ASE T7 HVAC (Medium/Heavy Truck) key terms

A fluid that absorbs heat when it evaporates at low pressure and releases heat when it condenses at high pressure, used to move heat out of the cab.

The quantity of heat energy absorbed by a liquid as it changes into a vapor without a temperature rise. In an A/C system this is the heat pulled from cab air.

The coil inside the cab where low-pressure liquid refrigerant boils and absorbs heat from air passing over its fins, cooling and dehumidifying the cab.

The coil mounted outside the cab where high-pressure refrigerant vapor gives up heat to outside air and condenses into a liquid.

A belt- or electrically-driven pump that raises refrigerant pressure and temperature, moving vapor from the evaporator to the condenser and driving the whole cycle.

A metering device that throttles high-pressure liquid refrigerant into the evaporator as a low-pressure spray, regulating flow to match cooling demand.

A fixed-restriction metering device with a calibrated passage that controls refrigerant flow into the evaporator; used with an accumulator instead of a receiver-drier.

A tank on the high-pressure side that stores liquid refrigerant and holds desiccant to remove moisture and filter debris before the metering device.

A canister on the low-pressure side of an orifice-tube system that traps liquid refrigerant and oil so only vapor reaches the compressor, and holds a desiccant bag.

The number of degrees a refrigerant vapor is above its saturation temperature at a given pressure; used to judge expansion valve and charge performance.

The number of degrees a liquid refrigerant is below its saturation temperature at a given pressure; used to judge condenser performance and system charge.

A two-gauge tool with hoses and valves that reads high-side and low-side pressures and allows controlled connection for testing, evacuation, and charging.

The high-side pressure measured between the compressor outlet and the metering device; abnormally high readings point to condenser or charge problems.

The low-side pressure measured between the evaporator outlet and compressor inlet; low readings can indicate low charge, restriction, or a faulty metering device.

Methods such as electronic sniffers, UV dye with a black light, soap bubbles, or pressure testing used to locate where refrigerant escapes from the system.

Pulling a deep vacuum on an empty system and watching whether it holds, to confirm the system is sealed and free of moisture before charging.

Using a vacuum pump to remove air and moisture from a sealed system before charging; a deep vacuum lowers the boiling point so trapped water vaporizes and is removed.

An electronic instrument that measures very low absolute pressure in microns, used to confirm the system has been evacuated to a level that removes moisture.

The process of removing refrigerant from a system and storing it in an approved container without intentionally releasing it to the atmosphere.

Processing recovered refrigerant through filters and separators to remove moisture, oil, and contaminants so it can be reused in the same or another system.

A DOT-approved, color-coded tank used to store recovered refrigerant; it must never be filled beyond its marked capacity and must be weighed during filling.

The EPA credential required for technicians who service motor vehicle air conditioning systems, covering refrigerant handling, recovery, and release prohibitions.

Special lubricant such as PAG or POE that circulates with refrigerant to lubricate the compressor; it must match the refrigerant type and is hygroscopic.

A device that analyzes the gas in a cylinder or system to confirm the refrigerant type and detect contamination before recovery or charging.

A pressure or temperature switch that engages and disengages the compressor clutch to keep evaporator temperature above freezing and maintain cab comfort.

A resistor pack that drops voltage to the blower motor to provide lower fan speeds; a failed resistor usually causes one or more speeds to stop working.

An electric fan that moves air across the evaporator and heater core and through the cab ducts; speed is controlled by resistors or a solid-state module.

A small radiator inside the HVAC case through which engine coolant flows, transferring heat to air that is blown into the cab.

A movable flap inside the HVAC case that mixes air passing through the heater core with air bypassing it to control cab temperature.

An electric motor or vacuum device that moves a door to direct air to the floor, panel, or defrost outlets based on the driver's selection.

The dash-mounted panel with switches, knobs, or a touchscreen that sends commands to blower, doors, and compressor controls to set cab comfort.

A valve that controls engine coolant flow through the heater core, often vacuum- or electrically-operated, so the heater can be shut off when not needed.

A switch that opens or closes based on refrigerant pressure to protect the compressor or signal the control module to cycle or disable the clutch.

A sensor that reports outside air temperature to the HVAC control module, used to inhibit compressor operation in cold weather and manage automatic climate control.

A pleated filter that traps dust, pollen, and debris from air entering the HVAC case, protecting the evaporator and improving cab air quality.

A vacuum-operated actuator that moves an air door using engine vacuum routed through the control head; a leaking diaphragm or hose prevents door movement.