For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• | Strategy Based Diagnosis

• | Diagnostic Procedure Instructions

### Diagnostic Fault Information

Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

### Circuit/System Description

When the A/C switch is pressed, the HVAC control module 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) 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 CLUTCH relay. With the relay contacts closed, battery voltage is supplied to the A/C compressor clutch assembly.

### Diagnostic Aids

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

• | Battery voltage is between 9-16 volts.

• | Engine coolant temperature (ECT) is less than 123°C (253°F).

• | Engine speed is less than 4760 RPM.

• | Engine speed is more than 600 RPM.

• | A/C high side pressure is between 2929-269 kPa (425-39 psi).

• | Throttle position is less than 100 percent.

• | Evaporator temperature is greater than 0°C (32°F)

• | ECM does not detect excessive torque load

• | ECM does not detect insufficient idle quality.

### Reference Information

#### Schematic Reference

HVAC Schematics

#### Connector End View Reference

HVAC Connector End Views

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### Scan Tool Reference

• | Scan Tool Data List

• | Scan Tool Data Definitions

• | Scan Tool Output Controls

### Circuit/System Verification

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.

### Circuit/System Testing

1. Ignition OFF, disconnect the harness connector C2 ground circuit terminal at the HVAC control module.
2. Ignition OFF, test for less than 1.0 ohm of resistance between the ground circuit terminal 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 a/c request signal circuit terminal and ground. With a scan tool, verify the A/C Request Switch parameter is ON.

⇒ | If not On, test the a/c request 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 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 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.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Air Conditioning Compressor Replacement

• | Control Module References for BCM, HVAC and ECM replacement, setup and programming.