A/C Compressor Control

A/C Compressor Control

The A/C (air conditioning) compressor is engaged by pressing either the automatic mode switch, defrost switch, A/C (air conditioning) switch or maximum A/C (air conditioning) switch. To prevent a dip in engine speed when the engine is at idle, a time delay of approximately 0.5 second is built into the A/C (air conditioning) compressor engagement process. The time delay allows the ECM (engine control module) to increase throttle angle and fueling in anticipation of the additional load on the engine when the A/C (air conditioning) compressor engages.

When it receives an input to engage the A/C (air conditioning) compressor, the ATC (automatic temperature control) module sends a message to the ECM (engine control module), via the K bus, instrument cluster and high speed CAN (controller area network) bus, to advise that it wants to engage the A/C (air conditioning) compressor. Provided there are no engine management problems, the ECM (engine control module) responds by increasing throttle angle and fueling and sending a message granting the request to the ATC (automatic temperature control) module over the CAN (controller area network) bus, instrument cluster and K bus. When it receives the grant message, the ATC (automatic temperature control) module energizes the A/C (air conditioning) compressor clutch provided the following conditions exist:
^ Engine speed is more than 400 rev/min.
^ Evaporator temperature is more than 3 degrees C (37 degrees F).
^ The refrigerant pressure is within limits.
^ Battery voltage is less than 16 V.
^ The blower is running.
^ There are no faults detected by the ATC (automatic temperature control) module.

The A/C (air conditioning) compressor remains engaged until selected off or the required conditions no longer exist. If the evaporator temperature decreases to approximately 2 degrees C (36 degrees F) the A/C (air conditioning) compressor is disengaged, then re-engaged when the evaporator temperature increases to more than 3 degrees C (37 degrees F) again. If battery voltage exceeds 16 V for more than 5 seconds the A/C (air conditioning) compressor is disengaged, then re-engaged when voltage decreases to less than 15.8 V.

The A/C (air conditioning) compressor can also be disengaged by the ECM (engine control module) changing the grant message on the high speed CAN (controller area network) network to a negative value, or by a 12 V hardwired signal to the ATC (automatic temperature control) module from the ECM (engine control module) (3.0L Diesel models) or the instrument cluster (4.2L and 4.4L models). Changing the grant message involves a time delay of up to 4 seconds before the ATC (automatic temperature control) module de-energizes the A/C (air conditioning) compressor clutch. The time delay allows the ECM (engine control module) to decrease throttle angle and fueling, in anticipation of the reduction in engine load when the A/C (air conditioning) compressor disengages, to prevent a sudden increase in engine speed if the engine is at idle. The hardwired signal is used to obtain instant disengagement of the A/C (air conditioning) compressor to ensure maximum acceleration when accelerator pedal demand suddenly goes from no load to full load.