Compressor Controls

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 430 psi and closes again when the A/C head pressure falls below 200 psi. The Pressure Cycling Switch opens if the refrigerant pressure drops below 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.

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.