PCM Controlled A/C Circuit Diagnosis

Circuit Description
The PCM controls the operation of the A/C clutch through the A/C clutch relay. When the operator selects an A/C mode on the HVAC control module, the HVAC module sends a B+ signal (A/C request) through the A/C high pressure switch to the PCM. The A/C high pressure switch is normally closed unless a high pressure condition exists. This signal is displayed on the scan tool as A/C REQUEST. The PCM supplies a B+ signal to the A/C compressor cycling switch (which is normally closed when A/C pressure is between 18-49 psi), to ground. When these two inputs are received by the PCM, the PCM then supplies a ground on the A/C relay control circuit closing the relay contacts.

The PCM controlled HVAC system consist of the following:
^ The A/C High Pressure Cutout Switch
^ The A/C Compressor Cycling Switch
^ The A/C Compressor Clutch Relay
^ The PCM

The PCM will engage the A/C clutch any time the engine speed is below 5000 RPM and the A/C is requested unless any of the following conditions exist:
^ Throttle angle is at 100%.
^ The A/C compressor cycling switch pressure is less than 18 psi or greater than 49 psi.
^ The A/C High Pressure Cutout Switch pressure is greater than 420 psi.
^ Engine speed is greater than 5500 RPM.
^ ECT is greater than 121°C (250°F).

Any of the conditions described above will inhibit A/C compressor relay operation.

Diagnostic Aids

Important: Remove any debris from the PCM connector surfaces before servicing the PCM. Inspect the PCM connector gaskets when diagnosing/replacing the PCM. Ensure that the gaskets are installed correctly. The gaskets prevent water intrusion into the PCM.

^ The following may cause an intermittent:
- Mis-routed harness
- Rubbed through wire insulation
- Broken wire inside the insulation
- Check the HVAC and PCM connectors for corroded or backed out terminals
^ For an intermittent, refer to Symptoms. Symptom Related Diagnostic Procedures

Test Description
The numbers below refer to the steps on the diagnostic table.
2. This step tests if the A/C compressor clutch is engaged when there is no command to be engaged.
3. This step tests if the A/C compressor clutch is functioning. If the A/C clutch operates, inspect for poor connections throughout the system.
7. The A/C clutch relay is located in the underhood electrical center. This step tests the PCM and the A/C clutch relay control circuit.
12. There should be no A/C request when the HVAC control module is in the OFF position.
13. The A/C request circuit is shorted to a voltage between the A/C high pressure switch and the PCM if the scan tool indicates an A/C request when the switch is disconnected.
18. This step tests for a short to ground on the A/C relay control circuit.
19. This step tests for a short between the A/C relay control circuit and any other circuit in the PCM harness connector.