ASE A7 key terms

A fluid that absorbs heat when it boils at low pressure and releases heat when it condenses at high pressure. Circulating refrigerant carries heat from the cabin to the outside air.

An engine- or electrically driven pump that draws low-pressure refrigerant vapor from the evaporator and compresses it to a high pressure and temperature so it can be condensed.

A heat exchanger mounted ahead of the radiator where high-pressure refrigerant vapor gives up heat to outside air and condenses into a liquid.

A heat exchanger inside the HVAC case. Low-pressure liquid refrigerant enters, absorbs heat from air passing over the fins, and boils into vapor, cooling the cabin air.

A metering device that senses evaporator outlet temperature and pressure and varies refrigerant flow to keep the evaporator properly filled without letting liquid reach the compressor.

A fixed-restriction metering device with a calibrated hole that controls refrigerant flow into the evaporator. Because it cannot modulate flow, the compressor cycles on pressure-switch control.

A canister on the low side of an orifice-tube system that separates and stores any liquid refrigerant, holds desiccant, and returns oil to the compressor.

A canister on the high side of a TXV system that stores liquid refrigerant, holds desiccant to absorb moisture, and often contains the sight glass used for charge checks.

A moisture-absorbing material, such as silica gel, inside the receiver-drier or accumulator that traps water so it cannot form corrosive acids or freeze inside the system.

The number of degrees a refrigerant vapor is above its saturation temperature at the measured pressure. Low superheat means the evaporator is flooded; high superheat means it is starved.

The number of degrees a liquid refrigerant is below its saturation temperature at the measured pressure. Measuring it at the condenser outlet helps judge charge and condenser performance.

Special lubricating oil circulated with the refrigerant to protect the compressor. PAG oil is matched to R-134a and POE oil to R-1234yf, and the two must not be mixed.

An electromagnetic assembly on the compressor nose that locks the pulley to the shaft when the control module supplies power, and releases to stop refrigerant flow.

Two gauges with hoses and hand valves used to read high- and low-side pressures and to connect a recovery, evacuation, or charging station to the system.

The process of drawing refrigerant out of a system into a DOT-approved container with a recovery machine so that none of it is released into the atmosphere.

Filtering recovered refrigerant to remove oil, moisture, and contaminants so it can be reused, either in the same vehicle or in another vehicle's system.

The EPA credential a technician must hold before servicing or repairing motor vehicle air conditioning systems, earned by passing an approved test on refrigerant handling rules.

An EPA program that lists which refrigerant substitutes are acceptable in motor vehicle air conditioning, judged by ozone depletion and global warming potential.

A hydrofluoroolefin refrigerant with very low global warming potential used in newer vehicles. It is classified A2L, meaning it is mildly flammable and requires special handling precautions.

An ASHRAE safety group for refrigerants of lower flammability, requiring specific service precautions, ventilation, and control of ignition sources while the system is open.

Pulling a deep vacuum on a sealed, repaired system with a vacuum pump so that moisture boils off and air is removed before the system is charged.

A vacuum-measuring instrument that reads in microns of mercury, used to confirm the system has been evacuated enough to remove moisture and non-condensable gases.

Finding where refrigerant escapes by using a heated-diode or infrared detector, UV dye with a black light, or soap bubbles at joints, hoses, and components.

A check that compares vent outlet temperature, high- and low-side pressures, and ambient conditions against specifications to judge whether the system is cooling properly.

A pleated filter at the HVAC intake that traps dust, pollen, and debris before air enters the blower and evaporator, protecting the cabin and the evaporator core.

An electric motor with a squirrel-cage fan that pushes outside or recirculated air through the HVAC case, past the evaporator and heater core, and out the ducts.

A set of resistance elements that drop voltage to the blower motor to produce the lower fan speeds; it is bypassed so the motor receives full voltage on high speed.

A solid-state module that pulse-width modulates blower motor voltage to give stepless fan speeds and is commanded by the HVAC control head.

A movable door in the HVAC case that proportions air through or around the heater core, setting the temperature of the air leaving the case.

A door that directs conditioned air to the selected outlets, panel, floor, or defrost, by routing air through different passages in the HVAC case.

A door that selects between fresh outside air and cabin air entering the blower, used for faster cooling and to keep out odors or exhaust.

A small radiator inside the HVAC case through which hot engine coolant flows, transferring heat to the air that passes over its fins and into the cabin.

A temperature-sensitive valve that stays closed while the engine warms and then opens to let coolant flow to the radiator, holding the engine at normal operating temperature.

An engine-driven or electric pump that circulates coolant through the block, heads, radiator, and heater core, moving heat to where it can be released.

An electric or engine-driven fan that pulls air through the condenser and radiator at low vehicle speed or whenever A/C operation raises underhood and coolant temperatures.

A valve in the coolant line feeding the heater core that opens or closes to control how much hot coolant reaches the core in systems that regulate heat by coolant flow.

A thermistor that measures cabin air temperature, often with a small aspirator fan to pull air across it, and reports the value to the automatic climate control module.

A photodiode mounted on top of the dash that measures solar intensity so the automatic climate control can add cooling when the cabin is being heated by the sun.

A sensor that converts refrigerant pressure into an analog voltage signal so the control module can cycle the compressor and protect the system from extreme pressures.