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.

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
A/C CLUTCH Relay Coil Ignition | 2 | 2 | -- | --
A/C CLUTCH Relay Switch B+ | 2 | 2 | -- | --
A/C CLUTCH Relay Control | P0645 | P0645 | P0645 | --
A/C Clutch Control | 2 | 2 | 1 | --
A/C Compressor Clutch Ground | -- | 2 | 2 | --
1. A/C Clutch Always Engaged 2. A/C Clutch Inoperative

### Circuit/System Description

When the A/C switch is pressed, the HVAC control assembly 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 CLTCH 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 9-18 volts

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

• | Engine speed is less than 4,760 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

Component Connector End Views

#### Description and Operation

• | Air Temperature Description and Operation

• | Air Delivery 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

Ignition ON, command the A/C relay ON and OFF with a scan tool. Observe the HVAC compressor operation, the A/C compressor clutch should engage and disengage when changing between the commanded states.

### Circuit/System Testing

1. Disconnect the A/C CLUTCH relay.
2. Ignition ON, verify that a test lamp does not illuminate between the control circuit terminal 87 and ground.

⇒ | If the test lamp illuminates, test the control circuit for a short to voltage.

3. 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. If the circuit tests normal and the ignition circuit fuse is open, test or replace the relay.

4. 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 a short to ground or an open/high resistance. If the circuit tests normal and the B+ circuit fuse is open, test the control circuit terminal for a short to ground.

5. Disconnect the harness connector at the A/C compressor clutch.
6. Test for less than 1.0 ohm between the A/C compressor clutch ground circuit terminal B and ground

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

7. Connect the harness connector at the A/C compressor clutch.
8. Connect a 10A fused jumper wire between the B+ circuit terminal 30 and the control circuit terminal 87. Verify the A/C compressor clutch is activated.

⇒ | 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 clutch.

9. Connect a test lamp between the ignition circuit terminal 85 and the control circuit terminal 86.
10. Command the A/C relay ON and OFF with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.

⇒ | If the test lamp is always ON, test the control circuit for a short to ground. If the circuit tests normal, replace the ECM.

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

11. If all circuits test normal, test or replace the A/C CLUTCH relay.

### Component Testing

#### Relay Test

1. Ignition OFF, disconnect the A/C CLUTCH relay.
2. Test for 60-180 ohms between terminals 85 and 86.

⇒ | If the resistance is not within the specified range, replace the relay.

3. Test for infinite resistance between the following terminals:

• | 30 and 86

• | 30 and 87

• | 30 and 85

• | 85 and 87

⇒ | If not the specified value, replace the relay.

4. Install a 30-amp fused jumper wire between relay terminal 85 and 12 volts. Install a jumper wire between relay terminal 86 and ground. Test for less than 2 ohms between terminals 30 and 87.

⇒ | If greater than the specified range, replace the relay.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Compressor Clutch Assembly Replacement

• | Relay Replacement

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