Compressor Controls

The Compressor for the Air Conditioning system is belt-driven by the Engine through the A/C Compressor Clutch. The Clutch allows the 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.

Whether or not the A/C Compressor should operate depends on both the particular mode selected at the Electronic A/C Control Head and the Control Head temperature setting.

When an A/C Defrost mode is selected, a ground signal is applied to one side of the A/C Control Relay which energizes the relay. When energized, the normally open contacts close, sending battery voltage to the Electronic Control Module (ECM) and to the A/C Compressor Control Relay through the A/C Pressure Cycling Switch. This battery voltage applied at the ECM (A/C Request Input) is used to signal the ECM to energize the A/C Compressor Control Relay as well as adjusting idle speed to compensate for the additional load placed on the Engine by the A/C Compressor.

The ECM energizes the A/C Compressor Control Relay by sending a ground signal (from the A/C Compressor Control Output) to the relay, thus closing the normally open contacts. When the contacts close, the same battery voltage applied to the ECM A/C Request Input is applied to one side of the A/C Compressor Clutch and engages the A/C Compressor. When a wide open throttle condition exists, the ECM will de-energize the A/C Compressor Control Relay and disengage the A/C Compressor Clutch.

When the A/C Compressor Clutch is disengaged, an undesirable voltage spike is produced by the Clutch coils collapsing magnetic field. The A/C Clutch Diode will not allow the magnitude of this voltage to be more than 0.7 volt, thus preventing harm to other electrical systems on the vehicle.

The Pressure Cycling Switch turns the compressor off when the evaporator
temperature is low enough to cause icing. It opens when the refrigerant pressure is less than 170 kPa (25 psi). This opens the circuit to the A/C Compressor Control Relay and the ECM, which will disengage the A/C Compressor Clutch. This switch also closes to cycle the compressor on again when the evaporator temperature and pressure rise enough to require more cooling. The Electronic A/C Control Head senses the opening and closing of the cycling switch. The Control Head uses this input as well as certain information sent by the ECM on a serial data line (RPM, Coolant Temp, etc.), to determine if a malfunction exists. If a malfunction does exist, the Control Head will set a malfunction code. Refer to Diagnostic Trouble Code Description for code descriptions. Testing and Inspection