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: A/C Clutch Relay Control Circuit

### Diagnostic Fault Information

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

### Circuit System Description

When the A/C switch is pressed, the HVAC control module sends a serial data message to the ECM. This input will request the engine control module (ECM) to ground the A/C compressor clutch relay control circuit, which will switch the A/C CMPRSR CLUTCH relay on. 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 greater than 600 RPM.

• | An A/C request is made.

### Conditions for Setting the DTCs

The ECM detects an open on the control circuit of the A/C compressor clutch relay when commanded OFF with the engine in crank or run status.

### Action Taken When the DTC Sets

The ECM will store conditions which were present when the DTC set as Failure Records data only. This information will not be stored as Freeze Frame data.

### Conditions for Clearing the DTC

A History DTC clears after 40 consecutive warm-up cycles have occurred without a malfunction.

### 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 greater than 600 RPM.

• | Engine speed is less than 4,760 RPM.

• | A/C high side pressure is between 269-2929 kPa (39-425 psi)--Refer to Air Conditioning (A/C) System Performance Test .

• | 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.

• | The ambient temperature is above 1°C (34°F).

### 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

Engine running place the mode switch in the defrost position. The A/C compressor clutch should engage.

⇒ | If the clutch does not engage refer to Air Conditioning (A/C) System Performance Test

### 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, test all components connected to the ignition circuit and replace as necessary.

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 Clutch Relay.

5. Disconnect the harness connector at the A/C clutch coil.
6. Test for less than 1 ohm between the ground circuit terminal 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 clutch relay.

### Component Testing

#### Relay Test

1. Ignition OFF, disconnect the A/C relay.
2. Test for 60-200 ohms between terminal 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 15A 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.

• | Air Conditioning Compressor Replacement

• | Relay Replacement

• | Compressor Clutch Assembly Replacement

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