For 1990-2009 cars

### Diagnostic Instructions

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

• | Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• | Diagnostic Procedure Instructions provides an overview of each diagnostic category.

### Circuit/System Description

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 serial data 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 5300 RPM.

• | Engine speed is more than 600 RPM.

• | A/C high side pressure is between 2951-310 kPa (428-44 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

Component Connector End Views

#### Description and Operation

• | Air Delivery Description and Operation

• | Air Temperature Description and Operation

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### Scan Tool Reference

Control Module References for scan tool information

### Circuit/System Verification

1. Engine running, 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 X3 at the HVAC control module.
2. Test for 558-12235 ohms between the signal circuit terminal C and the low reference circuit terminal A.

⇒ | If not within the specified range, test the signal circuit and the low reference for an open/high resistance. If the circuits test normal, test or replace the evaporator temperature sensor.

3. Ignition OFF, connect the harness connector X3. Disconnect the harness connector X2 at the HVAC control module.
4. Ignition OFF, test for less than 1.0 ohm between the ground circuit terminal L and ground.

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

5. Ignition ON, install a 3A fused jumper wire between the signal circuit terminal G and B+. Verify the scan tool A/C Request Switch parameter is ON.

⇒ | If not the specified value, test the signal circuit for an open/high resistance. If circuit tests normal, replace the BCM.

6. Connect the harness connector X2 at 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, replace the HVAC control module.

7. Disconnect the A/C compressor clutch relay.
8. Verify that a test lamp illuminates between the ignition circuit terminal 85 and ground.

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

9. Verify that a test lamp illuminates between the B+ circuit terminal 30 and ground.

⇒ | If the test lamp does not illuminate, test the 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.

10. Disconnect the harness connector at the A/C compressor clutch.
11. Ignition OFF, test for less than 1.0 ohm between the ground circuit terminal A and ground.

⇒ | If greater than the specified range, test the ground circuit for an open/high resistance.

12. Connect the harness connector at the A/C compressor clutch.
13. Ignition ON, connect a 10A fused jumper wire between the B+ circuit terminal 30 and the control circuit terminal 87. 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.

14. Connect a test lamp between the control circuit terminal 86 and the ignition circuit terminal 85.
15. 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 is always ON, test for a short to ground on the control circuit. If the circuit tests normal, replace the ECM.

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

16. If all circuits test normal, test or replace the A/C CLTCH relay.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Air Conditioning Compressor Replacement

• | Evaporator Temperature Sensor Replacement

• | Heater and Air Conditioning Control Replacement

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