GM Service Manual Online
For 1990-2009 cars only

The compressor for the air conditioning system is belt-driven by the engine through the A/C compressor clutch. The clutch controls the compressor in the following ways:

    • Disengages when air conditioning is not requested
    • Disengages when the engine load must be reduced

The heater and A/C control automatically requests A/C compressor clutch engagement with the system operating in the defrost mode

Under normal operating conditions, the following events occur:

    • The heater and A/C control sends an A/C operation desired message to the dash integration module (DIM)
    • The DIM sends a request for A/C compressor clutch engagement to the powertrain control module (PCM)
    • The PCM supplies a ground to the A/C compressor control relay coil
    • The A/C CLU relay energizes
    • Voltage is supplied to the A/C compressor clutch through the A/C CLU fuse
    • The A/C compressor clutch engages
    • The compressor runs

The PCM monitors A/C refrigerant line pressure and engine coolant temperature. This information is placed on the Class 2 communication link and is monitored by the DIM. The DIM looks at A/C line pressure, engine coolant temperature and battery voltage. The DIM will request A/C operation from the PCM if these parameters are within normal operation limits and the heater and A/C control is requesting A/C operation.

If the PCM determines that the engine load should be reduced, such as during full throttle conditions, the PCM will de-energize the A/C compressor control relay. This will disengage the compressor, even though the request for A/C compressor clutch engagement from the heater and A/C control is still present.

The A/C compressor clutch diode is connected across the A/C compressor clutch terminals. This diode provides a path for current resulting from voltage spikes. The collapsing magnetic field of the A/C compressor clutch coil generate these voltage spikes. Voltage spikes occur every time the coil is de-energized.