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
DTC P0646: Air Conditioning (A/C) Clutch Relay Control Circuit Low Voltage
DTC P0647: Air Conditioning (A/C) Clutch Relay Control Circuit High Voltage

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
A/C Compressor Clutch Relay Control | P0645 P0646 | P0645 P0646 P0647 | P0645 | --
A/C Relay coil side supply voltage circuit | 1 | 1 | -- | --
A/C Relay coil side ground circuit | -- | 1 | -- | --
Ignition 1 circuit | 1 | 1 | -- | --
B+ circuit | 1 | 1 | -- | --
1. Circuit Inoperative

### 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 fault detection circuit 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 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. Install a scan tool.
2. Ignition ON with the engine OFF, command the A/C Relay ON and OFF in the ECM Special Functions scan tool data list.
3. 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

1. Ignition OFF, disconnect the A/C Relay.
2. Ignition ON, verify that a test lamp does not illuminate between the controlled output circuit, and ground.

⇒ | If the test lamp illuminates, test the releay controlled output circuit for a short to voltage.

3. Verify that a test lamp illuminates between the relay ignition 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 or replace the A/C Compressor Clutch Relay.

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

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

⇒ | If the test lamp does not illumminate, test the relay switch ignition circuit for an open/high resistance.

5. Disconnect the harness connector at the A/C Compressor Clutch Coil.
6. Test for less than 1 ohm of resistance between the A/C Compressor Clutch Coil terminal B ground circuit terminal and ground.

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

7. Connect the harness connector at the A/C Compressor Clutch Coil.
8. Connect a 3-amp fused jumper wire between the relay switch B+ circuit and the relay controlled output circuit. Verify the A/C Clutch Relay is activated.

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

9. Connect a test lamp between the relay coil ignition circuit, and the relay coil control circuit.
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 Compressor Clutch Relay.

### Component Testing

#### Relay Testing

1. Ignition OFF, disconnect the A/C Compressor Clutch Relay.
2. Test for 137 ohms across the relay coil B+ and relay coil output controlled circuits.

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

3. Test for infinite resistance between the following terminals:

⇒ | relay switch B+ and the relay switch controlled output.

⇒ | If less than infinite resistance, replace the relay

4. Install a 20-amp fused jumper wire between relay coil B+ and 12 volts. Install a jumper wire between relay coil control and ground. Measure for less than 2 ohms of resistance between the relay switch B+circuit and relay switch controlled output circuit.

⇒ | 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 setup, replacement, and programming.

• | Relay Replacement