PCM Controlled Air Conditioning

DESCRIPTION
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 (WOT) performance.
^ Provides A/C compressor protection from operation with incorrect refrigerant pressures.

The A/C electrical system consists of the following components:
^ A/C control head.
^ A/C refrigerant pressure sensor
^ A/C compressor clutch.
^ A/C compressor clutch relay.
^ PCM.

OPERATION
When an A/C mode is selected at the A/C control head, the A/C control head provides a 12 volts signal to the A/C request circuit. The PCM monitors the A/C request circuit and controls the A/C compressor relay based on the A/C request signal. 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 also monitors the A/C refrigerant pressure sensor circuit. If the A/C refrigerant pressure is too high or low, the PCM will disable the A/C compressor relay.

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 414 psi (4.27 volts) or less than 34 psi (0.35 volt) (as determined by the A/C refrigerant pressure sensor).
^ Ignition voltage below 10.5 volts.
^ Engine speed greater than 4500 RPM for 5 seconds or 5400 RPM.
^ ECT greater than 125° C (257° F).
^ IAT less than 5° C (41° F).


DIAGNOSIS
A scan tool will be used in diagnosing the refrigerant portion of the HVAC system. The scan tool has the ability to read the AC request and refrigerant pressure inputs to the PCM. The scan tool can display when the PCM has commanded the AC clutch ON. The scan tool has the ability to override the AC request signal and energize the AC control relay.