PCM Controlled Air Conditioning

Locations:

The air conditioning system uses a compressor with variable displacement. This compressor is referred to as the V-5 type compressor. The V-5 compressor meets A/C requirements without cycling.

The A/C compressor operation is controlled by the Powertrain Control Module (PCM) for the following reasons:

^ Improves idle quality during compressor clutch engagement.

^ Improves wide open throttle performance (WOT).

^ Provides A/C compressor protection from operation with incorrect refrigerant pressures.

For a description of the system and an explanation of the components used, refer to Heating and Air Conditioning.

The A/C electrical system consists of the following components:

^ A/C control head

^ A/C refrigerant pressure switch

^ A/C compressor clutch

^ A/C compressor clutch relay

^ PCM

When A/C is selected through the A/C control head a 12 volt signal is supplied to the A/C request input of the PCM. The A/C compressor clutch relay is controlled through the PCM. This allows the PCM to increase the engine idle speed just prior to A/C clutch engagement for better idle quality. In addition the PCM will command the cooling fans on during A/C operation. The PCM monitors the A/C refrigerant pressure. If the A/C refrigerant pressure, and engine operating conditions are within a specific calibrated acceptable ranges the PCM will enable the A/C compressor relay. This is accomplished by providing a ground path for the A/C relay coil within the PCM. When the A/C compressor relay is enabled battery voltage is supplied to the compressor clutch coil. 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 greater than 90%

^ A/C head pressure greater 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 below 9.5 volts.

^ Engine speed greater than 6900 RPM for 5 seconds.

^ ECT greater than 121°C (250°F).

^ IAT less than 0°C (32°F).

A/C Clutch 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, refer to Heating and Air Conditioning.

A scan tool will be used in diagnosing the system. The scan tool has the ability to read the A/C 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.