Your FREE ASE T7 Flashcards 2026 – 200+ Cards

A two-gauge service tool: the blue low-side gauge reads suction pressure, the red high-side gauge reads discharge pressure, and the center hose is used for recovery, evacuation, or charging.

Connects to the suction line and reads evaporator (low) pressure — about 25–35 psi on a normal R-134a system at ~80°F.

Connects to the liquid/discharge line and reads condenser (high) pressure — about 150–250 psi on a normal R-134a system at ~80°F.

A low refrigerant charge or a restriction starving the system. Warm vents and little pressure differential.

Overcharge, air/non-condensable in the system, or poor condenser airflow (debris or a dead fan). Weak cooling, hot high side.

A worn or inefficient compressor that cannot build a pressure differential. Little temperature change across it.

A severely low or empty charge — the compressor cannot separate the pressures, so there is no cooling.

Degrees the vapor leaving the evaporator is above its saturation temperature (suction-line temp minus saturation temp). Confirms the compressor gets vapor, not liquid.

Degrees the high-side liquid leaving the condenser is below its condensing temperature (saturation temp minus liquid-line temp). Confirms full condensing.

Points to a low refrigerant charge or a starved evaporator — too little refrigerant boils off early and the vapor overheats.

Points to a flooded or overcharged evaporator — liquid may reach the compressor and cause slugging.

Suction-line surface temperature minus the saturation temperature for the low-side pressure. Example: 50°F line − 35°F saturation = 15°F superheat.

High-side saturation temperature minus the actual liquid-line temperature. A positive value shows the liquid was cooled below its condensing point.

Capturing refrigerant into EPA-approved equipment instead of venting it. Required by the Clean Air Act before opening any refrigerant line.

Releasing refrigerant to the atmosphere — illegal under the Clean Air Act. Always recover instead.

Cleaning recovered refrigerant of moisture, oil, and acid so it meets purity standards. Required before reuse, even back into the same vehicle.

Pulling a deep vacuum (about 28–30 inches of mercury) to remove air and boil off moisture before charging.

Removes air (a non-condensable that raises head pressure) and boils off moisture, which would form corrosive acids and freeze in the metering device.

About 28–30 inches of mercury. At ~29 inHg water boils near 76°F, so residual moisture vaporizes at shop temperature and is drawn out.

The most accurate way to charge — add the exact mass on the underhood label using a scale or metered machine. Gauge guessing and sight-glass charging are inaccurate.

The certification required of anyone who services motor-vehicle A/C for pay. Section 608 covers stationary equipment, not vehicles.

A spring-loaded core valve at each service port that the quick-connect coupler depresses to open the port for service.

An analyzer that detects the actual refrigerant and any contamination before you connect recovery equipment. Confirms the underhood label.

Air trapped in the A/C system that will not condense; it raises high-side pressure and reduces cooling. Removed by recovery and evacuation.

A sniffer that detects escaping refrigerant at fittings and components — a reliable way to pinpoint a leak.

Dye circulates with the oil and glows at the leak point under ultraviolet light, pinpointing the exact leak location.

Blue low-side gauge to the suction line, red high-side gauge to the liquid/discharge line, yellow center hose for service (recovery/evacuation/charging).

Blue = low side, red = high side, yellow = center service hose. Standardized to prevent cross-connection errors.

Suspect overcharge, air in the system, or restricted condenser airflow — all keep the refrigerant from condensing efficiently.

For a given refrigerant, each pressure corresponds to a saturation temperature. P-T charts let a tech convert a gauge reading to a temperature for superheat/subcooling.

The temperature at which a refrigerant boils/condenses for a given pressure. The baseline for measuring superheat and subcooling.

The tool that evacuates the system, pulling moisture and air out to a deep vacuum before charging.

Shop air introduces moisture and contaminants into an open A/C system. Use recovery and evacuation, not compressed air, to service it.

Recover the refrigerant with approved equipment first, then open the system. Never vent or use shop air.

EPA-approved equipment connected to the service ports that captures refrigerant until the system reaches a stable vacuum.

Often a clogged or contaminated cabin air filter or microbial growth on the evaporator; the filter traps odors and contaminants.

At ~80°F ambient: low side about 25–35 psi and high side about 150–250 psi with the system stabilized.

Read both gauges together — the relationship between high and low side reveals charge, compressor, restriction, or airflow faults before opening the system.

A restriction (kinked liquid line, plugged drier, or partly closed TXV) can show a low high side after the blockage and starve the evaporator, with frost at the restriction.

On some TXV systems, a window in the liquid line. Continuous bubbles can indicate a low charge, but charging by sight glass is not as accurate as charging by weight.

The low side — the vapor leaving the evaporator. It protects the compressor from liquid slugging.

The high side — the liquid leaving the condenser. It confirms the refrigerant fully condensed.

A safety valve that protects the A/C system from over-pressurization by venting excess pressure (a last-resort safety device).

When recovery is complete, the system holds a stable vacuum, indicating the refrigerant has been captured.

Modern systems hold a precise mass; even a small over- or undercharge hurts cooling. Charge by weight to the label, not by feel.

Form when moisture combines with refrigerant and oil; they corrode components. Proper evacuation and a fresh drier prevent them.

Indicates very cold, possibly low-pressure refrigerant; a rough field clue but not an accurate way to set the charge.

After service, recheck vent temperature and gauge pressures to verify the concern is gone and the system performs to spec.

High pressure on both gauges with poor cooling can be caused by overcharge, air in the system, OR poor condenser airflow — multiple causes can be correct.

Oil, moisture, or the wrong refrigerant in recovered gas must be cleaned out by recycling/reclaiming before the refrigerant is reused.

The sealed loop where refrigerant is compressed, condensed, expanded, and evaporated to carry heat from the cabin to outside air.

The engine-driven pump that compresses low-pressure vapor into hot, high-pressure vapor and circulates refrigerant around the loop.

Raises refrigerant pressure and temperature so the condenser can reject heat. It pumps vapor, not liquid.

A magnetic clutch that couples the compressor to the engine pulley when energized and lets the pulley free-spin when off.

An energized coil creates a magnetic field that pulls the armature plate against the spinning pulley, locking it to the compressor shaft.

Check for voltage and ground at the clutch coil, the low-pressure switch, the relay, and the clutch air gap — it is an electrical fault, not a charge fault.

The small gap between the armature and pulley. Too large a gap prevents engagement even with power at the coil.

A heat exchanger ahead of the radiator that rejects heat from high-pressure vapor and condenses it into high-pressure liquid.

Ram air and the condenser fan pull heat out of the condenser. Debris, a dead fan, or a bad fan clutch drives the high side up and weakens cooling.

Assists cooling of the condenser at idle and low speed; if it fails, high-side pressure climbs and cooling drops, often worse at idle.

The cabin heat exchanger where low-pressure liquid refrigerant boils into vapor, absorbing heat from cabin air and dehumidifying it.

Absorbs heat from cabin air as refrigerant boils; moisture condenses on the cold fins and drains away — so cooling also dehumidifies.

A metering device with a sensing bulb that varies refrigerant flow into the evaporator to hold a target superheat. Pairs with a receiver-drier.

A bulb on the evaporator outlet senses temperature; bulb pressure opens the valve while spring and evaporator pressure close it, balancing flow to hold superheat.

A fixed-size metering restriction used instead of a TXV. Because it cannot adjust flow, it is paired with an accumulator on the low side.

The TXV varies flow to control superheat; the orifice tube is a fixed opening. Both drop pressure; neither raises it.

A high-side component on TXV systems that stores liquid refrigerant and uses desiccant to remove moisture and debris before the metering valve.

A low-side (suction) component on orifice-tube systems that stores liquid, removes moisture with desiccant, and lets only vapor reach the compressor.

Receiver-drier = high-side liquid line in TXV systems; accumulator = low-side suction line in orifice-tube systems. Both contain desiccant.

A moisture-absorbing material inside the receiver-drier or accumulator that traps water before it can freeze or form acids.

A non-flammable HFC refrigerant with a GWP of about 1,430 and zero ozone depletion potential; the long-standing automotive refrigerant.

A low-GWP refrigerant (GWP under 1) replacing R-134a on newer trucks; classified A2L (mildly flammable) with unique, non-interchangeable fittings.

An older ozone-depleting refrigerant phased out of automotive use; replaced by R-134a and now R-1234yf.

R-1234yf uses uniquely sized quick-connect couplers so equipment and refrigerants cannot be cross-connected or contaminated.

Polyalkylene glycol lubricant specified for R-134a and R-1234yf systems; it is hygroscopic and circulates with the refrigerant to lubricate the compressor.

Mineral oil (for old R-12) or engine oil is not miscible with R-134a/R-1234yf and damages the system. Use the specified PAG (or POE) oil.

Read the underhood label and confirm with an identifier before connecting equipment — pressures overlap and smell is not a safe test.

A high side that stays only slightly warm with weak cooling while the charge is correct — the compressor isn't building pressure.

Raises pressure on both sides, can cause rapid clutch cycling, reduces cooling capacity, and risks system damage.

Low pressure on both sides; warm vents at idle that may improve at higher RPM. Confirm by gauges, then leak-test before recharging.

Classic low refrigerant charge — slow compressor speed at idle can't maintain flow. Also check for weak condenser-fan airflow.

An overcharged system or a faulty pressure switch can cause the clutch to cycle on and off quickly, giving inconsistent cooling.

A low-side switch that cuts the compressor clutch when pressure drops too low (to prevent evaporator icing) and re-engages it as pressure rises.

Compressor discharge → condenser → receiver-drier → metering device. Hot high-pressure vapor, then high-pressure liquid.

Evaporator → accumulator (orifice systems) → suction line → compressor inlet. Low-pressure liquid boiling into low-pressure vapor.

Drops high-pressure liquid into low-pressure liquid and controls how much enters the evaporator (TXV varies it; orifice tube is fixed).

The A/C does not 'make cold' — the evaporator absorbs heat from cabin air and the condenser rejects it outside.

One-way valves inside the compressor; if they leak or break, the compressor loses efficiency and the pressures fail to separate.

Add fluorescent dye and inspect with UV light, or use an electronic leak detector — both pinpoint the spot better than listening for a hiss.

O-ring fittings, the compressor shaft seal, condenser, evaporator, and hose connections. Replace O-rings and the drier when opening the system.

High-pressure vapor into high-pressure liquid by rejecting heat to outside air.

Low-pressure liquid into low-pressure vapor by absorbing heat from cabin air.

Refrigerant oil (PAG) carried with the refrigerant lubricates the compressor; loss of charge or oil leads to compressor wear and seizure.

Adjusts its output to match cooling demand instead of cycling a clutch, smoothing performance and reducing load.

Mildly flammable, low-toxicity (e.g. R-1234yf). Requires proper handling and dedicated, non-interchangeable service equipment.

A measure of a refrigerant's heat-trapping effect. R-134a is ~1,430; R-1234yf is under 1, which is why R-1234yf was adopted.

A measure of ozone-layer harm. R-134a and R-1234yf are zero ODP; old R-12 had a high ODP and was banned.

The system is opened for major repair or has been contaminated — desiccant becomes saturated once exposed to air and moisture.

The high-side line carrying high-pressure liquid from the condenser/drier to the metering device.

The low-side line carrying low-pressure vapor from the evaporator back to the compressor inlet.

The high-side line carrying hot, high-pressure vapor from the compressor to the condenser.

Too much liquid refrigerant in the evaporator (low superheat); liquid can reach and damage the compressor (slugging).

Too little refrigerant in the evaporator (high superheat); cooling is weak because the coil is not fully fed.

Starves the evaporator — low low-side pressure, high superheat, weak cooling, possible frost at the valve.

Floods the evaporator — high low-side pressure, low superheat, and liquid risk to the compressor.

Seals the rotating shaft where it exits the compressor; a common refrigerant and oil leak point.

Use the correct refrigerant-rated hoses and O-rings, lubricated with the system oil, to seal connections and prevent leaks.

Measuring discharge-air temperature at the dash vents confirms cooling performance after service.

On clutch-cycling systems, the clutch turns the compressor on and off to control evaporator temperature and prevent icing.

Vapor → (compressor) hot vapor → (condenser) liquid → (metering device) low-pressure liquid → (evaporator) vapor → repeat.

A temperature-controlled valve that blocks coolant flow to the radiator until the engine warms, then opens to hold operating temperature.

Causes engine overheating — coolant cannot flow to the radiator to shed heat.

Engine runs too cool — poor fuel economy and weak cab heat because the coolant (and heater core) never reach full temperature.

The belt- or gear-driven pump that circulates coolant through the engine, radiator, and heater core.

Stops coolant circulation, causing overheating; a leak at the weep hole is a classic sign of a failing pump seal.

Sheds engine heat to the air passing through its fins; ram air and the cooling fan move air across it.

Seals the cooling system and raises the coolant boiling point so it can run hotter without boiling; also lets coolant return from the overflow tank.

A small radiator inside the HVAC case; hot engine coolant flows through it and the blower pushes cabin air across it to make heat.

A valve that controls the flow of hot coolant to the heater core, setting cab heat (used with or instead of a blend door).

Usually a stuck-open thermostat, an air-bound cooling system, a plugged heater core, or a closed heater control valve — not the A/C.

A sweet (coolant) smell, fogged windows, and a greasy film inside the windshield point to a leaking heater core.

Draws air through the radiator at low speed and idle; a heavy-truck fan clutch engages the fan when coolant gets hot.

Air-actuated shutters in front of the heavy-truck radiator that close to speed warm-up and open to increase cooling.

A mix of antifreeze and water that lowers the freeze point, raises the boil point, and protects against corrosion. Use the specified type.

Engine overheating and potential damage; can also cause weak or intermittent cab heat as the heater core loses flow.

A damaged radiator, hose, water pump, heater core, or gasket. Pressure-test the system to locate it.

A stuck-closed thermostat, failed water pump, low coolant, plugged radiator, stuck fan clutch, or a blown head gasket.

Bubbles in the surge tank or a positive combustion-gas test point to a head-gasket or cylinder-liner leak.

Trapped air pockets block coolant flow through the heater core, causing no or intermittent heat; bleed the system to fix it.

Restricts coolant flow, so the cab blows cool air even with a warm engine; flushing or replacement restores heat.

Reports coolant temperature to the engine and HVAC controls, affecting fan operation, fueling, and the temperature gauge.

Holds extra coolant and allows for expansion as the engine heats; the cap and tank manage system pressure and air separation.

Cab heat is a branch of the engine cooling circuit — hot coolant routed through the heater core — so the two systems are diagnosed together.

Applies pressure to find external leaks and a failing cap; combustion-gas testing checks for internal head-gasket leaks.

Coolant boils and overflows, lowering the level and causing overheating.

Water pump → engine block & head → thermostat → radiator → back to the pump, with a branch through the heater core for cab heat.

A stuck-disengaged fan clutch causes overheating at low speed; a stuck-engaged one wastes power and noise.

Inspect for correct mix, contamination, and proper additive level; degraded coolant loses corrosion and boil/freeze protection.

With the engine at operating temperature, measure heater-vent temperature; low output points to flow, thermostat, or core problems.

After an overheat, air can enter the system and block the heater core; bleed the cooling system to restore both cooling and heat.

The temperature door that directs airflow between the heater core and evaporator to set discharge-air temperature.

An electric, vacuum, or cable actuator that moves the blend door on command from the control head; a failure causes wrong or stuck temperature.

Routes conditioned air to the defrost, panel, or floor vents in the climate-control system.

Causes uneven airflow or air coming out the wrong vents (e.g. defrost only), affecting comfort and defogging.

A stuck blend or mode door can imitate a refrigerant or coolant problem — verify door movement before chasing the system.

Closes the fresh-air intake and re-cools cabin air, reaching and holding lower temperatures faster than pulling in hot outside air.

Pulls outside air in for ventilation and to clear window fogging; the opposite of recirculation.

The fan that pushes conditioned air through the ducts and out the vents into the cabin.

Sets lower blower speeds by dropping voltage across resistive elements; high speed usually bypasses it.

Commonly loses the lower fan speeds while high speed still works (or vice versa), depending on which element fails.

A solid-state module that varies blower voltage electronically instead of using a resistor; a fault can run the fan off-speed.

Can cause the fan to run faster or slower than the selected setting, or not at all.

Measures refrigerant pressure and reports to the control module to manage compressor operation and protect the system.

Monitors refrigerant/coil temperature at the evaporator so the control system can prevent icing and manage cooling.

Reports cabin temperature so automatic climate control can hold the set temperature; a fault causes inaccurate control.

A filter ahead of the evaporator that cleans incoming air; when clogged it restricts airflow and weakens cooling and heating.

Reduces blower output and vent cooling/heating without changing refrigerant pressures or coolant temperature.

Manual sets blower, temperature, and mode by hand; automatic uses sensors to hold a set temperature with electronic actuators.

Older systems use engine vacuum to move mode doors; a vacuum leak can leave doors defaulting to defrost.

A second evaporator and blower (or dedicated system) that conditions the sleeper berth, adding extra mode/blend doors and controls.

A small diesel or battery unit that runs cab heating and cooling with the main engine off, cutting idle time and fuel use.

Runs an electric A/C or stored cooling from a battery bank to keep the bunk comfortable overnight without idling the engine.

A diesel-fired heater that warms the sleeper without running the engine, drawing from the truck's fuel tank.

Anti-idling rules and fuel savings push trucks toward APUs and no-idle systems for sleeper comfort — a T7-specific topic.

The driver interface that commands blower speed, temperature (blend door), mode (mode door), and recirculation.

Many electronic actuators self-calibrate their travel limits on startup; a miscalibrated or failed actuator gives wrong temperature or airflow.

Directs airflow to the windshield and usually runs the A/C to dehumidify, clearing fog and ice for safety.

Directs heated air to the floor vents for heating; common in cold-weather operation.

Directs conditioned air to the dash vents, used mainly for cooling the cab.

The control head signals the engine/HVAC module to engage the compressor clutch when cooling or defrost is selected.

Air coming from the wrong vents points to a mode-door or actuator fault (or a vacuum leak on vacuum systems), not the refrigerant charge.

A failed blend-door actuator can leave temperature stuck hot or cold, or split between sides on dual-zone systems.

An electrically controlled switch that powers the compressor clutch; a faulty relay can prevent engagement.

Opens the clutch circuit when refrigerant pressure is too low, protecting the compressor from running without charge/oil.

Disengages the compressor clutch if high-side pressure climbs dangerously high, protecting the system.

Diagnose an actuator by verifying it has command voltage, ground, and a position signal before condemning it.

Uses separate blend doors and actuators to set different temperatures for the driver and passenger sides.

Stored trouble codes for actuators, sensors, or the blower circuit help pinpoint electronic HVAC faults during diagnosis.

A second climate-control head in the bunk that commands the sleeper HVAC independently of the dash controls.

Leaves the system pulling hot outside air, hurting cooling — a control/actuator fault, not a charge problem.

On simple systems, a cable physically moves the blend or mode door from the control head; a broken cable jams the door.

Cycle through all blower speeds to isolate a resistor or control-module fault — note which speeds work and which don't.

A pressure or temperature input that tells the controller when to engage or cut the compressor to manage cooling and prevent icing.

On automatic climate systems, senses sunlight and biases cooling toward the sunny side or raises blower output.

Stripped plastic gears inside an electronic actuator cause clicking, stuck doors, and wrong temperature or airflow.

On a control complaint, confirm door movement, actuator command, and sensor input before replacing refrigerant or coolant parts.

The bunk A/C uses its own evaporator and blower; diagnose its doors, actuators, and controls separately from the dash system.

Battery or thermal-storage systems keep the bunk cool without idling; have their own compressor or stored-cooling components.

An APU has its own small compressor, condenser, evaporator, and controls to provide cab climate while the main engine is off.

Many areas limit truck idling, driving adoption of APUs, bunk heaters, and electric no-idle HVAC for sleeper comfort.

Corroded or loose connectors at actuators, sensors, the blower, or the clutch are common causes of intermittent HVAC control faults.

Some HVAC modules run a door-actuator self-test or recalibration after a battery disconnect or repair to relearn door positions.

Points to a blocked cabin filter, a stuck mode door, or a duct obstruction rather than a blower fault.

The controller cuts the clutch on low charge, high pressure, or wide-open throttle to protect the compressor and engine.

The electronic brain that reads sensors and switches, drives the actuators and blower, and commands the compressor clutch.

The temperature (blend) door sets how hot/cold the air is; the mode door sets where the air goes. Two different doors and actuators.

Check the charge and clutch first, but also verify the blend door isn't stuck on heat and the recirc door isn't stuck on fresh air.

A drifting cabin or evaporator sensor makes automatic climate control hold the wrong temperature even though the A/C works.

Modern trucks send HVAC commands over a data bus; a network or module fault can disable controls that test fine individually.

Blend door = temperature, mode door = airflow direction, blower control = speed, plus the sensors, switches, and sleeper/no-idle systems that run them.