ASE T7 HVAC (Medium/Heavy Truck) cheat sheet

A/C loop order: compressor → condenser → receiver-drier or accumulator → expansion device (orifice tube/TXV) → evaporator → back to compressor.

Compressor raises refrigerant temperature above ambient so the condenser can reject heat; the evaporator absorbs cab heat as refrigerant boils at low pressure.

Orifice-tube systems use an accumulator on the evaporator outlet; TXV systems use a receiver-drier on the condenser outlet.

TXV modulates refrigerant flow to hold evaporator superheat; a stuck-open TXV causes flooding and low superheat, stuck-closed causes no cooling.

Accumulator stores liquid and holds desiccant plus a pickup tube; a restricted accumulator oil bleed causes compressor oil starvation.

Condenser subcooling confirms liquid seal at the outlet; low subcooling with high discharge pressure suggests condenser airflow or overcharge issues.

Blower motor speed is set by resistors or a PWM module; a failed blower resistor leaves only high speed or no speed depending on design.

Heater core receives engine coolant flow controlled by a blend-air door or coolant valve; no heat with hot hoses indicates blend door or airflow fault.

Static pressure with system off roughly equals saturation pressure at ambient temperature (R-134a ≈ 68-72 psi at 70°F).

Low side below normal with high side below normal indicates low charge; both sides high with poor cooling indicates overcharge or condenser airflow restriction.

Low side high and high side high, with compressor cycling quickly, indicates a weak compressor or faulty expansion device.

Superheat = evaporator outlet temp − evaporator saturation temp; normal TXV superheat is roughly 5-10°F.

Subcooling = condenser saturation temp − liquid line temp; a low reading with bubbles in the sight glass signals undercharge.

A pressure drop across the suction line or a frost pattern at the orifice tube indicates a restriction; verify with temperature differential.

Electronic leak detection plus dye and UV inspection locate leaks; never braze on a pressurized or charged system.

Evacuation to 500 microns or lower with a standing vacuum hold confirms no leaks and removes moisture before recharge.

No heat with both heater hoses hot points to a blend-air door, actuator, or control head fault; cool hoses indicate coolant flow or thermostat issues.

Heater core restriction shows as a large temperature difference between inlet and outlet hoses and reduced cabin heat.

Air in the cooling system or a failed thermostat can cause erratic heat and overheating; bleed per manufacturer procedure.

Cooling fan clutch or electric fan failure raises condenser and radiator temperatures, degrading A/C performance at idle.

Blend door actuator calibration and mode door position feedback are checked with a scan tool before condemning the actuator.

Cabin air filter restriction reduces airflow across evaporator and heater core, mimicking blower or refrigerant faults.

Coolant type and concentration affect heater performance; 50/50 mix protects against freeze and boil-over.

Evaporator freeze-up from a faulty evaporator temperature sensor or cycling switch causes low airflow after extended operation.

HVAC control modules communicate over CAN or LIN; loss of communication can disable blower, blend, and compressor functions.

Compressor clutch control may be direct, via relay, or through a module; verify power and ground at the clutch coil before replacement.

Pressure switches and transducers protect the system: high-pressure cutoff disables the clutch on overpressure, low-pressure cutoff prevents operation when undercharged.

Blower motor PWM modules receive a duty-cycle command; check command signal and module ground before replacing the motor.

Actuator position sensors provide feedback; a code for out-of-range position often indicates a stuck door or failed actuator, not the module.

Use a wiring diagram and measure voltage drops under load; a 0.5V drop on a ground circuit can cause intermittent blower operation.

Ambient and evaporator temperature sensors influence compressor cycling; a skewed sensor can cause no-cool or freeze-up complaints.

Scan tool data (clutch request, pressure sensor voltage, actuator counts) guides diagnosis before parts replacement.

Recover refrigerant before opening any system component; venting refrigerant to atmosphere is prohibited.

Use EPA Section 609 certified equipment and personnel for motor vehicle A/C; recovery machines must meet SAE J2788 or J2810 standards.

Recovery cylinder must be labeled, in hydrostatic test date, and never filled above 80% capacity.

Never mix refrigerants; identify refrigerant with a refrigerant identifier before recovery to avoid contaminating the recovery cylinder.

Wear safety glasses and gloves; liquid refrigerant causes frostbite, and refrigerant vapor displaces oxygen in enclosed spaces.

Evacuate to 500 microns and hold to confirm moisture and non-condensables are removed; a rising vacuum indicates a leak or moisture.

Recovered refrigerant may be recycled on-site; reclaimed refrigerant must meet AHRI 700 and be processed by a certified facility.

Keep refrigerant away from open flame; burning refrigerant produces toxic phosgene-type decomposition products.