ASE H7 HVAC (Transit Bus) key terms

The fluid that circulates through an HVAC system, absorbing heat as it evaporates at low pressure and releasing heat as it condenses at high pressure. Transit buses typically use R-134a or R-1234yf.

The energy absorbed or released when a substance changes phase, such as liquid refrigerant boiling into vapor, without a change in temperature. This phase change is what moves heat in an HVAC system.

The number of degrees a refrigerant vapor is above its saturation temperature at a given pressure. Measured at the evaporator outlet, it confirms the refrigerant is fully vaporized before reaching the compressor.

The number of degrees a liquid refrigerant is below its saturation temperature at a given pressure. Measured at the condenser outlet, it indicates how much liquid is stacked in the condenser to feed the metering device.

A reference table listing the saturation pressure of a refrigerant at each temperature. Technicians use it to convert gauge readings into expected temperatures when checking superheat and subcooling.

Safety gear such as gloves, goggles, and face shields worn when handling refrigerant or servicing pressurized lines. Liquid refrigerant can cause frostbite and eye injury, so PPE is required before opening any system.

The federally required credential from the EPA that allows a technician to service motor vehicle air conditioning systems. It covers refrigerant recovery, recycling, and safe handling rules.

Tissue damage caused by contact with liquid refrigerant or escaping vapor, which boils at very low temperature. Affected skin turns pale and numb, and eyes must be flushed immediately with water.

The process of removing refrigerant from a system and storing it in an approved container rather than venting it to the atmosphere. Federal law prohibits intentional venting of refrigerant during service.

A motor-driven unit that pulls refrigerant from an HVAC system, separates oil and contaminants, and pushes the vapor into a storage cylinder. It must be used whenever a system is opened for service.

A DOT-approved, color-coded tank used to store recovered refrigerant. It must never be filled beyond its rated capacity and is never used as a charging source for a different system.

Cleaning recovered refrigerant by removing moisture, acid, and particulates so it can be reused in the same vehicle. Recycling differs from recovery, which only removes the refrigerant from the system.

The pump that raises refrigerant pressure and temperature so heat can be rejected at the condenser. In transit buses it is typically belt-driven from the engine or an electric motor.

An electrically controlled assembly that engages or disengages the compressor from the drive belt. The coil magnetizes the hub to lock the drive plate to the pulley when cooling is requested.

The electromagnetic winding inside the clutch pulley that creates the magnetic field to pull the drive plate in. It is energized by the control module and must be checked for resistance and supply voltage.

The clearance between the clutch drive plate and the pulley friction surface. Too large a gap prevents engagement; too small a gap causes drag and rapid wear.

Specialized lubricant, commonly PAG or POE, circulated with the refrigerant to lubricate moving parts. Oil is hygroscopic, so containers must stay sealed and the correct type must be used for the refrigerant.

The heat exchanger mounted at the front of the bus that rejects heat from high-pressure refrigerant vapor, causing it to condense into a liquid. Airflow across its fins is essential to performance.

The heat exchanger inside the cabin where low-pressure refrigerant absorbs heat from passenger air and boils into vapor. Condensate drained from its fins removes humidity from the cabin.

A tank on the high side that stores liquid refrigerant and holds a desiccant bag to absorb moisture and filter debris. It feeds the expansion valve and must be replaced when the system is opened.

A canister on the low side of an orifice-tube system that traps liquid refrigerant and oil so only vapor reaches the compressor. It also contains a desiccant to remove moisture.

Water contamination in an HVAC system that forms acids, corrodes metal, and can freeze at the expansion device. It is controlled by desiccant in the receiver-drier or accumulator and by proper evacuation.

A thermostatic metering device that controls refrigerant flow into the evaporator by sensing outlet temperature and pressure. It maintains a set superheat and blocks flow when the compressor is off.

A fixed-restriction metering device that controls refrigerant flow by a calibrated passage rather than a moving valve. It is used with an accumulator and cannot adjust to load changes.

The low-side pressure measured between the evaporator and the compressor. It reflects evaporator temperature and load, and is used with ambient temperature to judge system performance.

The high-side pressure measured between the compressor and the condenser. It rises with ambient temperature, condenser airflow restriction, or overcharge and falls when the system is undercharged.

A tool with high-side and low-side gauges and hoses used to read system pressures and to connect a recovery or charging station. Hoses must be purged to avoid mixing refrigerant.

Pulling a deep vacuum on an open system to boil off air and moisture before charging. A vacuum that holds without rising confirms the system is leak-free and dry.

A pump used to remove air and moisture from an HVAC system through a deep vacuum. Its oil must be changed regularly because it absorbs moisture and loses sealing ability.

The measured amount of refrigerant in a system, specified by weight. Undercharge or overcharge both degrade cooling and can damage the compressor.

The electronic unit that reads sensors and switches and commands the clutch, blower, and blend doors. It may be a dedicated HVAC controller or a body controller on the bus network.

A sensor that converts refrigerant pressure into an electrical signal for the control module. The module uses it to protect the compressor and to cycle the clutch.

A small electric motor that moves a door to mix heated and cooled air for temperature control. A failed actuator causes one temperature setting regardless of the requested value.

The electric fan that moves air across the evaporator and heater core and into the cabin. Speed is controlled by a resistor pack or a pulse-width-modulated module.

A small radiator inside the cabin that uses engine coolant to warm passenger air. A leaking core produces a sweet smell, fogging, and coolant loss.

The HVAC mode that directs warm, dry air to the windshield to clear fog and ice. It typically engages the compressor to dehumidify air and may force outside air intake.

The process of finding a leak using electronic sniffers, UV dye, or soap bubbles. Federal rules require repairing leaks above a specified rate before recharging a system.