PCM Controlled A/C Circuit 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 Heating, Ventilation and Air Conditioning (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 the HVAC section 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 defective 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.