Compressor HVAC: Description and Operation

The climate control unit receives battery voltage at all times through fuse 39. With the ignition switch in ON (II), voltage is supplied to the climate control unit. The control unit is grounded at G403.

When you push the A/C ON/OFF button with the ignition switch in ON (II), the climate control unit will ground the A/C ON input of the fan control unit. The fan control unit then signals the powertrain or engine control module (PCM or ECM) that A/C has been requested and applies a ground signal to the PCM through the PNK wire (see Fans).

The PCM or ECM, grounds the A/C compressor clutch relay when the ignition switch is in ON (II). The A/C compressor clutch relay then energizes the compressor clutch, which engages and begins turning the compressor.

The climate control unit monitors the operation of the air conditioning system in the car through the in-car temperature sensor, outside the car with the outside air temperature sensor, and in the evaporator outlet with the evaporator temperature sensor.

The climate control unit monitors the amount of direct sunlight in the car with the sunlight sensor, and monitors the engine coolant temperature with the engine coolant temperature sensor (ECT) in the engine coolant outlet.

When refrigerant pressure becomes too high (due to blockage), or too low (due to leakage), the A/C pressure switch will open and the PCM or ECM will then disengage the A/C compressor clutch.