Compressor HVAC: Description and Operation

Battery voltage is supplied through fuse 56 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 17. 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 (ECM) or powertrain control module (PCM) through the A/C thermostat and the A/C pressure switch.

The A/C compressor clutch relay is grounded by the engine control module (ECM) or Powertrain Control Module (PCM). When energized, the A/C compressor clutch relay allows battery voltage to turn on the A/C compressor clutch.

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

A/C Thermostat
The A/C thermostat is located on the evaporator housing. The A/C thermostat turns off the A/C compressor clutch if the temperature at the evaporator goes below 3 °C (37 °F). This prevents condensation from freezing on the evaporator fins and blocking the air delivery into the passenger compartment. The blower motor will keep running when the sensor turns off the compressor.

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.

Thermal Protector
The thermal protector, located on the A/C compressor, opens and turns the compressor off if it senses high temperature. Once the compressor cools, the switch will close and the compressor will begin running again.