When the A/C switch is pressed, the HVAC control assembly grounds the A/C request signal circuit. The body control module (BCM) receives the ground input and sends a GMLAN message to the engine control module (ECM) or powertrain control module (PCM) for an A/C request. This input will request the PCM/ECM to ground the A/C compressor clutch relay control circuit, which will switch the A/C CLUTCH 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 PCM/ECM to turn on the compressor clutch:

Schematic Reference

HVAC Schematics

Connector End View Reference

HVAC Connector End 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 C2 pin K ground circuit terminal at the HVAC control assembly.
2. Ignition OFF, test for less than 1.0 ohm of resistance between the ground circuit terminal K 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 C2 pin C and ground. With a scan tool, verify the A/C Request 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 assembly. With a scan tool, verify the A/C Request 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 assembly.

5. Remove the A/C compressor clutch relay.
6. Verify that a test lamp illuminates between the relay coil B+ circuit pin G3 and ground.

⇒	If the test lamp does not illuminate, test the B+ circuit pin G3 for a short to ground or an open/high resistance.

7. Verify that a test lamp illuminates between the relay switch B+ circuit pin G1 and ground.

⇒	If the test lamp does not illuminate, test the relay switch B+ circuit pin G1 for an open/high resistance. If the A/C fuse 5 is open, test the relay switch control circuit pin H3 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 pin B 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 pin G1 and the relay switch control circuit pin H3. 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 pin H3 and the relay coil B+ circuit pin G3.
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 PCM/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 PCM/ECM.

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

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.