Compressor Clutch Control

COMPRESSOR CLUTCH CONTROL
The compressor clutch is controlled by the PCM from inputs to both the PCM and ACM. Inputs to the PCM are as follows:
^ ACM request for compressor.
^ ACM's A/C possible flag.
^ Inputs to the ACM are as follows:
^ Low refrigerant pressure switch.
^ High refrigerant pressure switch.
^ A/C high side temperature.
^ A/C low side temperature.
^ Rear defog switch from Instrument Panel Cluster (IPC).
^ Outside air temperature.
^ Vehicle speed.
^ System mode.

The voltage source to the compressor clutch is from the 10 amp A/C fuse through the A/C compressor clutch relay which is controlled by the PCM.

When the following inputs to the ACM are at the indicated values, the ACM will signal the PCM that compressor operation is possible:
^ Climate Control setting is not "OFF" or "ECON."
^ Outside temperature is above 4°C (40°F).
^ No compressor disabling DTCs are set.

Once engagement is deemed possible, there are two modes of compressor clutch control for the minimum time the clutch will stay on and these are based on vehicle speed.

Extended Compressor at Idle
This mode allows for smoother idle conditions by increasing the minimum clutch "ON" time to a maximum of 45 seconds whenever the vehicle speed drops below 15 mph.

Minimum Compressor On Time
Vehicles operating above 20 mph will be controlled to a minimum "ON" time of from 4 to 6 seconds before the ACM will allow the clutch to disengage.

After the minimum "ON" time has expired the ACM will request the PCM to disengage the compressor clutch only if the low side temperature is below -2°C (28°F). After reaching this turn-off temperature, the ACM waits for the low side to reach a turn-on temperature of 10°C (50°F) and then repeats the cycle. During the compressor on cycle any of the following conditions will force the PCM to disengage the clutch:

DTC Code B1347 -
Repeated occurrences of:
^ A/C low side temperature below -7°C (20°F). and/or A/C low pressure switch open.
DTC Code B1348 -
^ A/C low pressure switch open (due to catastrophic loss of refrigerant).
DTC Code B1350-
^ Coolant temperature above 126°C (259°F).

Except for the wide open throttle, all of the above conditions will keep the compressor "OFF" until the condition no longer exists. The wide open throttle condition will keep the compressor "OFF" for a maximum of 20 seconds.

On initial crank, the ACM will check for indications of liquid refrigerant which may have collected at the compressor and if found, the A/C clutch will engage. After the engine is started or shortly thereafter, the clutch will disengage and wait until the normal calibrations and parameters are met to signal re-engagement of the clutch.

Idle Speed Control
In order to minimize the effects of compressor clutch operation on engine idle speed, the PCM is able to modify the engine rpm during A/C operation. When the PCM receives the clutch engagement signal from the ACM in anticipation of the engine load, the PCM makes an adjustment in the idle speed control motor to compensate.

Low Refrigerant Message
In the process of controlling the compressor clutch, the ACM monitors certain system inputs for an indication of low refrigerant charge. If the system's refrigerant state of charge should fall below approximately one fourth of its capacity, a "VERY LOW A/C REFRIGERANT- COMPRESSOR OFF" message will be displayed and the ACM will store a DTC Code (B1347 and/or B1348) in memory.