transmit thrust loads and cause vibration or seal damage. Balancing, setting joint angle, and supporting a two-piece shaft are handled by balance weights, mounting geometry, and the center support bearing respectively.
- A truck has a speed-related vibration. Before condemning the driveshaft, a technician mounts a dial indicator against the machined face of the rear axle companion flange and rotates it. An excessive reading here MOST directly indicates:
- A low axle lubricant level
- Worn input shaft bearings
- Worn synchronizers
- Companion flange or pinion runout that can induce driveline vibration
Correct answer: Companion flange or pinion runout that can induce driveline vibration
Companion flange or pinion runout that can induce driveline vibration is what an excessive dial-indicator reading on the flange face indicates. The companion flange must run true within a tight tolerance (often a few thousandths of an inch); runout there throws the attached driveshaft out of true and creates a vibration that can be mistaken for an unbalanced shaft. If reindexing the shaft 180 degrees does not move the high spot, the flange or pinion stem is at fault. This test does not assess input-shaft bearings, lube level, or synchronizers.
- A driver complains of a clunk on acceleration and a vibration that increases with speed. To check the U-joints for wear, a technician grasps the driveshaft near a joint and attempts to rock and twist it by hand. Which finding confirms a worn U-joint?
- The shaft is completely rigid with no movement
- The slip yoke slides smoothly in and out
- Noticeable play, looseness, or a gritty feel at the joint
- Fresh grease purging from the seals
Correct answer: Noticeable play, looseness, or a gritty feel at the joint
Noticeable play, looseness, or a gritty feel at the joint confirms a worn U-joint. A serviceable joint should rock and twist with only minimal, smooth movement; detectable free play or a rough, gritty sensation means the bearings and trunnion are worn, which produces clunking on acceleration and a speed-sensitive vibration. A rigid joint with no play and smooth slip-yoke travel are signs of health, and purging grease relates to lubrication, not bearing wear.
- On a tandem-drive truck, a driver-controlled, air-actuated device allows the front (forward-rear) and rear-rear axles to turn at slightly different speeds during cornering while still splitting torque between them. What is this device called?
- The interaxle differential (power divider)
- The wheel differential lock
- The driveline phasing damper
- The limited-slip clutch pack
Correct answer: The interaxle differential (power divider)
The interaxle differential, commonly called the power divider, is the component that splits engine torque between the two axles of a tandem set while permitting them to rotate at slightly different speeds for cornering, uneven surfaces, and minor tire-diameter differences. A wheel differential handles speed differences between the left and right wheels of one axle, not between the two axles, so that choice describes a different mechanism.
- A driver notices reduced traction on an icy grade. Engaging the interaxle differential (power divider) lock improves traction because, when locked, the unit:
- Disconnects the forward-rear axle so all torque goes to the rear-rear axle
- Acts as a solid through-drive so both tandem axles must turn at the same speed
- Reduces pinion bearing preload to lower driveline drag
- Increases ring-and-pinion backlash to soften torque delivery
Correct answer: Acts as a solid through-drive so both tandem axles must turn at the same speed
When locked, the interaxle differential acts as a solid through-drive, forcing both tandem axles to rotate at the same speed so a spinning axle cannot rob torque from the axle with grip. It should be locked before wheels begin to spin and only used in low-traction conditions; leaving it locked on dry, high-traction pavement causes driveline wind-up and tire scuffing in turns.
- A two-speed drive axle gives the driver high and low ratio choices within a single axle by shifting a planetary or sliding-clutch assembly. The actual ratio change inside a two-speed axle occurs at the:
- Interaxle differential between the two tandem axles
- Auxiliary gear set that engages high or low through the differential carrier
- Wheel hubs through a separate planetary at each wheel end
- Slip yoke on the inter-axle driveshaft
Correct answer: Auxiliary gear set that engages high or low through the differential carrier
In a two-speed axle, the ratio change happens at an auxiliary gear set (often planetary) within the carrier that engages either the high or low reduction, doubling the available final ratios from one axle. This is different from a power divider, which splits torque between two axles of a tandem rather than changing the reduction ratio at one axle.
- During differential carrier overhaul, the technician must set pinion depth before adjusting backlash. Pinion depth in a hypoid drive axle is controlled by:
- The shim pack behind (or under) the pinion bearing cage
- The pinion nut torque value
- The thrust block clearance
- The carrier-bearing adjusting rings
Correct answer: The shim pack behind (or under) the pinion bearing cage
Pinion depth is set by the shim pack located behind the pinion bearing cage (or under the inner pinion bearing cup), which moves the pinion toward or away from the ring gear centerline to place the tooth contact pattern correctly along the tooth depth. The carrier-bearing adjusting rings move the ring gear sideways to set backlash and carrier preload, which is a separate, later step.
- After setting pinion depth, a technician needs to set ring-and-pinion backlash on a heavy-truck drive axle. Backlash is adjusted by:
- Moving the ring gear toward or away from the pinion using the carrier-bearing adjusting rings
- Replacing the thrust block with a thicker unit
- Tightening the pinion nut to collapse the crush sleeve further
- Changing the shim pack behind the pinion bearing cage
Correct answer: Moving the ring gear toward or away from the pinion using the carrier-bearing adjusting rings
Backlash is set by moving the ring gear laterally with the carrier-bearing adjusting rings: loosening the adjuster on the tooth side and tightening the opposite adjuster moves the ring gear away from the pinion to increase backlash, and the reverse decreases it, while keeping carrier preload. Changing the pinion shim alters pinion depth, not backlash, and the crush sleeve sets pinion bearing preload only.
- A typical published backlash specification for a heavy-truck hypoid ring-and-pinion set, measured with a dial indicator at the ring gear, falls in approximately which range?
- 0.008 to 0.018 inch
- 0.150 to 0.200 inch
- 0.060 to 0.090 inch
- 0.0005 to 0.001 inch
Correct answer: 0.008 to 0.018 inch
Ring-and-pinion backlash for heavy-truck drive axles is typically specified in roughly the 0.008 to 0.018 inch range (always confirm the exact figure stamped on the ring gear or in the OEM manual). The very small 0.0005 to 0.001 inch figure is far too tight and is closer to a bearing-clearance value, while 0.060 inch and larger would represent gross wear or misadjustment.
- A technician measures pinion bearing preload (rolling torque) on a newly assembled drive-axle pinion before the carrier is installed. This reading is checked with a/an:
- Foot-pound (ft-lb) breaker bar at the pinion nut
- Inch-pound (in-lb) torque wrench while rotating the pinion
- Dial indicator placed against the ring gear face
- Feeler gauge between the pinion bearing cups
Correct answer: Inch-pound (in-lb) torque wrench while rotating the pinion
Pinion bearing preload is verified as a rotating (rolling) torque using an inch-pound torque wrench while turning the pinion, with specs commonly in roughly the 15 to 30 in-lb range for new bearings (confirm OEM value). A foot-pound wrench is far too coarse for this small measurement, and a dial indicator measures backlash or runout, not rolling resistance.
- On a pinion assembly that uses a collapsible spacer (crush sleeve), the spacer's purpose is to:
- Set the pinion depth relative to the ring gear
- Lock the ring gear against the differential case
- Limit ring-gear deflection like a thrust block
- Establish pinion bearing preload as the pinion nut is tightened
Correct answer: Establish pinion bearing preload as the pinion nut is tightened
The crush sleeve (collapsible spacer) sits between the inner and outer pinion bearings and is deliberately collapsed by tightening the pinion nut until the correct rolling preload is reached. Because it can be crushed only once, it must be replaced whenever the pinion nut is loosened; pinion depth is set by shims and is independent of the crush sleeve.
- A drive axle whines under acceleration (drive load) but is quiet on deceleration (coast). This pattern most often points to:
- A failed wheel bearing on the same axle
- A pinion-depth/contact-pattern problem on the drive side of the ring gear teeth
- A worn slip yoke on the driveshaft
- Excessive axle-shaft endplay
Correct answer: A pinion-depth/contact-pattern problem on the drive side of the ring gear teeth
A whine that appears under acceleration but not on coast indicates a gear-mesh (ring-and-pinion) contact-pattern issue on the drive side of the teeth, usually traceable to incorrect pinion depth or backlash. Coast (deceleration) whine implicates the opposite tooth flank; isolating which condition produces the noise tells the technician which side of the pattern to correct. A wheel bearing typically produces a speed-related rumble that does not change between drive and coast.
- Inside the differential case, the small gears that turn on the cross (spider) shaft and allow the left and right axle shafts to rotate at different speeds during a turn are the:
- Carrier bearings
- Helical reduction gears
- Spider (pinion) gears meshing with the side gears
- Thrust washers on the ring gear
Correct answer: Spider (pinion) gears meshing with the side gears
The spider gears (also called differential pinion gears) ride on the cross shaft and mesh with the two side gears splined to the axle shafts, allowing the wheels to turn at different speeds in a corner while still transmitting torque. In a thrust-block design, a thrust block between the spider gears limits side-gear movement and supports the gears under load.
- In some heavy-truck differentials, a thrust block is fitted between the differential side gears or behind the spider gears. Its primary function is to:
- Limit outward movement of the side gears and support the gears under load
- Lock the interaxle differential when traction is lost
- Provide pinion bearing preload
- Set ring-and-pinion backlash automatically
Correct answer: Limit outward movement of the side gears and support the gears under load
The thrust block limits how far the side gears can move outward and helps support the differential gears against the heavy thrust loads developed in service, keeping the gear mesh stable. It plays no role in setting backlash (carrier adjusting rings) or pinion preload (crush sleeve or shims), nor does it lock the tandem axles together.
- A technician needs to confirm the drive-axle gear ratio without the data tag. Rotating the input pinion yoke and counting how many turns it makes per one full turn of the wheel (with both wheels equal) yields the ratio. If the pinion turns 4.11 times for one wheel revolution, the axle ratio is:
Correct answer: 4.11:1
The drive-axle ratio equals the number of pinion (driveshaft) revolutions per one revolution of the driven wheel, so 4.11 pinion turns to one wheel turn is a 4.11:1 ratio, expressed driveshaft-to-wheel. The parenthetical 'with both wheels equal' is essential: with both wheels raised and turning together (as in a locked or limited-slip axle test), one full wheel revolution equals one ring-gear revolution and the count is direct. For an open differential with only one wheel raised, two full wheel turns are needed to get one ring-gear revolution, so the raw count must not be halved here.
- On a two-plate (twin-disc) heavy-duty truck clutch, what is the function of the intermediate (center) plate?
- It provides a second clamping friction surface so the assembly can transmit more torque using two driven discs
- It replaces the pilot bearing by centering the input shaft
- It dampens torsional vibration in place of the disc springs
- It applies clamping force in place of the pressure plate springs
Correct answer: It provides a second clamping friction surface so the assembly can transmit more torque using two driven discs
Correct answer: It provides a second clamping friction surface so the assembly can transmit more torque using two driven discs. A two-plate clutch adds an intermediate plate between two friction discs, doubling the clamping surfaces so the unit handles the high torque of heavy trucks without an oversized single disc. The intermediate plate does not center the input shaft, dampen vibration, or supply the clamping force, which remain the jobs of the pilot bearing, disc dampers, and pressure plate springs respectively.
- When installing a clutch disc and pressure plate on a flywheel, why must a technician use a clutch alignment tool (pilot shaft) before final torquing of the pressure plate bolts?
- To pre-load the pressure plate springs to the correct clamping force
- To center the disc splines with the pilot bearing bore so the transmission input shaft will slide in and the disc engages without runout
- To set release bearing travel to specification
- To measure flywheel face runout during assembly
Correct answer: To center the disc splines with the pilot bearing bore so the transmission input shaft will slide in and the disc engages without runout
Correct answer: To center the disc splines with the pilot bearing bore so the transmission input shaft will slide in and the disc engages without runout. The alignment tool holds the disc hub concentric with the crankshaft/pilot bearing while the bolts are tightened; if the disc is off-center the input shaft cannot be installed and engagement is rough. The tool does nothing to spring pre-load, release bearing travel, or flywheel runout, which are separate steps.
- A technician finds the clutch disc facings soaked with gear oil and traces it to a leak. Which seal is the MOST likely source of oil contaminating the clutch from the transmission side?
- The engine rear main seal
- The transmission input shaft (front) seal in the bearing retainer
- The differential pinion seal
- The valve cover gasket
Correct answer: The transmission input shaft (front) seal in the bearing retainer
Correct answer: The transmission input shaft (front) seal in the bearing retainer. Gear oil reaching the clutch from behind comes through the transmission input shaft seal located in the front bearing retainer, where the release bearing rides. The engine rear main seal leaks engine oil from the front of the bellhousing, the pinion seal is at the axle, and the valve cover gasket leaks at the top of the engine, so none of those deliver gear oil to the clutch.
- What is the primary purpose of the torsional (damper) springs built into a heavy-duty clutch driven disc?
- To absorb engine torsional vibration and cushion shock as the clutch engages
- To supply the clamping force that holds the disc against the flywheel
- To return the clutch pedal to its rest position
- To slow the input shaft when starting from a stop
Correct answer: To absorb engine torsional vibration and cushion shock as the clutch engages
Correct answer: To absorb engine torsional vibration and cushion shock as the clutch engages. The coil torsional springs around the disc hub flex slightly to damp the firing pulses of the engine and soften engagement, protecting the driveline from shock loads. Clamping force comes from the pressure plate springs, pedal return is handled by the return spring, and slowing the input shaft from a stop is the clutch brake's job.
- A heavy truck originally equipped with a ceramic-button clutch disc is being serviced. Compared with a standard organic-facing disc, the ceramic-button disc is characterized by:
- Lower torque capacity and the smoothest possible engagement
- Higher torque capacity but more aggressive, less smooth engagement
- Identical engagement feel but a much shorter service life
- No torsional springs and no clamping requirement
Correct answer: Higher torque capacity but more aggressive, less smooth engagement
Correct answer: Higher torque capacity but more aggressive, less smooth engagement. Ceramic-button facings grip harder and tolerate higher heat and torque than organic facings, which is why they are used in severe-duty applications, but the tradeoff is a more abrupt, grabby engagement. They do not lower torque capacity, do not inherently shorten life, and they still rely on disc torsional springs and pressure plate clamping just like organic discs.
- Before installing a new clutch, a technician inspects the flywheel friction surface and finds heat checks, hard spots, and deep scoring. What is the correct course of action?
- Install the new clutch as-is because the disc will quickly wear the surface smooth
- Resurface (machine) or replace the flywheel to restore a flat, smooth friction surface within specification
- Apply grease to the flywheel face to mask the scoring
- Increase pressure plate bolt torque to compensate for the rough surface
Correct answer: Resurface (machine) or replace the flywheel to restore a flat, smooth friction surface within specification
Correct answer: Resurface (machine) or replace the flywheel to restore a flat, smooth friction surface within specification. A damaged flywheel face must be machined flat or replaced so the new disc seats properly and does not chatter, slip, or wear prematurely. Installing the clutch on a damaged surface, greasing the friction face, or over-torquing bolts would all cause poor engagement or rapid failure.
- A two-plate clutch is dragging and will not fully release even though release bearing travel is correct. A common cause unique to two-plate designs is:
- Worn pedal return spring
- The intermediate plate drive pins or slots are worn or sticking, so the center plate does not free up when the pedal is pressed
- Excessive flywheel ring gear wear
- Low engine idle speed
Correct answer: The intermediate plate drive pins or slots are worn or sticking, so the center plate does not free up when the pedal is pressed
Correct answer: The intermediate plate drive pins or slots are worn or sticking, so the center plate does not free up when the pedal is pressed. In a two-plate clutch the intermediate plate must float free on its drive pins or slots during release; if those bind, the center plate keeps gripping a disc and the clutch drags. A pedal return spring, ring gear wear, or idle speed would not specifically prevent the intermediate plate from releasing.
- On a heavy truck with a hydraulically actuated clutch, the pedal slowly sinks to the floor under steady foot pressure and the clutch is hard to keep disengaged. The MOST likely cause is:
- A worn clutch disc
- An internal leak past the master cylinder cup seal
- A warped flywheel
- Excessive clutch brake squeeze
Correct answer: An internal leak past the master cylinder cup seal
Correct answer: An internal leak past the master cylinder cup seal. A clutch pedal that sinks under steady pressure with no external fluid loss points to fluid bypassing the master cylinder cup seal internally, so pressure cannot be held. A worn disc causes slip, a warped flywheel causes chatter, and clutch brake squeeze affects only the very bottom of pedal travel, none of which make the pedal slowly drop on its own.
- When replacing a clutch release (throwout) bearing on a heavy truck, what is the correct lubrication and handling practice?
- Soak the sealed bearing in solvent to flush old grease before installation
- Pack the sealed bearing fully with chassis grease through its seals
- Leave the sealed bearing as supplied and lightly lubricate the carrier bore and release fork contact points per the service manual
- Run the bearing dry with no lubrication anywhere
Correct answer: Leave the sealed bearing as supplied and lightly lubricate the carrier bore and release fork contact points per the service manual
Correct answer: Leave the sealed bearing as supplied and lightly lubricate the carrier bore and release fork contact points per the service manual. A sealed release bearing comes pre-lubricated and must not be opened, washed in solvent, or repacked, since that contaminates or removes the internal grease. The technician only lightly lubricates the sliding carrier bore and the fork contact pads as the manual specifies; running everything dry causes rapid wear.
- A heavy-duty clutch slips only briefly during hard upshifts but holds the rest of the time, and pedal free play measures slightly less than specification. What should the technician check or correct FIRST?
- Restore the correct pedal free play so the release bearing is not riding the pressure plate fingers
- Replace the flywheel ring gear
- Replace the pilot bearing
- Add friction modifier to the transmission lubricant
Correct answer: Restore the correct pedal free play so the release bearing is not riding the pressure plate fingers
Correct answer: Restore the correct pedal free play so the release bearing is not riding the pressure plate fingers. Slightly low free play lets the release bearing partially load the fingers, relieving some clamping force and causing momentary slip under high-torque shifts; correcting free play is the simplest first step before tearing the clutch apart. The ring gear, pilot bearing, and lubricant friction modifier are unrelated to a free-play-induced slip.
- When using a dial indicator to measure mainshaft gear end play in a heavy-duty manual transmission, an excessive reading MOST directly indicates:
- Worn thrust washers or worn snap-ring grooves on the mainshaft
- Incorrect range air pressure to the auxiliary section
- A burned synchronizer cone in the auxiliary box
- An overfilled lubricant condition in the main case
Correct answer: Worn thrust washers or worn snap-ring grooves on the mainshaft
Worn thrust washers or worn snap-ring grooves on the mainshaft is correct. Mainshaft gear end play is the axial float a gear has between its locating washers or snap rings, so an excessive dial-indicator reading means those thrust washers are worn thin or the snap-ring grooves have spread. Range air pressure, synchronizer cones, and lube level do not control mainshaft axial float and would not show up as excessive end play.
- A technician inspects gear teeth in a heavy-duty transmission and finds pitting and flaking confined to the pitch line of several teeth. This surface damage is BEST described as:
- Fatigue spalling caused by repeated high contact stress
- Adhesive scuffing from a momentary lube starvation
- Plastic deformation from a single shock overload
- Corrosion etching from water-contaminated lubricant
Correct answer: Fatigue spalling caused by repeated high contact stress
Fatigue spalling caused by repeated high contact stress is correct. Pitting and flaking concentrated at the pitch line, where rolling contact stress is highest, is the classic appearance of surface fatigue (spalling) from many load cycles. Scuffing leaves smeared metal, a single shock overload bends or breaks teeth, and corrosion produces rust-colored etching across surfaces rather than localized pitch-line pitting.
- Before disassembling a heavy-duty transmission for an internal noise complaint, the technician should FIRST:
- Perform a road test and isolate whether the noise occurs in neutral, in a specific gear, or only under load
- Verify the lubricant level and condition and inspect for external leaks or loose mounts
- Remove the PTO cover to inspect the countershaft gear teeth
- Replace the input shaft bearing as a precaution
Correct answer: Verify the lubricant level and condition and inspect for external leaks or loose mounts
Verifying the lubricant level and condition and inspecting for external leaks or loose mounts is correct as the first step. Low or wrong lube and loose mountings are quick, inexpensive checks that can cause or mimic internal noise, so they are ruled out before any teardown. A road test is part of diagnosis but follows the basic fluid and mounting checks, and removing the PTO cover or replacing a bearing are invasive actions taken only after the simple checks point that way.
- A heavy-duty transmission shows a steady stream of lubricant weeping from the breather/vent on the top cover. The MOST likely cause is:
- A plugged or restricted breather causing internal pressure buildup
- An overfilled lubricant level pushing oil out as it heats and expands
- A worn input shaft seal drawing oil forward
- Excessive mainshaft gear backlash
Correct answer: An overfilled lubricant level pushing oil out as it heats and expands
An overfilled lubricant level pushing oil out as it heats and expands is correct. When the case is filled above the fill-plug level, the extra oil is churned and thermally expands until it escapes through the highest opening, which is the breather. A plugged breather traps pressure and forces oil past seals rather than out the vent itself, a worn input seal leaks at the front of the case, and backlash does not move oil out the breather.
- While shimming a bearing during a heavy-duty transmission rebuild, the technician must set bearing preload primarily to:
- Increase the gear backlash to reduce operating noise
- Remove internal clearance so the shaft is properly located and the bearing carries load without excessive play
- Allow the gears to free-wheel more easily in neutral
- Compensate for a worn synchronizer blocker ring
Correct answer: Remove internal clearance so the shaft is properly located and the bearing carries load without excessive play
Removing internal clearance so the shaft is properly located and the bearing carries load without excessive play is correct. Preload shimming takes the slack out of tapered or angular-contact bearings so the shaft stays centered and the rollers share the load evenly. Preload is not used to add backlash, to make gears free-wheel, or to make up for a worn synchronizer, which is a separate component requiring its own replacement.
- A technician measures ring-and-pinion-style gear backlash inside the auxiliary section and finds it well below specification. Insufficient backlash is MOST likely to cause:
- Overheating and rapid wear because the teeth bind with no room for a lube film
- A loud clunk only when reversing direction of load
- The transmission to jump out of gear under acceleration
- Delayed engagement when selecting a starting gear
Correct answer: Overheating and rapid wear because the teeth bind with no room for a lube film
Overheating and rapid wear because the teeth bind with no room for a lube film is correct. Backlash is the small clearance between meshing teeth that allows for thermal expansion and an oil film, so too little of it makes the teeth bind, run hot, and wear quickly. A clunk on load reversal and noise on coast are symptoms of excessive backlash, while jumping out of gear and delayed engagement stem from other causes such as worn detents or clutch drag.
- A driver reports that a heavy truck is hard to shift and the transmission feels notchy only during the first few miles on a cold morning, then improves. The MOST likely cause is:
- Lubricant that is too heavy or the wrong viscosity for cold operation
- A worn clutch release bearing
- An air leak in the range-shift system
- A cracked transmission case
Correct answer: Lubricant that is too heavy or the wrong viscosity for cold operation
Lubricant that is too heavy or the wrong viscosity for cold operation is correct. An oil that is too thick when cold resists the movement of gears and sliding clutches, producing stiff, notchy shifts that ease once friction warms the lube. A worn release bearing makes noise during clutch use, a range air leak prevents range changes regardless of temperature, and a cracked case leaks lube rather than causing temperature-dependent shift effort.
- When checking the shift fork-to-sliding-clutch (collar) clearance during a transmission inspection, a measurement greater than specification indicates:
- A bent or worn shift fork or a worn clutch collar groove
- Correctly set bearing preload
- Proper synchronizer cone contact
- An overfilled main case
Correct answer: A bent or worn shift fork or a worn clutch collar groove
A bent or worn shift fork or a worn clutch collar groove is correct. Excess fork-to-collar clearance means the fork pads or the groove they ride in have worn, or the fork has bent, which lets the collar move sloppily and can let the gear creep out of engagement. Bearing preload and synchronizer cone contact are unrelated measurements, and lube level has no effect on the physical clearance between the fork and the collar.
- A heavy-duty transmission produces a growling noise that is present in every forward gear and in reverse, but the noise disappears when the transmission is shifted to neutral with the truck stopped. The MOST likely source is:
- A worn mainshaft (output) bearing that is loaded whenever the truck is moving in any gear
- A worn input shaft bearing
- A worn synchronizer for second gear only
- A loose shift tower cover gasket
Correct answer: A worn mainshaft (output) bearing that is loaded whenever the truck is moving in any gear
A worn mainshaft (output) bearing that is loaded whenever the truck is moving in any gear is correct. Because the output (mainshaft) bearing turns and carries load in every gear, its noise is heard in all gears and in reverse but stops once the output stops turning in neutral at a standstill. An input bearing whines most in neutral with the clutch engaged, a single worn synchronizer affects only its own gear, and a cover gasket leaks rather than makes a growl.
- Technician A says that a transmission countershaft (counter gear) carries no load when the transmission is in direct drive on a single-countershaft design. Technician B says that in a twin countershaft design the load is split between the two countershafts in the indirect gears. Who is correct?
- Technician A only
- Technician B only
- Both Technician A and Technician B
- Neither Technician A nor Technician B
Correct answer: Both Technician A and Technician B
Both Technician A and Technician B is correct. In direct drive the input and output are locked together one-to-one, so power bypasses the counter gear and it is essentially unloaded (Technician A). In a twin countershaft transmission the indirect-gear load is shared by two countershafts, which is why those gearboxes handle high torque in a compact, balanced package (Technician B). Both statements describe accepted heavy-duty transmission behavior.
- During reassembly, the technician finds that a tapered bearing cup has spun in its bore in the transmission case. The CORRECT repair is to:
- Apply extra preload to clamp the cup tighter
- Repair or sleeve the bore (or replace the case) so the cup has the proper press fit, then install a new bearing
- Coat the cup with high-strength thread locker and reuse the case as is
- Increase the lubricant viscosity to keep the cup from moving
Correct answer: Repair or sleeve the bore (or replace the case) so the cup has the proper press fit, then install a new bearing
Repairing or sleeving the bore (or replacing the case) so the cup has the proper press fit, then installing a new bearing, is correct. A cup that has spun has enlarged or polished its bore, destroying the interference fit, so the bore must be restored to spec before a new bearing will stay put. Adding preload, gluing the cup, or thickening the lube are improper shortcuts that do not restore the required press fit and will let the cup spin again.
- A heavy-duty transmission has been overheating. The technician confirms correct lube level and viscosity. The NEXT item to check is:
- The condition and flow of the transmission oil cooler and its lines, if equipped
- The clutch pedal free travel
- The driveshaft slip yoke spline wear
- The rear axle ratio tag
Correct answer: The condition and flow of the transmission oil cooler and its lines, if equipped
The condition and flow of the transmission oil cooler and its lines, if equipped, is correct. Once correct lube level and viscosity are verified, a restricted or non-circulating oil cooler is the next likely reason a transmission runs hot, since it removes the heat the lube absorbs. Clutch free travel is a clutch-system check, slip-yoke wear is a driveshaft concern, and the axle ratio tag has nothing to do with how the transmission sheds heat.
- When pressing a new bearing onto a transmission shaft, the technician should apply the pressing force to:
- The inner race only, so force is not driven through the rolling elements
- The outer race, allowing the rollers to seat the inner race
- The bearing cage to keep the rollers aligned
- The rolling elements directly for an even fit
Correct answer: The inner race only, so force is not driven through the rolling elements
Applying the pressing force to the inner race only, so force is not driven through the rolling elements, is correct. When a bearing is installed onto a shaft, the inner race takes the interference fit, so pushing on it keeps the load off the balls or rollers that would otherwise be brinelled. Pressing on the outer race, the cage, or the rolling elements forces the load through the delicate components and damages the bearing before it is ever used.
- A technician must confirm the exact internal ratios and component build of a heavy-duty transmission before ordering parts. The BEST single source for this information is:
- The transmission identification/model tag matched to the manufacturer's service data
- Counting the splines on the output shaft
- Measuring the overall length of the case
- Reading the rear axle ratio stamped on the carrier
Correct answer: The transmission identification/model tag matched to the manufacturer's service data
The transmission identification/model tag matched to the manufacturer's service data is correct. The model and serial tag tells the technician the exact assembly, gear ratios, and parts list when cross-referenced to the maker's published data, which is essential for ordering correct components. Spline counts and case length only hint at family or fit, and the rear axle ratio describes the axle, not the transmission's internal build.
- A two-piece driveshaft on a medium-duty truck produces a vibration that is felt strongest at moderate cruising speeds. Inspection shows the rubber-insulated center support bearing carrier is cracked and the rubber is deteriorated. Why does a worn center bearing cause a speed-related driveline vibration?
- It allows the intermediate shaft to sag and shift, changing the U-joint working angles and letting the shaft whip
- It increases the splined slip-joint travel so the shaft becomes too long
- It adds resistance that slows the driveshaft below engine speed
- It causes the transmission output seal to leak fluid onto the shaft
Correct answer: It allows the intermediate shaft to sag and shift, changing the U-joint working angles and letting the shaft whip
Correct answer: It allows the intermediate shaft to sag and shift, changing the U-joint working angles and letting the shaft whip. The center support bearing locates and supports the junction between the two driveshaft sections; when its rubber isolator deteriorates, the shaft is no longer held on its true rotational centerline. The resulting sag alters the U-joint operating angles and permits the shaft to whip, producing a speed-sensitive vibration. It does not change overall shaft length, slow the shaft, or cause seal leakage.
- During U-joint replacement, a technician needs to remove the bearing caps from the cross (spider). After removing the snap rings, what is the recommended method to press the caps out of the yoke?
- Heat the yoke ears with a torch until the caps drop free
- Pry the caps out with a screwdriver between the cap and yoke ear
- Use a press or U-joint tool to push the cross and one cap through the yoke, then remove the opposite cap
- Strike the center of the cross directly with a hammer until both caps eject
Correct answer: Use a press or U-joint tool to push the cross and one cap through the yoke, then remove the opposite cap
Correct answer: Use a press or U-joint tool to push the cross and one cap through the yoke, then remove the opposite cap. The accepted procedure after removing the snap rings is to support the yoke in a press (or a dedicated U-joint puller) and drive the cross to push one bearing cap out of its bore, then reverse to free the opposite cap. Torching the yoke can damage it, prying scores the bores, and hammering the cross damages the cross and races.
- A rear-wheel-drive vehicle exhibits a low-frequency driveline vibration that gets worse as engine torque increases, and inspection reveals incorrect pinion-yoke angle after a leaf-spring shop replaced the rear springs. To correct excessive U-joint working angles caused by the new ride height, a technician should:
- Install a longer driveshaft to span the new distance
- Install tapered shims (wedges) between the axle and spring to re-set the pinion angle
- Add balance weights to the front of the driveshaft tube
- Replace both U-joints with greasable units
Correct answer: Install tapered shims (wedges) between the axle and spring to re-set the pinion angle
Correct answer: Install tapered shims (wedges) between the axle and spring to re-set the pinion angle. When a ride-height change alters the pinion (rear) yoke angle and creates unequal or excessive U-joint working angles, tapered shims placed between the axle housing and the leaf springs rotate the pinion to restore the correct angle. Changing shaft length, adding balance weights, or swapping U-joint type does not correct a pinion-angle error.
- A technician is checking driveline runout and finds that the driveshaft tube itself is bent. According to standard service practice, what is the correct repair for a bent steel driveshaft tube?
- Straighten the tube in a hydraulic press and re-balance it
- Weld a reinforcing sleeve over the bent section
- Replace the driveshaft assembly
- Add balance weights opposite the bend to offset it
Correct answer: Replace the driveshaft assembly
Correct answer: Replace the driveshaft assembly. A bent driveshaft tube cannot be reliably straightened because the tube wall is thin and any attempt to press it true leaves residual stress and runout that cause vibration; the accepted practice is to replace the shaft. Welding a sleeve, adding offset weights, or pressing it straight are not approved repairs for a bent tube.
- A driveline uses a double-Cardan (constant-velocity) joint at the transfer case output of a four-wheel-drive truck. Compared with a single Cardan U-joint, what is the PRIMARY advantage of the double-Cardan joint in this application?
- It eliminates the need for a slip yoke in the driveline
- It delivers nearly constant output velocity even at the large operating angles found on lifted or short-coupled drivelines
- It allows the driveshaft to be run without any lubrication
- It lets the driveshaft spin faster than the transfer case output
Correct answer: It delivers nearly constant output velocity even at the large operating angles found on lifted or short-coupled drivelines
Correct answer: It delivers nearly constant output velocity even at the large operating angles found on lifted or short-coupled drivelines. A double-Cardan joint combines two Cardan joints with a centering ball so the two halves split the total angle, canceling the speed fluctuation of a single U-joint and providing essentially constant velocity at high angles. It still requires a slip yoke for length change, still needs lubrication, and does not change the shaft's rotational speed ratio.
- A technician applies marking compound to a hypoid ring gear and rotates it to read the tooth-contact pattern. The pattern shows contact concentrated toward the small (toe) end of the tooth near the top (face). According to standard drive-axle setup, the technician should correct this by:
- Increasing backlash only and leaving pinion depth alone
- Moving the pinion deeper (toward the ring gear) and then resetting backlash
- Adding more axle lubricant to soften the contact
- Replacing the ring gear because the pattern cannot be corrected
Correct answer: Moving the pinion deeper (toward the ring gear) and then resetting backlash
Moving the pinion deeper and resetting backlash is correct. A toe/face (high and toward the small end) pattern indicates the pinion is too far out; moving it deeper centers the pattern, and backlash is rechecked afterward because shimming the pinion changes the mesh. Simply changing backlash, adding lube, or replacing the ring gear does not address pinion depth.
- A heavy-truck drive axle calls for a hypoid gear lubricant. Which lubricant specification is the MOST appropriate for the hypoid ring-and-pinion gearing in this drive axle?
- API GL-1 straight mineral oil
- API GL-5 gear oil of the viscosity specified by the axle maker
- Automatic transmission fluid (ATF) only
- Engine oil of any current API SN service grade
Correct answer: API GL-5 gear oil of the viscosity specified by the axle maker
API GL-5 gear oil of the specified viscosity is correct. Hypoid gears slide heavily under high tooth pressure and require the extreme-pressure additive level of GL-5. GL-1 lacks EP additives, ATF is not a hypoid gear lubricant, and engine oil is not formulated for hypoid axle service.
- A technician is checking the lubricant level in a heavy-truck drive-axle housing through the side fill plug. The correct full level is indicated when the lubricant is:
- Two inches above the fill-plug opening
- Just touching the ring gear at the bottom of the case
- Level with the bottom of the fill-plug opening
- Halfway up the differential carrier
Correct answer: Level with the bottom of the fill-plug opening
Level with the bottom of the fill-plug opening is correct. Drive-axle housings are filled until lubricant reaches the lower edge of the fill hole, where excess simply runs out. Filling above the hole, or judging by ring-gear or carrier height, leads to incorrect levels and possible seal leakage or starvation.
- A drive axle is leaking lubricant at the pinion (input) yoke. After confirming the breather is clear and the lube level is correct, the technician should FIRST suspect:
- A cracked ring gear
- Worn differential spider gears
- Excessive ring-and-pinion backlash
- A worn pinion (input shaft) seal
Correct answer: A worn pinion (input shaft) seal
A worn pinion seal is correct. Lubricant escaping at the input yoke points to the seal that rides on the yoke or pinion shaft, often worn by a grooved yoke or excessive pinion bearing play. A cracked ring gear, worn spider gears, or excess backlash cause noise or operating problems, not a yoke seal leak.
- A driver reports a clunk only at the moment of acceleration and again when releasing the throttle, with no whine while cruising. In the drive axle, this on/off-throttle clunk MOST likely indicates:
- A pinion bearing set with too much preload
- Lubricant viscosity that is too high
- Excessive backlash or worn differential/axle drivetrain clearance
- A ring gear contact pattern that is centered correctly
Correct answer: Excessive backlash or worn differential/axle drivetrain clearance
Excessive backlash or worn drivetrain clearance is correct. A clunk that occurs only on throttle application and release is the slack in the gear mesh and driveline taking up in each direction. Excessive pinion preload, heavy lube, or a correctly centered contact pattern do not produce that on/off lash clunk.
- On a full-floating heavy-truck rear axle, the wheel hub turns on its own bearings and the axle shaft transmits only torque. Because of this design, removing the axle shaft on a full-floating axle:
- Causes the wheel to fall off because the shaft holds the wheel on
- Does not unload or release the wheel bearings, which still support the wheel
- Releases the wheel-bearing preload that the shaft normally carries
- Requires the differential to be removed first
Correct answer: Does not unload or release the wheel bearings, which still support the wheel
The axle shaft can be removed without unloading the wheel bearings is correct. In a full-floating axle the hub and bearings carry the vehicle weight, so pulling the shaft leaves the wheel supported. The shaft does not hold the wheel on or set bearing preload, and the differential need not come out to pull a shaft.
- A technician mounts a dial indicator against the back face of an installed ring gear and rotates it one full turn, observing the total indicator movement. This measurement checks ring gear:
- Runout (lateral wobble), which can come from a warped gear or dirt on the case flange
- Backlash between the ring gear and pinion
- Pinion bearing rolling torque
- Tooth contact pattern depth
Correct answer: Runout (lateral wobble), which can come from a warped gear or dirt on the case flange
Ring gear runout is correct. A dial indicator on the gear's back face reading total movement per revolution measures lateral wobble, caused by a bent gear, debris between the gear and case, or a distorted case flange. Backlash is read at the tooth flank, preload with a torque wrench, and the contact pattern with marking compound.
- Many heavy-truck differential carriers use an adjustable thrust screw (thrust block screw) positioned behind the ring gear. The thrust screw is adjusted to:
- Set ring-and-pinion backlash to its final value
- Hold the pinion bearing preload
- Lock the differential case against rotation
- Limit ring-gear deflection under heavy load while leaving a small running clearance
Correct answer: Limit ring-gear deflection under heavy load while leaving a small running clearance
Limiting ring-gear deflection while keeping a small clearance is correct. The thrust screw is set close to the back of the ring gear so it supports the gear under load yet does not drag during normal running. It is not used to set backlash, hold pinion preload, or lock the case.
- A driver reports the truck rolls forward in gear but makes no drive at all, and a loud bang was heard beforehand. The axle lube shows large chunks of metal. The MOST likely failure inside the drive axle is:
- A slightly low lubricant level
- A clogged axle housing breather
- Broken ring-and-pinion or differential gears that no longer transmit torque
- Slightly excessive ring-gear backlash
Correct answer: Broken ring-and-pinion or differential gears that no longer transmit torque
Broken ring-and-pinion or differential gears are correct. A loud bang followed by complete loss of drive plus large metal chunks in the lube indicates gear teeth or differential parts have failed and can no longer carry torque. Low lube, a clogged breather, or excess backlash cause wear, leaks, or noise, not an instant total loss of drive with broken metal.
- After replacing the pinion seal on a drive axle, the technician finds the housing breather (vent) is plugged with dirt and paint. Leaving the breather plugged is likely to:
- Build internal pressure that forces lubricant past the seals, causing leaks
- Improve sealing by keeping moisture out permanently
- Reduce ring-and-pinion backlash over time
- Increase pinion bearing preload
Correct answer: Build internal pressure that forces lubricant past the seals, causing leaks
Building internal pressure that forces lube past the seals is correct. As the axle heats up, air and vapor must vent; a plugged breather lets pressure rise and push lubricant out at the seals. A clogged vent does not improve sealing, change backlash, or alter pinion preload.
- On a tandem-drive truck, the interaxle differential (power divider) lock must NOT be engaged when:
- The wheels are spinning or slipping with a large speed difference, or when cornering on dry pavement
- The truck is stopped before starting up a slick grade
- All wheels have equal traction on a straight, icy road
- The driver expects to need extra traction shortly
Correct answer: The wheels are spinning or slipping with a large speed difference, or when cornering on dry pavement
Engaging the lock while wheels are spinning or while cornering on dry pavement is the wrong time, making this correct. Locking the power divider when there is a large wheel-speed difference shock-loads the unit, and locking it on high-traction turns causes driveline windup. It is properly engaged before reaching slick conditions while moving slowly or stopped, not during slip or dry-pavement turns.
- A driveshaft uses a constant-velocity (double-Cardan) joint at the front and a single Cardan U-joint at the rear axle. To minimize vibration, how should the technician set the rear pinion angle relative to the transmission output?
- The pinion centerline should be roughly parallel to the front driveshaft section so the single rear U-joint operates at a small, nearly equal working angle
- The pinion should point straight up so the U-joint operates at a 90-degree angle
- The pinion angle is irrelevant because the CV joint cancels all vibration in the system
- The pinion should be set 15 degrees higher than the transmission output to preload the slip yoke
Correct answer: The pinion centerline should be roughly parallel to the front driveshaft section so the single rear U-joint operates at a small, nearly equal working angle
The correct setup is to keep the pinion centerline roughly parallel to the front driveshaft section so the single rear U-joint runs at a small, nearly equal working angle. In a CV/double-Cardan arrangement the CV joint cancels its own speed fluctuation, so cancellation no longer depends on matched front and rear angles; instead the lone rear single-Cardan joint must operate at the smallest practical angle, which happens when the pinion is aimed nearly parallel to the front shaft. Pointing the pinion straight up creates an extreme angle, the CV joint does not cancel the rear single joint, and adding 15 degrees of preload would force a large unequal working angle that causes vibration.
- A technician must replace a U-joint that is retained by injected nylon rather than snap rings. What is the correct removal procedure?
- Apply heat or press the caps out to shear the nylon retainer, then clean the residual nylon from the yoke bores before installing a snap-ring service joint
- Pry off the external snap rings and tap the caps out with a brass drift
- Drill out the bearing caps and leave the nylon in place to act as a cushion
- Soak the joint in solvent for several hours so the nylon dissolves and the caps fall free
Correct answer: Apply heat or press the caps out to shear the nylon retainer, then clean the residual nylon from the yoke bores before installing a snap-ring service joint
The correct method is to apply heat or press the caps out to shear the injected nylon retainer, clean the residual nylon from the yoke bores, and install a snap-ring service joint. Factory nylon-injected joints have no snap rings, so prying for rings is pointless; the nylon flows into grooves and must be sheared by pressing or softened with heat. Leaving nylon in place prevents proper cap seating, and solvent soaking will not reliably remove the cured nylon.
- On a rear-wheel-drive car, a driveline vibration is present only during hard acceleration and disappears when coasting. Which condition is the MOST likely cause?
- Excessive U-joint working angle change caused by worn or collapsed engine/transmission mounts allowing the powertrain to rise under torque
- An out-of-balance driveshaft, which always vibrates the same regardless of throttle
- A bent driveshaft tube, which produces vibration only at steady cruise speeds
- Low transmission fluid level reducing slip-yoke lubrication
Correct answer: Excessive U-joint working angle change caused by worn or collapsed engine/transmission mounts allowing the powertrain to rise under torque
The most likely cause is excessive change in U-joint working angle from worn or collapsed powertrain mounts that let the engine and transmission lift under acceleration torque. Because the symptom is load- and throttle-dependent rather than speed-dependent, it points to angle change under torque, not a constant fault. An out-of-balance shaft or bent tube produces speed-related vibration that does not vanish when coasting, and slip-yoke lubrication does not cause a throttle-sensitive vibration.
- After setting up a hypoid ring-and-pinion, a technician paints the ring gear teeth and reads a contact pattern that is concentrated toward the toe and near the root (deep) of the tooth. Which adjustment corrects this pattern?
- Move the pinion away from the ring gear (decrease pinion depth) and adjust backlash to recenter the pattern toward the center of the tooth
- Increase pinion preload only, leaving depth and backlash unchanged
- Add carrier preload by tightening both side bearing adjusters equally
- Replace the ring gear because a toe/root pattern cannot be corrected by shims
Correct answer: Move the pinion away from the ring gear (decrease pinion depth) and adjust backlash to recenter the pattern toward the center of the tooth
The correct fix is to move the pinion away from the ring gear (decrease pinion depth) and reset backlash to bring the pattern toward the center of the tooth. A deep, root-and-toe pattern indicates the pinion is set too far in, so reducing pinion depth via a thinner shim moves the contact toward the flank/center, and backlash is then trimmed to center it heel-to-toe. Pinion preload and carrier preload control bearing load, not the depth pattern, and a misadjusted pattern is corrected with shims rather than by replacing the gear.
- A vehicle with a limited-slip (clutch-type) differential chatters and shudders when turning slowly in a parking lot. What is the MOST likely cause and correct first action?
- The friction modifier in the gear lube is depleted or missing; drain and refill with the specified lube plus the correct limited-slip friction additive
- The ring-and-pinion backlash is excessive; tighten the side adjusters to remove it
- The axle shaft bearings are worn; replace both axle bearings
- The pinion seal is leaking; replace the seal and refill to correct the chatter
Correct answer: The friction modifier in the gear lube is depleted or missing; drain and refill with the specified lube plus the correct limited-slip friction additive
The most likely cause is depleted or missing limited-slip friction modifier, and the correct first action is to drain and refill with the specified gear lube plus the proper friction additive. Clutch-type limited-slip units rely on friction modifier so the clutch packs release smoothly during turns; without it the plates grab and release, producing the characteristic low-speed turning chatter. Backlash, axle bearings, and a pinion seal leak do not cause friction-additive turning shudder and would present with different symptoms.