When the A/C switch is pressed, the HVAC control assembly sends a GMLAN message to the Engine control module (ECM) for an A/C request. This input will request the ECM to ground the A/C compressor clutch relay control circuit, which will switch the A/C CMPR relay. With the relay contacts closed, battery voltage is supplied to the A/C compressor clutch assembly.

The following conditions must be met in order for the ECM to turn on the compressor clutch:

Schematic Reference

HVAC Schematics

Connector End View Reference

HVAC Component Views

Electrical Information Reference

Scan Tool Reference

1. Press the A/C request switch. The A/C compressor clutch should engage.
2. Place the mode switch in the defrost position. The A/C compressor clutch should engage.

1. Ignition OFF, disconnect the harness connector at the HVAC control module.
2. Ignition OFF, test for less than 1.0 ohm of resistance between the ground circuit terminal A5 and ground.

⇒	If greater than 1.0 ohm, test the ground circuit for an open/high resistance.

3. Install a 3-amp fused jumper wire between the signal circuit terminal A1 and ground. With a scan tool, verify the BCM, A/C Switch parameter is ON.

⇒	If not On, test the signal circuit for a short to voltage or an open/high resistance. If circuit tests normal, replace the BCM.

4. Reconnect the harness connector of the HVAC control module. With a scan tool, verify the BCM A/C Switch parameter cycles between On and Off with each press of the A/C switch.

⇒	If the parameter does not cycle, test the signal circuit for a short to ground. If the circuit tests normal, test or replace the HVAC control module.

5. Remove the A/C compressor clutch relay.
6. Verify that a test lamp illuminates between the relay coil B+ circuit and ground.

⇒	If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance.

7. Verify that a test lamp illuminates between the relay switch B+ circuit and ground.

⇒	If the test lamp does not illuminate, test the relay switch B+ circuit for an open/high resistance. If the A/C fuse is open, test the relay switch control circuit for a short to ground. If all circuits test normal, test or replace the A/C compressor clutch.

8. Disconnect the harness connector at the A/C compressor clutch.
9. Ignition OFF test for less than 1.0 ohm of resistance between the A/C compressor ground circuit and ground.

⇒	If greater than 1.0 ohm, test the ground circuit for an open/high resistance.

10. Connect the harness connector at the A/C compressor clutch.
11. Connect a 10-amp fused jumper wire between the relay switch B+ circuit and the relay switch control circuit. Verify the A/C compressor clutch engages.

⇒	If the A/C compressor clutch does not activate, test the control circuit for an open/high resistance. If the circuit tests normal, test or replace the A/C compressor.

12. Connect a test lamp between the relay control circuit and the relay coil B+ circuit.
13. Using a scan tool, command the A/C relay output ON and OFF. The test lamp should turn ON and OFF when changing between the commanded states.

⇒	If the test lamp remains ON all the time, test for a short to ground on the control circuit. If the circuit tests normal, replace the ECM.

⇒	If the test lamp remains OFF all the time, test for a short to voltage or an open/high resistance on the control circuit. If the circuit tests normal, replace the ECM.

14. If all circuits test normal, test or replace the A/C compressor.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.