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 Compressor Clutch Relay Control | P0645 | P0645 | P0645 | --
A/C Relay Coil Side Supply Voltage Circuit | -- | P0645 | -- | --

### Circuit/System Description

Ignition voltage is supplied directly to the A/C compressor clutch relay. The engine control module (ECM) controls the relay by grounding the A/C clutch relay control circuit via an internal solid state device called a driver. The primary function of the driver is to supply the ground for the component being controlled. The driver has a fault line which is monitored by the ECM. When the ECM is commanding a component ON, the voltage of the control circuit should be near 0 volts. When the ECM is commanding the control circuit to a component OFF, the voltage potential of the circuit should be near battery voltage. If the ECM senses a voltage other than what is expected, a DTC will set.

### Conditions for Running the DTC

• | The ignition voltage is between 11.0-18.0 volts.

• | The engine speed is less than 600 RPM.

• | The ECM A/C Compressor Clutch Relay Control driver transitions from ON to OFF or from OFF to ON.

### 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.

• | The DTC can be cleared by using a scan tool.

### Reference Information

#### Schematic Reference

HVAC Schematics

#### Connector End View Reference

HVAC 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

• | Scan Tool Output Controls

• | Scan Tool Data List

### Circuit/System Verification

1. Ignition ON with the engine OFF, command the A/C Relay ON and OFF with a scan tool.
2. Feel/listen for the A/C relay to click. Observe the scan tool A/C Relay Circuit Status function or A/C relay solenoid turns on and off clicks.

### Circuit/System Testing

The A/C clutch relay terminal numbers called out per the following testing were verified on an actual vehicles underhood fuseblock and A/C clutch relay.

1. Ignition OFF, disconnect the A/C Relay.
2. Ignition ON, verify that a test lamp does not illuminate between the control circuit 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, relay coil feed circuit 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 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 of resistance between the A/C Clutch Coil ground circuit terminal ground.

⇒ | If greater than 1 ohm, test the ground circuit for an open/high resistance.

7. Connect the harness connector at the A/C Clutch Coil.
8. Connect a 3-amp fused jumper wire between the B+ circuit and control circuit terminal of the A/C Clutch Relay. Verify the A/C Clutch Relay is activated.

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

9. Connect a test lamp between the coil side of the A/C Clutch Relay ignition circuit and the control circuit terminal.
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, replace the A/C Clutch Relay.

### Component Testing

#### Relay Testing

1. Ignition OFF, disconnect the A/C Relay.
2. Test for 137 ohms across the relay coil terminals.

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

3. Test for infinite resistance between switch side of the relay

⇒ | If less than infinite resistance, replace the relay

4. Install a 20-amp fused jumper wire between the relay coil side and 12 volts. Install a jumper wire between relay the coil side and ground. Measure for less than 2 ohms of resistance between the relay switch side.

⇒ | If greater than 2 ohms, replace the relay.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

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

• | Relay Replacement