Compressor HVAC: Testing and Inspection

Diagnostic Chart (Part 1 Of 6):

Diagnostic Chart (Part 2 Of 6):

Diagnostic Chart (Part 3 Of 6):

Diagnostic Chart (Part 4 Of 6):

Diagnostic Chart (Part 5 Of 6):

Diagnostic Chart (Part 6 Of 6):

Schematic:

CIRCUIT DESCRIPTION
The Powertrain Control Module (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 Heating, Ventilation, And Air Conditioning (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 percent.
- The A/C compressor cycling switch pressure is less than 18 psi or more than 49 psi.
- The A/C high pressure cutout switch pressure is more than 420 psi.
- Engine speed is more than 5500 RPM.
- Engine Coolant Temperature (ECT) is more 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 or 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.