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.

### DTC Descriptor

DTC P0645: Air Conditioning (A/C) Clutch Relay Control Circuit

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
A/C Relay Coil Ignition | 2 | 2 | -- | --
A/C Relay Switch B+ | 2 | 2 | -- | --
A/C 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 discrete signal 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 energize the A/C relay. With the relay contacts closed, battery voltage is supplied to the A/C compressor clutch assembly.

### Conditions for Running the DTC

• | The ignition voltage is between 11-18 volts.

• | The engine speed is more than 400 RPM.

• | An A/C request is made.

### Conditions for Setting the DTC

• | A short to ground on the A/C compressor clutch relay control circuit

• | A short to voltage on the A/C compressor clutch relay control circuit

• | An open circuit on the A/C compressor clutch relay control circuit or relay

• | An internally shorted or excessively low resistance A/C clutch relay coil

### Action Taken When the DTC Sets

The A/C relay is commanded OFF.

### Conditions for Clearing the DTC

The history DTC will clear after 40 fault-free ignition cycles.

### Reference Information

#### Schematic Reference

HVAC Schematics

#### Connector End View Reference

Component Connector End Views

#### 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 Testing

1. Ignition OFF, disconnect the A/C relay.
2. Ignition ON, verify that a test lamp does not illuminate between the control circuit terminal 86 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 a open/high resistance. If the circuit tests normal and ignition circuit fuse is open, replace the A/C relay.

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

⇒ | If the test lamp does not illuminate, test the B+ circuit for a short to ground or a open/high resistance. If the circuit tests normal and B+ circuit fuse is open, replace the A/C relay.

5. Disconnect the harness connector at the A/C clutch coil.
6. Test for less than 1 ohm between the ground circuit terminal 2 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 clutch coil.
8. Connect a 10-amp fused jumper wire between the B+ circuit terminal 30 and control circuit terminal 87. Verify the A/C clutch is activated.

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

9. Connect a test lamp between the ignition circuit terminal 85 and the control circuit terminal 86.
10. Command the A/C relay output function 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 relay.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Relay Replacement

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