PCM Controlled Air Conditioning

The A/C compressor clutch is controlled by the Powertrain Control Module (PCM) The HVAC controller sends the A/C request (10) to the BCM. The BCM relays the request to the PCM through the Class II serial data circuit. This allows the PCM to manage idle quality, improve wide open throttle performance, and to protect the A/C system from excessively high and low pressures.

The PCM controlled air conditioning system uses a variable displacement compressor. The compressor meets the A/C requirements without cycling. For a description of the system and an explanation of the components used, refer to Cooling Insufficient, A/C System in Heating and Air Conditioning. Cooling Insufficient - A/C System

The A/C electrical system consists of the following components:
^ A/C Control Assembly
^ A/C Refrigerant Pressure Sensor
^ A/C Compressor Clutch
^ A/C Compressor Clutch Relay
^ Class 2 Serial Data Circuit
^ Powertrain Control Module (PCM)
^ Body Control Module (BCM)

The PCM will enable the A/C compressor clutch whenever the engine is running and the A/C has been requested, unless any of the following conditions are met:
^ Throttle more than 90 percent
^ A/C head pressure more than 427 psi (4.27 volts) or less than 41 psi (0.35 volt), as determined by the A/C refrigerant pressure sensor
^ Ignition voltage is below 9.5 volts
^ Engine speed is more than 6900 RPM for 5 seconds
^ ECT more than 121°C (250°F)
^ IAT less than 0°C (32°F)

The PCM Controlled A/C Circuit Diagnosis should be used for diagnosing the electrical portion of the A/C compressor clutch circuit. When diagnosing the refrigerant portion of the HVAC system, the Cooling Insufficient, A/C System in Heating Ventilation, and Air Conditioning should be used.

A scan tool will be used in diagnosing the system. The scan tool has the ability to read the A/C request and refrigerant pressure inputs to the PCM. The scan tool can display when the PCM has commanded the A/C clutch on. The scan tool should have the ability to override the A/C request signal and energize the A/C control relay.