ASE A7 cheat sheet

The A/C system has a high-pressure side including the compressor outlet, condenser, and receiver-drier, and a low-pressure side including the evaporator, accumulator, and compressor inlet.

The compressor raises refrigerant pressure and temperature; it moves heat rather than creating cold directly.

The condenser transfers heat from high-pressure refrigerant vapor to outside air and changes the refrigerant to liquid.

The expansion valve or orifice tube meters refrigerant and separates the high-pressure side from the low-pressure side.

The evaporator absorbs heat from cabin air as low-pressure liquid refrigerant boils into vapor inside the evaporator core.

TXV systems use a receiver-drier on the high side; orifice-tube or cycling-clutch systems use an accumulator on the low side.

The blower motor moves air through the evaporator and heater core; mode doors direct air to floor, panel, or defrost outlets.

The blend door controls cabin temperature by varying airflow through and around the heater core, and the recirculation door selects outside or cabin air.

A restricted cabin air filter reduces airflow through the HVAC case and can cause low blower output at all speeds.

For a given refrigerant, saturation pressure corresponds to one saturation temperature, so pressure changes alter boiling and condensing temperatures.

Superheat equals evaporator outlet temperature minus evaporator saturation temperature; low superheat can indicate overcharge or evaporator flooding.

Subcooling equals condenser saturation temperature minus liquid-line temperature; low subcooling can indicate undercharge.

The compressor clutch engages when controls request A/C and pressure and temperature inputs permit operation.

Fixed-orifice-tube systems cycle the compressor clutch to control evaporator temperature, and the accumulator prevents liquid slugging.

A TXV modulates refrigerant flow to maintain evaporator superheat, while the receiver-drier stores liquid and removes moisture.

PAG oil is specified for R-134a and R-1234yf automotive systems, and mineral oil must not be substituted in R-134a systems.

High-side pressure rises with high ambient temperature, poor condenser airflow, or overcharge; low-side pressure falls with low charge or a restriction.

The heater core transfers heat from engine coolant to cabin air; reduced coolant flow causes reduced or no cabin heat.

The thermostat regulates coolant flow to the radiator and helps the engine reach and maintain proper operating temperature.

The radiator pressure cap raises the coolant boiling point and releases pressure above its rated value.

Coolant mixture ratio affects freeze protection, boil-over protection, corrosion protection, and heat transfer; the specified ratio must be used.

A water valve or blend door controls heater performance by regulating coolant flow or airflow through the heater core.

The electric cooling fan may run with an A/C request; fan failure can raise high-side pressure and reduce cooling performance.

Ventilation brings outside air into the cabin, while recirculation reduces outside air intake and can speed cooling but increases humidity.

Low coolant level or air trapped in the cooling system can cause no heat even when the heater core is not clogged.

Manual HVAC systems use cables or mechanical linkages for doors, while automatic HVAC systems use sensors, actuators, and a control module.

A thermistor changes resistance with temperature; most automotive HVAC temperature sensors have negative temperature coefficient behavior.

A potentiometer or position sensor provides feedback so the control module can determine actuator door position.

A blower resistor provides lower fan speeds by adding resistance, while a PWM blower controller varies motor speed by switching power rapidly.

The A/C compressor clutch circuit includes a fuse, relay, pressure switch, PCM driver, and clutch coil; an open circuit in any part prevents clutch engagement.

An evaporator temperature sensor prevents evaporator freeze-up by cycling the compressor or controlling refrigerant flow.

Ambient temperature, interior temperature, sunload, and evaporator temperature inputs let automatic climate control adjust blower speed, airflow mode, and temperature.

A/C pressure switches protect the compressor and may control cooling fans by disabling the clutch at low or high pressure limits.

Section 609 certification is required for technicians who service or repair motor vehicle A/C refrigerant systems, and venting refrigerant is prohibited.

Refrigerant must be recovered before a system is opened; releasing refrigerant to the atmosphere is not permitted.

Recovery and recycling equipment must be certified for the refrigerant being serviced, and R-134a and R-1234yf must not be mixed.

Identify refrigerant by system label or refrigerant analyzer before recovery; unknown or mixed refrigerant requires special handling.

Evacuation removes air and moisture; achieving and holding the specified vacuum verifies the system is ready for charging and helps reveal leaks.

Charge refrigerant by weight using a scale and the manufacturer-specified capacity; oil quantity must also match the repair and system specification.

R-1234yf is mildly flammable, so dedicated equipment and ignition-source control are required during service.

Compressed air must not be used to pressure-test an A/C system; dry nitrogen and approved leak detection methods are used instead.

A performance test sets engine speed, blower speed, temperature controls, and cabin conditions as specified, then compares vent temperature and system pressures.

Low suction and low discharge pressures commonly indicate low refrigerant charge, along with weak cooling and possible compressor short-cycling.

High suction and high discharge pressures can indicate overcharge, poor condenser airflow, or a compressor or expansion-device issue.

Electronic leak detection, UV dye, and soap bubbles locate leaks; the system is evacuated and recharged after repair.

If the compressor clutch does not engage, check the fuse, relay, pressure switch, PCM request, and clutch coil circuit.

If the blower works only on high speed, suspect an open blower resistor or faulty blower controller in the lower-speed circuit.

No cabin heat can result from low coolant, a stuck-open thermostat, clogged heater core, failed blend door, or closed water valve.

A restricted expansion device or orifice tube can cause low suction pressure and frost or condensation at the restriction.