The Instrument Panel Integration Module (IPM) sends a Class 2 message to the Dash Integration Module (DIM). The DIM monitors, A/C refrigerant pressure, low side temperature, and engine coolant and battery voltage to ensure conditions are correct for compressor operation. If all conditions are correct the DIM sends a Class 2 message to the Powertrain Control Module (PCM) with the system operating in any mode but OFF.
Under normal operating conditions, the PCM then grounds the A/C compressor control relay coil and the relay energizes. Voltage is supplied to the A/C compressor clutch through a fuse and the A/C compressor clutch relay contacts. The A/C compressor clutch engages and the compressor operates.
If the PCM determines that engine load should be reduced, such as during full throttle conditions, it will de-energize the A/C compressor relay to disengage the compressor even though the message signal from the Dash Integration Module (DIM) is still present.
The compressor contains an internal thermal switch. This switch opens when the compressor temperature exceeds 155°C (311°F). The compressor thermal switch closes when the compressor temperature reaches approximately 125°C (257°F). The compressor thermal switch is serviced by replacement of the compressor.
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 DIM will allow the clutch to disengage.
After the minimum ON time has expired the DIM 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 DIM 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:
| • | Battery voltage low. |
| • | A/C Pressure Sensor detects a low or high pressure condition. |
| • | 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 DIM 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, the clutch will disengage and waits for normal conditions to re-engage of the clutch.
If the conditions are not met for engaging the compressor, a service A/C system message will appear on the driver's information center.
