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 Compressor Clutch Relay Coil Ignition | P0645, P0646 | P0645, P0647 | -- | --
A/C Compressor Clutch Relay Switch B+ | 1 | 1 | -- | --
A/C Compressor Clutch Relay Coil Control | P0645, P0646 | P0645, P0647 | P0645, P0647 | 3
A/C Compressor Control | 1 | 1 | 2 | --
A/C Compressor Ground | 1 | 1 | -- | --
1. A/C Compressor Inoperative 2. A/C Compressor Always ON 3. A/C Compressor Malfunction

### Circuit/System Description

When the A/C switch is pressed, the HVAC control module sends an A/C request message to the engine control module (ECM) via CAN-Bus. Therefore the ECM grounds the A/C compressor clutch relay control circuit, which will switch the A/C compressor clutch relay. With the relay contacts closed, battery voltage is supplied to the A/C compressor clutch. The A/C compressor clutch will be activated.

### Diagnostic Aids

The following conditions must be met in order for the ECM to turn on the A/C compressor clutch:

• | Battery voltage is between 9-18 V.

• | Engine coolant temperature is less than 124°C (255°F).

• | Engine speed is greater than 600 RPM.

• | Engine speed is less than 5 500 RPM.

• | A/C high side pressure is between 269-2 929 kPa (39-425 PSI).

• | Throttle position is less than 100 percent.

• | Evaporator temperature is greater than 3°C (38°F).

• | ECM does not detect immoderate 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

Manual HVAC 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

Note: Before executing Circuit/System Testing, all conditions shown in Diagnostic Aids must be met.

1. Ignition ON, activate and deactivate the A/C switch. Verify the scan tool A/C Switch parameter changes between Active and Inactive.

⇒ | If not the specified value, replace the A26 HVAC Control.

2. Activate and deactivate the A/C switch. Verify the engine control module (ECM) scan tool A/C Request Signal parameter changes between Active and Inactive.

⇒ | If not the specified value, replace the K33 HVAC Control Module.

3. Ignition OFF, disconnect the KR29 A/C Compressor Clutch Relay.
4. Ignition ON, 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.

5. 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.

6. Ignition OFF, disconnect the harness connector at the Q2 A/C Compressor Clutch.
7. Test for less than 5 Ω between the Q2 A/C Compressor Clutch ground circuit terminal A and ground.

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

8. Connect the harness connector at the Q2 A/C Compressor Clutch.
9. Ignition ON, connect a 10 A fused jumper wire between the B+ circuit terminal 30 and the control circuit terminal 87. Verify the Q2 A/C Compressor Clutch is activated.

⇒ | If the Q2 A/C Compressor Clutch does not activate, test the control circuit for an open/high resistance. If the circuit test normal, replace the Q2 A/C Compressor Clutch.

10. Connect a test lamp between the ignition circuit terminal 85 and the control circuit terminal 86.
11. Command the engine control module (ECM) A/C Relay output function ON and OFF with the scan tool. The test lamp should turn ON and OFF when changing between the commanded states.

⇒ | If the test lamp always illuminates or always does not illuminate, replace the K20 Engine Control Module.

12. If all circuits test normal, replace the KR29 A/C Compressor Clutch Relay.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Air Conditioning Compressor Replacement

• | Heater and Air Conditioning Control Replacement

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