PCM Controlled A/C Circuit Diagnosis

Diagnostic Chart (Part 1 Of 2):

Diagnostic Chart (Part 2 Of 2):

Refer to Engine Controls Schematic A/C Compressor Control, and HVAC Schematic HVAC Compressor Controls Schematics in HVAC.

CIRCUIT DESCRIPTION
When A/C is selected through the HVAC controller a 12 volt signal is supplied to the BCM. The BCM in turn signals the PCM through the Class II serial data circuit that A/C has been requested. The PCM then determines if conditions suited for A/C operation are present. If the PCM determines that the engine operating conditions are within acceptable ranges the PCM will enable the A/C relay. The PCM provides a ground path for the A/C relay coil. This action closes that A/C compressor relay contacts and battery positive voltage is supplied to the compressor clutch coil, and the compressor clutch coil has its own ground path.

The PCM will enable the A/C unless any of the following conditions are met:
^ Throttle more than 96 percent.
^ A/C pressure sensor more than 472 psi (4.9 volts) or less than 9 psi (0.1 volt) as determined by the A/C pressure sensor.
^ Ignition voltage below 10.0 volts.
^ Engine speed more than 4700 RPM.
^ ECT more than 124°C (255°F).
^ IAT less than 5°C (41°F).
^ Class II communication error between the HVAC controller and the PCM.

DIAGNOSTIC AIDS
Inspect for the following conditions:

IMPORTANT: Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.

^ Poor terminal connection - Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Use a corresponding mating terminal to test for proper tension.
^ Damaged harness - Inspect the wiring harness for damage. If the harness inspection does not reveal a problem, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the scan tool display may indicate the location of the fault.
^ PCM and engine grounds for clean and secure connections

If the DTC is determined to be intermittent, reviewing the Failure Records can be useful in determining when the DTC was last set.

TEST DESCRIPTION
The numbers below refer to the step numbers on the diagnostic table:
6. If the scan tool indicates that an A/C request is not present the PCM will not engage the A/C relay. Verify that the HVAC controller is providing the 12 volt signal to the BCM. If the 12 volt signal is being supplied to the BCM inspect the BCM for DTCs. A malfunction in the Class II serial data circuit will prevent the PCM from receiving the A/C request.