Heating, Ventilation and Air Conditioning
The refrigeration cycle
Refrigerant leaves the compressor as a hot high-pressure gas, condenses to liquid in the condenser, passes through a receiver-drier or accumulator, is metered through an expansion valve or orifice tube, and boils in the evaporator, absorbing heat from cabin air. It then returns to the compressor as a low-pressure gas. Know the difference between R134a and R1234yf systems: different fittings, oils and service equipment, and never interchange them.
Service and safety
Refrigerant must be recovered and recycled, never vented. Evacuation removes air and moisture before charging by weight; non-condensable gas raises head pressure and reduces performance. Add the specified oil type and quantity, and check for leaks with electronic detectors or dye. Compressor operation depends on pressure switches and a clutch or control valve, so a non-running compressor can simply be a low-charge protection. The heater side is separate: coolant flows through the heater core, and blend doors mix hot and cold air, while the blower and cabin filter handle airflow.
Reading pressures
Low suction and low discharge often means low charge; high discharge and high suction suggests overcharge or poor condenser airflow; a fast-cycling compressor points to low charge.
Rule of thumb: vent temperature from a centre outlet on a correctly charged system is typically around 3 to 10 degrees Celsius on a warm day, and the suction line should be cold and sweating. Traps include charging liquid into the suction side of a running system, skipping evacuation, and mixing refrigerant types.