Compressor Clutch Control

COMPRESSOR CLUTCH CONTROL
The compressor clutch is controlled by the PCM from inputs to both the PCM and Air Conditioning Programmer (ACP) Inputs to the PCM are as follows:
-> Air Conditioning Programmer (ACP) request for compressor.
-> Air Conditioning Programmer (ACP)"s A/C possible flag.

Inputs to the Air Conditioning Programmer (ACP) 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 "PC).
-> Windshield Wiper Switch.
-> Outside Air Temperature.
-> Vehicle Speed (from "PC).
-> System Mode (from "PC).

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 Air Conditioning Programmer (ACP) are at the indicated values, the Air Conditioning Programmer (ACP) 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 codes 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 Air Conditioning Programmer (ACP) will allow the clutch to disengage.

After the minimum "ON" time has expired the Air Conditioning Programmer (ACP) will request the PCM to disengage the compressor clutch only if the low side temperature is below - 1 °C (30°F). After reaching this turn-off temperature, the Air Conditioning Programmer (ACP) waits for the low side to reach a turn-on temperature between 7°C (45°F) and 10°C (50°F) (depending on outside air temperature) and then repeats the cycle. During the compressor on cycle any of the following conditions will force the PCM to disengage the clutch:

Code A047 - Repeated occurrences of:
-> A/C low side temperature below -7°C (20°F).

AND/OR

-> A/C low pressure switch open.

Code A048 - A/C low pressure switch open (due to catastrophic loss of refrigerant).

Code A050 -
-> Coolant temperature above 126°C (259°F).

Code A012 -
-> Open or shorted A/C low side sensor circuit.
-> Wide open throttle signal from PCM.

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 Air Conditioning Programmer (ACP) 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 reengagement of the clutch.

IDLE SPEED CONTROL
In order to minimize the effects of compress 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 Air Conditioning Programmer (ACP) 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 Air Conditioning Programmer (ACP) 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 Air Conditioning Programmer (ACP) will store a diagnostic trouble Code (A047 and/or A048) in memory.