- Preventive maintenance inspection (PMI)
- A scheduled, systematic check of a commercial vehicle's safety and operating systems to find and document defects before failure or violation — inspect and report, not repair on the spot.
- Checking engine oil level (correct method)
- On level ground, a few minutes after shutdown so oil drains back to the pan. Reading while idling or right after shutdown reads falsely low.
- Milky, light-tan, frothy oil
- Coolant or water in the crankcase — usually a head gasket, cracked head, or leaking oil cooler. Flag for diagnosis; do not just change the oil.
- Dark, thick oil vs. milky oil
- Dark = oxidized/over-extended oil. Milky/tan = coolant contamination. Different problems with different actions.
- Refractometer
- The most accurate tool for coolant freeze protection; reads the bend of light and is largely unaffected by temperature.
- Floating-ball antifreeze hydrometer accuracy
- Only about ±8°F and temperature-sensitive — less accurate than a refractometer; ASTM moved to the refractometer method.
- Conventional coolant mix ratio
- A 50/50 mix of ethylene-glycol antifreeze and water gives standard freeze and boil-over protection.
- Straight antifreeze (no water)
- Reduces heat transfer and actually raises the freeze point — never run nearly straight concentrate.
- Supplemental coolant additive (SCA / nitrite)
- Protects wet cylinder liners from cavitation erosion; level is checked with test strips on a diesel PMI.
- Cylinder liner cavitation
- Pitting of wet liners caused by collapsing vapor bubbles; prevented by maintaining SCA/nitrite level.
- Radiator cap function
- Seals the cooling system and holds pressure to raise the boiling point; a vacuum valve returns coolant from the overflow as the engine cools.
- Radiator cap replacement signs
- Brittle/cracked sealing gasket or a weak/corroded pressure-relief spring — the cap can no longer hold rated pressure.
- Cooling-system pressure test
- Pump the system to its rated pressure to confirm it holds, and use a cap adapter to verify the cap releases at its stamped rating.
- Radiator hose inspection (by hand)
- A serviceable hose feels firm yet pliable. Spongy/collapsing or hard/brittle = deteriorated — replace before it ruptures.
- Water-pump weep hole leak
- A crusty dried coolant trail at the weep hole means the shaft seal is leaking — a failing pump.
- Serpentine belt replacement signs
- Glazing, multiple cracks across the ribs, or missing chunks of rib material; EPDM belts also wear thin (check with a wear gauge).
- Automatic belt tensioner check
- Read the built-in wear-indicator marks; the pointer must fall within the acceptable range or the belt/tensioner is worn.
- Exhaust leak (clearest sign)
- Black soot streaking around a joint, clamp, or manifold flange where pressurized gas escapes a failed seal.
- Why exhaust leaks are a priority
- Carbon monoxide (colorless, odorless) can enter the cab and endanger the driver — repair promptly.
- Restricted air filter consequences
- Reduced airflow → low power, black smoke, and higher fuel consumption; replace when the restriction indicator reaches its point.
- Loose air-intake boot clamp
- Lets unfiltered, abrasive air bypass the filter (clean side) and reach the engine, causing rapid cylinder and ring wear.
- Air-filter restriction indicator
- A gauge/manometer showing intake restriction; tells the technician when the element must be replaced.
- Glow plugs (diesel)
- Preheat the combustion chamber to aid cold starting by ensuring proper ignition.
- Turbocharger (diesel)
- Compresses intake air to raise power and efficiency; check for shaft play, oil leaks, and boost-leak signs.
- Excessive diesel exhaust smoke (worn rings)
- Worn piston rings let oil into the chamber, raising oil consumption and emissions (blue/black smoke).
- PCV (Positive Crankcase Ventilation)
- Routes blow-by gases from the crankcase back to the intake to be burned, relieving pressure and cutting emissions.
- EGR (Exhaust Gas Recirculation)
- Recirculates inert exhaust into the intake to lower peak combustion temperature and reduce NOx.
- Engine oil leak (PMI action)
- Trace the wet path upstream to its highest point to find the source, then document the gasket/seal leak and its severity for repair.
- Coolant level check safety
- Check only when the engine is cool and open the pressurized cap cautiously to avoid scalding; top off with the correct premix, not plain water.
- DEF (diesel exhaust fluid)
- A urea solution injected into the SCR system to reduce NOx; check level and for crystallization/contamination.
- Engine coolant temperature reading source
- Use the dipstick/gauge correctly — the dash temperature gauge shows operating temperature, not coolant level.
- Charge-air cooler (CAC) inspection
- Check the intercooler and its hoses/clamps for leaks; a CAC leak loses boost and hurts power and economy.
- Engine mounts
- Support the engine/transmission and absorb vibration; worn mounts cause movement and noise.
- Fan clutch / cooling fan
- Check operation and shroud condition; a fan that won't engage causes overheating under load.
- Air compressor (function)
- Engine-driven pump that builds the air supply for the brake system; governed by the governor.
- Oil-pressure check at PMI
- Verify pressure at idle and at speed against spec; low pressure can be thin oil, worn pump, or worn bearings.
- Diesel knock vs. detonation
- A diesel naturally compression-ignites; abnormal knock can point to injection timing or mechanical wear — diagnose, don't ignore.
- Coolant filter (spin-on)
- Many diesels use a coolant filter that meters SCA; replace per interval and confirm additive chemistry.
- Belt-driven accessory inspection
- Check alternator, A/C, water-pump, and fan pulleys for wobble, bearing noise, and proper belt tracking.
- Air-induction system seal (filter to turbo)
- Must stay sealed end-to-end; any leak on the clean side lets dirt into the engine.
- Cut or frayed seat-belt webbing
- A defect requiring replacement, even if the buckle still latches and releases — webbing damage cannot be compensated by a working buckle.
- Seat-belt standard reference
- Belt assemblies must comply with the FMVSS referenced by 49 CFR 393.93; CVSA lists cuts/fraying as defect conditions.
- Windshield wiper requirement
- A power-driven (automatic) wiping system that meets its required speeds — a single manual blade does not pass.
- Wiper speed minimums (FMVSS 104)
- Highest speed at least 45 cycles/min and the lower at least 20 cycles/min, regardless of engine load.
- Windshield crack inspection
- Cracks or chips in the driver's critical viewing area can fail; check the swept (wiped) area especially.
- Cab air suspension
- Adjusts cab ride height for stability and clearance; check air bags for leaks and the leveling linkage.
- Cab mounts
- Provide isolation and absorb vibration; worn mounts cause noise, movement, and uneven door/hood gaps.
- Cab tilt cylinder (cab-over)
- Raises and lowers the cab for engine access; inspect for leaks and proper operation.
- Cab tilt lock mechanism
- Secures the cab in the tilted position during service — must be engaged before working underneath.
- Cab tilt safety procedure
- Ensure the cab is securely supported and follow lockout/tagout before inspecting the tilt system.
- Hood latch defect
- A malfunctioning latch leaves the hood unsecured — a safety hazard that can lead to it opening in motion.
- Truck hood material (modern)
- Often aluminum or composite for light weight and corrosion resistance; check mounting and hinges.
- Cab air filter
- Filters fresh-air intake into the cab (HVAC); replace when restricted for ventilation and defrost.
- Cab insulation
- Provides sound insulation and temperature control inside the cab.
- Mirror inspection
- Verify all required mirrors are present, secure, and adjustable, and provide the required field of view.
- Horn check
- Confirm the horn operates — a required warning device.
- Gauge and warning-lamp check
- Verify oil pressure, coolant temp, air pressure, voltmeter, and warning lamps read and function correctly.
- Steps, grab handles & fuel-tank straps
- Check cab access steps and grab handles for security and that fuel-tank mounting straps are tight and uncracked.
- Door latches and hinges
- Doors must latch securely and open/close properly; check weatherstripping and glass operation.
- Defroster / HVAC operation
- Confirm the defroster clears the windshield — required for visibility in cold/wet conditions.
- Windshield washer
- Confirm washer fluid sprays and the reservoir holds fluid; supports the wiper system.
- Driver seat condition
- Check the suspension seat for proper adjustment, function, and secure mounting.
- Battery-disconnect / PPE under hood
- Disconnect the battery and wear appropriate PPE when servicing the engine compartment.
- Backup alarm
- Confirm the backup (reverse) alarm sounds when reverse is selected, where equipped/required.
- Open-circuit voltage (fully charged 12 V)
- About 12.6–12.7 V at rest after surface charge dissipates; ~12.4 V is only about 75% charged.
- Charging voltage (engine running)
- About 13.8–14.4 V — this is charging output, not a resting state-of-charge value.
- Accurate state-of-charge reading
- Remove the surface charge and let the battery rest with no charge or load before reading open-circuit voltage.
- Carbon-pile load test load
- Apply about one-half the battery's rated cold cranking amps (CCA) for 15 seconds.
- Carbon-pile load test pass
- Terminal voltage stays at or above about 9.6 V at the end of the 15-second test (at 50°F+).
- 600 CCA load-test value
- Apply about 300 amps (half of 600 CCA) for 15 seconds.
- Corroded/loose battery terminals
- White-green powdery buildup; causes dim lights and slow cranking. Clean and tighten; protect the connections.
- Loose/corroded ground connections
- Cause dim lights, slow cranking, and odd electrical symptoms — a common, easily missed fault.
- Slow cranking with a good battery
- Suspect high resistance in the cables/grounds or a worn starter, not the battery.
- Battery hold-down / box
- Batteries must be secured against movement; loose batteries are a defect.
- Alternator output check
- Verify charging voltage and that output supports the loads; dimming when revved suggests a loose belt or weak alternator.
- Required rear trailer lamps
- Tail, stop (brake), turn signal, and required marker/clearance lamps plus reflectors — all must function.
- Flickering marker/clearance lamps on rough roads
- Points to a loose connection or ground that intermittently opens — find and secure it.
- Headlamp aim and operation
- Verify high/low beams operate and are aimed correctly; both required for night operation.
- Required lighting devices
- Headlamps, turn signals, brake lamps, marker/clearance lamps, license-plate lamp, reflectors — confirm all required lamps work.
- Governor cut-out pressure
- The governor stops the compressor loading at no higher than about 135 psi.
- Governor cut-in pressure
- The compressor resumes building air at no lower than about 80–100 psi.
- Static air-leakage (leak-down) test
- Engine off, brakes released: max ~2 psi/min for a single vehicle, ~3 psi/min for a combination.
- Applied air-leakage allowance
- With brakes applied: about 3 psi/min single, 4 psi/min combination.
- Low-air warning activation
- The low-air warning light/buzzer must activate at about 60 psi.
- Air pressure build-up time
- Time how fast pressure builds from a low reservoir; slow build-up suggests a weak compressor or restriction.
- ABS warning lamp lit after bulb-check
- Indicates an ABS fault — the system is degraded/disabled; document and diagnose (often a wheel-speed sensor).
- Trailer ABS lamp
- A side-of-trailer ABS lamp works like the dash lamp — lit after bulb-check means a trailer ABS fault.
- Wheel-speed sensor
- Feeds wheel speed to the ABS ECU; a bad sensor or air gap commonly triggers the ABS lamp.
- Air dryer
- Removes moisture from the air supply to protect valves and prevent freeze-ups; check the purge and desiccant.
- Tank drain / moisture check
- Drain reservoirs to check for oil or excessive water, which signals air-dryer or compressor issues.
- Short to ground (inoperative circuit)
- A short can blow fuses or kill an accessory circuit; isolate with a test light/meter and repair the fault.
- OBD / fault-code reading
- Use a scan tool to read stored codes rather than guessing the cause of an electronic fault.
- Voltmeter vs. test light
- A voltmeter reads exact voltage/drops; a test light only shows presence of power — use the right tool.
- Spring-brake (parking) control
- Verify the parking-brake (spring-brake) control applies and releases the spring brakes properly.
- Trailer cord / gladhands
- Check the electrical trailer cord and air gladhand connections and seals for damage and secure connection.
- Battery cable inspection
- Check for corrosion, chafing, loose terminals, and proper routing/securement.
- Applied pushrod stroke (how to measure)
- Mark the rod at rest, apply the brakes at ~90–100 psi, then measure how far the rod travels out of the chamber.
- Pushrod stroke meaning
- Stroke at or beyond the chamber's readjustment limit means that brake is out of adjustment.
- 20% defective brakes rule
- A vehicle is out of service when 20% or more of its service brakes are defective (e.g. out of adjustment).
- Type 30 standard readjustment limit
- 2 inches of applied pushrod stroke.
- Long-stroke Type 30 readjustment limit
- 2-1/2 inches — more travel than the 2-inch standard Type 30.
- Type 24 standard readjustment limit
- 1-3/4 inches.
- Type 24 long-stroke readjustment limit
- 2 inches.
- Type 16 / 20 readjustment limit
- 1-3/4 inches (standard clamp-type).
- Type 36 readjustment limit
- 2-1/4 inches.
- Identify chamber type first
- 2-3/8" passes a long-stroke Type 30 (limit 2-1/2") but fails a standard Type 30 (limit 2") — identify the chamber before judging stroke.
- Slack adjuster (function)
- Connects the chamber pushrod to the S-cam, setting braking leverage and taking up lining wear.
- Automatic slack adjuster (still checked)
- Self-adjusts but must still have stroke checked at PMI — a failing one can let stroke drift out of limit.
- Manual slack adjuster correction
- Turn the adjusting nut to bring applied pushrod stroke back within spec.
- Never back off an automatic slack adjuster
- Long stroke on an automatic slack adjuster signals a mechanical problem to diagnose, not a manual readjustment.
- S-cam
- The S-shaped cam the slack adjuster rotates to spread the brake shoes against the drum.
- Foundation brake
- The mechanical brake at the wheel (drum-and-shoe or disc) that the air system operates.
- Brake-lining thickness check
- Inspect through the inspection slot without full disassembly; condemn when below the minimum lining/pad thickness.
- Brake drum inspection
- Check for cracks, heat checking, scoring, and that diameter is within the maximum; a cracked drum is a defect.
- Brake chamber damage
- Inspect chambers for cracks, dents, corrosion, and audible air leaks.
- Steer-axle minimum tread depth
- 4/32 inch, measured in a major tread groove (49 CFR 393.75).
- Drive/trailer minimum tread depth
- 2/32 inch — less strict than steer axles; measured in a major groove.
- '4 up front, 2 in the rear'
- Memory aid: steer (front) tires need 4/32", all other (rear) positions 2/32".
- Tire out-of-service conditions
- Flat tire, audible leak, fabric/cords exposed through the tread, or a regrooved tire on a steer axle.
- Regrooved tire on a steer axle
- Generally prohibited on the steer axle of a heavy truck.
- Measuring tread depth
- Use a tread-depth gauge in a major groove — not on a wear bar or the tread edge.
- Tire inflation check
- Check with a gauge against spec; both over- and under-inflation cause uneven wear and failure risk.
- Wheel/rim inspection
- Look for cracks, especially at the disc/hand-hole area, bent flanges, and elongated bolt holes.
- Loose wheel fasteners (signs)
- Rust streaks, shiny wear, or dust 'tails' around lug nuts indicate looseness; torque to spec.
- Mismatched / damaged tires
- Check for mismatched sizes on duals and cuts, bulges, or sidewall damage.
- Manual wheel-bearing adjustment
- Seat the bearing, back the nut off, then set a slight specified end play (small axial free movement).
- Wheel-bearing end play (too tight/loose)
- Too tight overheats the bearing; too loose lets the wheel wobble and wear.
- Wheel-seal leak
- Grease or oil at the hub/seal is a defect; inspect for leaks and proper lubricant level on oil-bath hubs.
- Hub oil level (oil-bath)
- Check the sight glass/level; low oil or contamination signals a seal or bearing problem.
- Multi-leaf spring defect
- A broken, shifted, or missing leaf — especially a cracked main leaf — is a reportable suspension defect.
- U-bolt
- Clamps the leaf spring to the axle; a broken or loose U-bolt lets the axle shift.
- Spring hangers, shackles & bushings
- Inspect for cracks, wear, and looseness that let the axle move out of position.
- Center (spring) bolt
- Locates the spring on the axle; a sheared center bolt lets the spring shift and the axle move.
- Air-spring (air bag) inspection
- Check for leaks, chafing, and cracks; a leaking bag drops ride height and load support.
- Height-control valve / linkage
- Maintains ride height on air suspension; damaged linkage causes incorrect height and leveling.
- Shock absorber inspection
- Check for leaks, broken mounts, and that they dampen movement; worn shocks let the suspension rattle/bounce.
- Torque rods / radius rods
- Locate the axle fore-aft; worn bushings cause axle misalignment and tire wear.
- Kingpin play (steer axle)
- Raise the axle and pry the tire; vertical play commonly ~0.060–0.125" per OEM spec — beyond it, replace pin and bushings.
- Tie-rod ends
- Transmit steering input to the wheels; check for looseness, which causes wander and toe wear.
- Drag link
- Connects the steering arm to the linkage; check for play and secure, cottered fasteners.
- Pitman arm / steering box
- Recirculating-ball steering box and pitman arm transmit steering motion; check for free play and leaks.
- Power-steering inspection
- Check fluid level, hoses, the pump, and for leaks; the pump provides hydraulic steering assist.
- Excess steering free play
- Indicates wear in the gearbox, kingpins, tie-rod ends, or linkage — diagnose the source.
- Wheel alignment / control arm
- Control arms and bushings hold alignment; uneven or cupped tire wear signals an alignment/suspension problem.
- Stabilizer (sway) bar
- Reduces body roll during cornering; check the bar and end links for damage and looseness.
- Frame-rail crack
- A crack running up a rail web near a stressed area (crossmember gusset) is a reportable defect — document for repair.
- Frame/cross-member inspection
- Check rails, crossmembers, and gussets for cracks, bends, loose rivets/bolts, and corrosion.
- Loose frame fasteners
- Loose or missing rivets/bolts at crossmembers and brackets are defects to flag.
- Fifth-wheel horizontal play (OOS)
- Movement over 1/2 inch between the upper and lower halves is out of service — worn jaws or kingpin.
- Fifth-wheel locking jaws
- Must fully engage and lock around the trailer kingpin; worn jaws let the trailer separate.
- Fifth-wheel mounting
- Check mounting bolts/brackets, the pivot, and proper lubrication of the plate.
- Sliding fifth wheel verification
- After repositioning, verify the locking plungers are fully engaged before service/operation.
- Kingpin (trailer)
- The pin that locks into the fifth wheel; check for wear, cracks, and proper diameter.
- Pintle hook / drawbar
- On combination units, check the pintle, lunette eye, and safety chains for wear and security.
- Driveline / U-joint
- Check U-joints for play and the slip yoke for wear; a banging 'slap' on accel/decel signals worn joints.
- Driveshaft guard / carrier bearing
- Check the center support (carrier) bearing and any driveshaft guard for wear and security.
- Differential / drive axle leaks
- Check axle housings for leaks, the breather, and lubricant level.
- Air brake governor location in system
- Mounted to control the compressor; verify cut-out/cut-in during the brake portion of the inspection.
- Brake balance / timing
- All brakes should apply together; uneven application or timing causes pull and uneven wear.
- Spring (parking) brake removal/caging
- Cage the spring brake (or follow the safe procedure) before servicing a chamber so the powerful spring can't release.
- Service vs. parking (spring) brakes
- Service brakes are air-applied; parking/emergency spring brakes are spring-applied and air-released.
- ABS modulator valve
- Cycles air pressure to a wheel during ABS events; part of the chassis air-brake electronics.
- Brake-hose / tubing inspection
- Check air hoses and tubing for chafing, cracks, kinks, and leaks; secure and away from heat.
- Disc brake (air) inspection
- Check pad thickness, rotor condition, and caliper/guide-pin operation on air-disc-braked axles.
- Brake adjustment limit varies by chamber
- The readjustment limit depends on chamber type AND size — always match the limit to the chamber.
- Steering wheel free-play (spec)
- Excessive lash beyond spec is a defect; measure at the rim with the wheels straight.
- Axle alignment (tracking)
- Misaligned axles cause dog-tracking and rapid tire wear; check for bent or shifted axles.
- Fuel-tank mounting & leaks
- Check tank straps, brackets, and for fuel leaks; a fuel leak is a serious defect.
- Exhaust-system routing (chassis)
- Check that the exhaust is secure, leak-free, and routed away from fuel lines and the cab.
- Mud flaps / splash guards
- Required where applicable; check for presence and security.
- Landing gear (trailer)
- Check the trailer landing gear/dollies for damage, crank operation, and security.
- Slack-adjuster angle warning
- A pushrod operating at a sharp angle to the slack adjuster reduces effective stroke/leverage — inspect geometry.
- Brake fade / heat damage
- Look for heat checking and blued drums indicating overheating; investigate dragging brakes.
- Wheel-end seal types
- Oil-bath hubs use a sight glass; grease-packed hubs are inspected for leaks and proper repack.
- Camshaft (S-cam) bushings
- Worn S-cam bushings cause excessive play and uneven braking — check for radial movement.
- Suspension ride-height check
- Confirm correct ride height; wrong height alters geometry, braking, and driveline angles.
- Tractor protection valve
- Protects tractor air if a trailer breaks away; verify proper operation during the air check.
- Spring-brake priority valve
- Ensures parking brakes apply on air loss; part of the safe-failure design — verify function.
- Tire matching on duals (diameter)
- Mismatched dual diameters overload the larger tire and cause scuffing — check and match.
- Bent or cracked wheel rim
- A cracked rim or elongated stud holes is a defect requiring replacement.
- Steering gear mounting
- Check that the steering gearbox is securely mounted and not leaking.
- Leaf-spring lubrication / wear pads
- Check slipper/wear pads at the rear hangers and that leaves aren't excessively rusted or fretted.
- Brake-adjustment check at 90-100 psi
- Always make the full brake application at about 90-100 psi reservoir pressure before measuring applied pushrod stroke.
- Equalizer / suspension equalizer beam
- On tandem suspensions, the equalizer balances load between axles; check its bushings and pivots for wear.
- Road (operational) test purpose
- Confirms braking, steering, shifting, instrumentation, and ride under load, turning symptoms into a component diagnosis.
- Vehicle pulls to one side (released wheel)
- Low front-tire pressure, brake drag, or an alignment problem causing unequal traction.
- Brake squeal on application
- Usually worn brake linings/pads — replace.
- Headlights dim when revved
- A loose alternator belt or weak charging system.
- Hard, grinding manual shifting
- Clutch master-cylinder/hydraulic fault or clutch adjustment affecting engagement.
- Engine stalls at a stop
- An idle-control problem affecting idle speed/stability.
- Vibration/shudder in motion
- Tire imbalance/misalignment or driveline (U-joint) wear.
- Delayed acceleration response
- A clogged air filter or fuel-delivery restriction limiting airflow/fuel.
- ABS lamp on when braking (road test)
- An ABS wheel-speed-sensor or system fault — diagnose with codes.
- Rattle over bumps
- Worn shock absorbers or loose suspension components.
- Erratic/delayed transmission shifting
- Transmission fluid level or quality issues affecting performance.
- Excessive steering play (road feel)
- A loose feeling/excess play points to worn steering linkage or components.
- Documenting findings (PMI output)
- Record each defect's location and severity, then a return-to-service, repair, or out-of-service determination.
- Review DVIRs first
- Check open driver vehicle inspection reports before starting so known defects are verified.
- Brake performance check (road)
- Confirm the truck stops straight and within distance, with no pull, grab, or excessive pedal travel.
- Out-of-service determination
- If a defect meets FMCSA/CVSA out-of-service criteria, the truck is removed from service until repaired.
