A/C Compressor Control

WARNING: This page is about a different car, the 2006 Land Rover Range Rover. However, it is still accessible from the selected car via links, so may be relevant.

The A/C compressor is engaged by pressing either the automatic mode switch, defrost switch, air conditioning (A/C) switch or maximum air conditioning (A/C) 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 compressor engagement process. The time delay allows the engine control module ECM to increase throttle angle and fueling in anticipation of the additional load on the engine when the A/C compressor engages.

When it receives an input to engage the A/C compressor, the ATC module sends a message to the engine control module ECM, via the K bus, instrument cluster and high speed controller area network CAN bus, to advise that it wants to engage the A/C compressor. Provided there are no engine management problems, the engine control module ECM responds by increasing throttle angle and fueling and sending a message granting the request to the ATC module over the controller area network CAN bus, instrument cluster and K bus. When it receives the grant message, the ATC module energizes the A/C compressor clutch provided the following conditions exist:

Engine speed is more than 400 rev/min.
Evaporator temperature is more than 3°C (37°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 module.

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

The A/C compressor can also be disengaged by the engine control module ECM changing the grant message on the high speed controller area network CAN network to a negative value, or by a 12 V hardwired signal to the ATC module from the engine control module ECM (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 module de-energizes the A/C compressor clutch. The time delay allows the engine control module ECM to decrease throttle angle and fueling, in anticipation of the reduction in engine load when the A/C 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 compressor to ensure maximum acceleration when accelerator pedal demand suddenly goes from no load to full load.