Compressor HVAC: Description and Operation

The A/C Compressor for the Air Conditioning System is belt-driven by the engine by way of the A/C Compressor Clutch. The Clutch allows the A/C Compressor to be disengaged when air conditioning is not required or when the air conditioning load should be removed from the engine, such as during full throttle.
Operation of the A/C Compressor depends on the A/C mode selected. With the driver-selected mode in BI-LEV, UPPER, DEFOG, DEF, or AUTO, the Heater and A/C Control Assembly communicates an A/C Compressor request through the E & C Data Line to the HVAC Programmer. The Programmer then supplies voltage to the A/C Compressor Clutch request input of the PCM.
The voltage path to the PCM is through the A/C High Pressure Cut-Out Switch and the Pressure Cycling Switch, both normally closed. The A/C High Pressure Cut-Out Switch opens if A/C head pressure rises above 2960 kPa (430 psi) and closes again when the A/C head pressure falls below 1379 kPa (200 psi). The Pressure Cycling Switch opens if the refrigerant pressure drops below 172 kPa (25 psi); the point at which icing of the evaporator may take place. When refrigerant pressure rises again, the Pressure Cycling Switch closes, re-engaging the A/C Compressor Clutch. In this way, the A/C Compressor is cycled on and off to maintain a low temperature in the evaporator without allowing condensation to freeze.
The PCM then energizes the A/C Compressor Control Relay by grounding terminal C3/C2. When the A/C Compressor Control Relay is energized, voltage is applied through the closed contacts of the relay to the A/C Compressor Clutch.
If the PCM determines that the A/C Compressor Clutch should be disengaged, such as during full throttle or if a low refrigerant condition is present, the PCM de-energizes the A/C Compressor Control Relay to remove voltage from the A/C Compressor Clutch. The clutch disengages and PCM code 66 is stored to prevent clutch operation.
The A/C Compressor Clutch Diode, connected across the A/C Compressor Clutch, provides a path for the induced voltage spikes that are generated each time the Clutch is turned off. The voltage spikes are generated by the collapsing magnetic field of the clutch coil.