Compressor Clutch Control

The compressor clutch is controlled by the PCM from inputs to both the PCM and heater and A/C programmer. Inputs to the PCM are as follows:

^ Heater and A/C programmer request for compressor.
^ Heater and A/C programmer's A/C possible flag.

Inputs to the heater and A/C programmer 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 IPC).
^ Ambient air temperature.
^ Vehicle speed.
^ System mode.

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

When the following inputs to the heater and A/C programmer are at the indicated values, the heater and A/C programmer 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 24 km (15 mph).

Minimum Compressor On Time

Vehicles operating above 32 km (20 mph) will be controlled to a minimum ON time of 4 to 6 seconds before the heater and A/C programmer will allow the clutch to disengage.

After the minimum ON time has expired the heater and A/C programmer 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 heater and A/C programmer 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 heater and A/C programmer 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 engage 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 heater and A/C programmer 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 heater and A/C programmer 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 heater and A/C programmer will store a DTC (B1347 and/or B1348) in memory.