Compressor Controls

COMPRESSOR CLUTCH CONTROL
The compressor clutch is controlled by the Body Control Module (BCM)from inputs to both the Electronic Control Module (ECM) and BCM.

INPUTS TO THE ECM
- Engine Coolant Temperature
- Throttle Position Sensor
- Power Steering Pressure Switch

INPUTS TO THE BCM
- In-Car Temperature
- Sunload Temperature
- Vehicle Speed
- Low Refrigerant Pressure Switch
- A/C High Side Temperature
- A/C Low Side Temperature


The voltage source to the compressor clutch is from the 15 amp A/C fuse, which is switched through a relay, controlled by an IGN-3 signal fed through the low refrigerant pressure switch.


When all inputs to the ECM and ECM are within the calibrated values, the compressor clutch will engage if the following parameters are met:
- Climate Control set to "AUTO", or "DEF."
- Outside temperature is above 4°C (40°F).

TWO MODES OF COMPRESSOR CLUTCH CONTROL
Once engaged there are two modes of compressor clutch control for the minimum time the clutch will stay on and these are based on vehicle speed:

1. EXTENDED COMPRESSOR AT IDLE - This mode allows for smoother idle conditions by increasing the minimum clutch "on" time to about 45 seconds whenever the vehicle speed drops below 15 MPH.
2. NORMAL COMPRESSOR "ON" TIME - Vehicles operating above 15 MPH will be controlled to a minimum "on" time of from 2 to 6 seconds before the BCM will allow the clutch to disengage.

After the minimum on time has expired the BCM will disengage the compressor clutch only if the low side temperature is below - 1°C (30°F). After reaching this turn off temperature the BCM waits for the low side to reach a temperature between 7°C (20°F) and O°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 BCM to disengage the clutch:
- A/C low pressure switch open.
- A/C low side temp. below -7°C (20°C).
- A/C high side temp. above 93°C (200°F).
- Coolant temp. above 126°C (259°F).
- Wide open throttle signal or high steering fluid pressure signal from ECM.
- Open or shorted A/C low side sensor circuit.

Except for the wide open throttle or high steering fluid pressure signal, 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.


IDLE SPEED CONTROL
In order to minimize the effects of compressor clutch operation on engine idle speed, the ECM is able to modify the engine RPM during A/C operation. When the ECM receives the clutch engagement signal from the BCM, in anticipation of the engine load, the ECM makes an adjustment in the Idle Speed Control Motor to compensate.