Compressor HVAC: Description and Operation

CIRCUIT OPERATION
The compressor for air conditioning is belt driven by the engine. The A/C Compressor Clutch engages and disengages the compressor to remove the air conditioning load from the engine when needed. The A/C Compressor Clutch engages the compressor when the A/C mode selector is in MAX, NORM, BI-LEVEL, DEFOG or DEFROST.

Compressor Clutch Diode
The A/C Compressor Clutch Diode connected across the A/C Compressor Clutch suppresses high voltage spikes that are generated by the collapsing magnetic field of the clutch coil when the clutch is turned off.

Compressor Relay
The A/C Compressor Relay opens and closes the circuit to the A/C Compressor Clutch. The coil of the relay is grounded by the Electronic Control Module (ECM). This enables the ECM to disengage the air conditioning to reduce the load on the engine when required. Wide open throttle, high power steering pressure or coolant temperature will also prevent the A/C system from engaging.

ECM and clutch engagement
The ECM receives an air conditioning ON signal, which is battery voltage from the Heater & A/C Control Assembly. It signals the ECM that A/C compressor operation is required. This voltage is supplied through the A/C High Pressure Switch. The Low Pressure Switch is connected in series with the A/C Compressor Clutch. It opens if refrigerant pressure becomes low enough that further operation may damage the compressor.