Compressor HVAC: Description and Operation

Battery voltage is supplied through fuse 34 to the A/C compressor clutch relay contacts at all times.

With the ignition switch in ON (II), voltage is applied to the coil of the A/C compressor clutch relay through fuse 8. When you push the A/C switch ON, and the heater fan switch is in position 1, 2, 3, or 4, a "ground" input is provided to the Engine Control Module (PCM or ECM) through the A/C thermostat and the A/C pressure switch.

The A/C compressor clutch relay is grounded by the Engine Control Module (PCM or ECM). When energized, the relay allows battery voltage to energize the compressor clutch coil which then engages the clutch.

The A/C ON indicator light comes ON when the A/C system is running.

A/C Pressure Switch
The A/C pressure switch is located in the condenser outlet line where refrigerant is in a high temperature/high pressure liquid state. The switch will sense abnormally high or low pressure, and open the circuit. This removes ground, and the compressor will stop running.