Heating, Ventilation and Air Conditioning Systems
Refrigeration cycle and diagnostics
An air-conditioning system moves heat; it does not create cold. Refrigerant leaves the compressor as a high-pressure, high-temperature gas, gives up heat in the condenser, leaves the receiver-drier as a liquid, passes through the expansion valve or orifice tube where pressure and temperature drop, absorbs heat in the evaporator, and returns to the compressor. Understand superheat and subcooling, and why charge must be set by weight rather than by gauge reading alone.
Diagnose by pressure pairs. Both sides low = undercharge. Low side low with high side normal or high = restriction at the expansion device or a plugged drier. Both sides high = overcharge, poor condenser airflow, or non-condensables. High side normal with low side high = weak compressor or a stuck-open expansion valve. A clutch that will not engage is usually a pressure switch, relay, fuse or low charge, not a failed clutch.
Refrigerant handling is regulated: recover and recycle, never vent. Evacuate to a deep vacuum (near 29 inHg) and hold it to prove there is no leak, then charge to the specified weight. Leak detection: electronic detector, UV dye, or nitrogen pressure with soap solution - never compressed shop air.
Heater and ventilation: coolant flow valve, blend door actuator, heater core, cab filters, blower resistor or module, and mode/defrost doors.
Worked example. The A/C cools at highway speed but not at idle. Condenser airflow is the prime suspect - check the fan clutch, electric fan operation, and debris on the condenser. High-side pressure climbing at idle confirms it.