PCM Controlled Air Conditioning Diagnosis

A/C SYSTEM
A/C operation is requested by the ACM using a serial data (Class 2) message to the PCM. The PCM will ground the A/C relay allowing current to flow through the A/C Compressor clutch coil. The PCM will disable or inhibit A/C compressor clutch operation during the following conditions:
^ High power steering load indicated.
^ Engine not running.
^ Engine at wide open throttle.
^ Outside temperature below approximately 4°C (40°F).
^ Certain PCM DTCs set.

The PCM will also engage the A/C compressor clutch during cranking in colder weather to prevent A/C slugging.

TEST DESCRIPTION
Number(s) below refer to the step number(s) on the Diagnostic Table.
1. Since many DTCs disable A/C clutch operation, any DTCs must be repaired and cleared before using this table.
2. An A/C Relay status of OFF indicates that the PCM is not allowing A/C clutch operation (DTCs set or low outside temp. etc.) or the ACM is not requesting A/C operation from the PCM. Check for HVAC DTC's which may inhibit the ACM from requesting A/C clutch operation.
To verify PCM control of the A/C clutch, use the output test for the A/C relay and command the A/C clutch ON and OFF with the engine RUNNING. Watch for the A/C clutch to engage and disengage.
4. Refer to Heating and Air Conditioning for diagnosis of intermittent operation. Check for proper system pressures and for HVAC DTC's that may be stored indicating a previous fault condition.
5. A lighted test light indicates the PCM can control the A/C relay ON. No A/C clutch operation could be caused by a blown fuse, a detective relay, A/C system pressure faults which open one of the A/C pressure switches or a defective A/C compressor clutch coil or ground. Refer to Heating and Air Conditioning for diagnosis of these components.