ASE T7 HVAC (Medium/Heavy Truck) flashcards
ASE T7 HVAC (Medium/Heavy Truck) · Build recall with a focused review session.
32 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Compressor function
- Pumps refrigerant vapor from the low-pressure evaporator side to the high-pressure condenser side, raising its temperature above ambient so heat can be rejected.
- Condenser role
- Rejects heat absorbed by the evaporator plus compressor work, condensing high-pressure vapor into liquid before it reaches the receiver-drier or accumulator.
- Accumulator purpose
- Located on the evaporator outlet in orifice-tube systems, it stores excess liquid refrigerant, holds desiccant, and prevents liquid slugging of the compressor.
- Receiver-drier purpose
- Mounted on the condenser outlet in TXV systems, it stores liquid refrigerant, removes moisture with desiccant, and provides a sight glass for charge checks.
- TXV function
- Thermostatic expansion valve meters refrigerant into the evaporator and modulates flow to maintain a set superheat at the evaporator outlet.
- Orifice tube operation
- A fixed restriction that meters refrigerant flow; system charge and accumulator design control performance because the tube cannot adjust to load.
- Blower resistor function
- Drops voltage to the blower motor in steps to provide lower speeds; failure typically leaves only high-speed operation or no blower at all.
- Static pressure rule
- With the system off and stabilized, gauge pressure approximates refrigerant saturation pressure at ambient temperature, about 70 psi for R-134a at 70°F.
- Low charge symptoms
- Both gauges read below normal, cooling is poor, and the compressor may cycle rapidly; bubbles may appear in the sight glass on TXV systems.
- Overcharge symptoms
- Both high and low side pressures are above normal with reduced cooling; high discharge pressure and possible clutch cycling from high-pressure cutoff.
- Superheat formula
- Superheat equals evaporator outlet temperature minus evaporator saturation temperature; normal TXV superheat is roughly 5 to 10°F.
- Subcooling formula
- Subcooling equals condenser saturation temperature minus liquid line temperature; low subcooling with bubbles indicates undercharge or a restriction.
- Restriction indicator
- A temperature drop across a component, such as a frosted orifice tube or cold suction line section, signals a restriction in that area.
- Evacuation target
- Pull the system to 500 microns or lower and hold with the pump isolated; a rising vacuum indicates a leak or residual moisture.
- Leak detection methods
- Electronic leak detectors, UV dye with black light, and soap solution locate leaks; never braze or weld on a charged system.
- No heat diagnosis
- If both heater hoses are hot but the cabin is cold, suspect the blend door, actuator, or control head rather than coolant flow.
- Heater core restriction
- A large temperature difference between heater core inlet and outlet hoses with poor cabin heat indicates restricted coolant flow through the core.
- Blend door function
- An air door that proportions airflow through the heater core and evaporator to set cabin temperature; actuator or calibration faults cause temperature errors.
- Cabin air filter effect
- A clogged filter reduces airflow across the evaporator and heater core, causing weak cooling and heating that mimics refrigerant or blower faults.
- Evaporator freeze-up
- Ice forms on the evaporator when the temperature sensor or cycling switch fails, blocking airflow and causing cooling to fade after extended operation.
- Fan clutch effect
- A slipping fan clutch reduces airflow through the condenser and radiator, raising discharge pressure and degrading A/C performance at idle.
- Thermostat and heat
- A thermostat stuck open keeps engine coolant too cool, reducing heater output; verify engine operating temperature before diagnosing the HVAC system.
- Clutch control circuit
- Compressor clutch may be driven directly, through a relay, or by a module; verify coil power, ground, and command before replacing the clutch.
- High-pressure cutoff
- A switch or sensor that disengages the compressor clutch when discharge pressure exceeds a set limit, protecting the system from overpressure damage.
- Low-pressure cutoff
- Disengages the compressor when suction pressure falls too low, preventing operation with an undercharge or at low ambient temperatures.
- PWM blower module
- Receives a duty-cycle command from the control head and varies blower motor speed; check command signal and grounds before replacing the motor.
- Actuator feedback code
- An out-of-range door position code often indicates a stuck door or failed actuator rather than a faulty control module.
- CAN communication loss
- Loss of module communication on the bus can disable blower, blend, and compressor functions; check network codes before component replacement.
- Voltage drop testing
- Measuring voltage lost across a circuit under load reveals high resistance; even 0.5V on a ground can cause intermittent blower or clutch operation.
- Recovery requirement
- Refrigerant must be recovered before opening the system; venting to atmosphere is prohibited and requires EPA Section 609 certified equipment and technicians.
- Cylinder fill limit
- Recovery cylinders must never be filled above 80 percent capacity and must carry a current hydrostatic test date and proper labeling.
- Refrigerant safety
- Liquid refrigerant causes frostbite, and vapor displaces oxygen in enclosed spaces; wear gloves and eye protection and ventilate the work area.