Buick Regal: DTC P0645, P0646, or P0647

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 Descriptors

DTC P0645

Air Conditioning (A/C) Compressor Clutch Relay Control Circuit

DTC P0646

Air Conditioning (A/C) Compressor Clutch Relay Control Circuit Low Voltage

DTC P0647

Air Conditioning (A/C) Compressor Clutch Relay Control Circuit High Voltage

Diagnostic Fault Information

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 serial data. The ECM then grounds the A/C compressor clutch relay coil control circuit, closing the relay contacts. The closed relay contacts provide voltage to the compressor clutch causing it to engage.

Conditions for Running the DTC

• Engine is running.
• The HVAC control module is ON.
• The A/C switch is active.

Conditions for Setting the DTC

P0645 or P0646

The ECM detects a short to ground or an open at the A/C compressor clutch relay control circuit.

P0647

The ECM detects a short to voltage at the A/C compressor clutch relay control circuit.

Action Taken When the DTC Sets

P0645 or P0646

The A/C compressor is inoperative or always active. The A/C compressor solenoid is commanded to 0% displacement. This causes warm air from the ducts, even with unwanted clutch engagement.

P0647

The A/C compressor is inoperative.

Conditions for Clearing the DTC

The condition for setting the DTC is no longer present.

Reference Information

Schematic Reference

HVAC Wiring Schematics

Connector End View Reference

Component Connector End View Index

Description and Operation

Automatic HVAC Description and Operation

Electrical Information Reference

• Circuit Testing
• Connector Repairs
• Testing for Intermittent Conditions and Poor Connections
• Wiring Repairs

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Testing

1. Ignition OFF, disconnect the KR29 A/C Compressor Clutch Relay, ignition ON.

2. Verify a test lamp illuminates between the B+ circuit terminal 30 and ground.

• If the test lamp does not illuminate and the circuit fuse is good

1. Ignition OFF, remove the test lamp.
2. Test for less than 2 Ω in the B+ circuit end to end.
   • If 2 Ω or greater, repair the open/high resistance in the circuit.
   • If less than 2 Ω, verify the fuse is not open and there is voltage at the fuse.

• If the test lamp does not illuminate and the circuit fuse is open

1. Ignition OFF, remove the test lamp.
2. Test for infinite resistance between the B+ circuit and ground.
   • If less than infinite resistance, repair the short to ground on the circuit.
   • Go to next step: If infinite resistance
3. Disconnect the harness connector at the Q2 A/C Compressor Clutch.
4. Test for infinite resistance between the control circuit terminal 87 and ground.
   • If less than infinite resistance, repair the short to ground on the circuit.
   • If infinite resistance, test or replace the Q2 A/C Compressor Clutch.

• Go to next step: If the test lamp illuminates

3. Ignition ON.

4. Verify a test lamp illuminates between the ignition circuit terminal 86 and ground.

• If the test lamp does not illuminate

Refer to Power Mode Mismatch.

• Go to next step: If the test lamp illuminates

5. Connect a test lamp between the ignition circuit terminal 86 and the control circuit terminal 85.

6. Verify the test lamp turns ON and OFF when commanding the ECM A/C Compressor Clutch Relay On and Off with a scan tool.

• If the test lamp is always OFF

1. Ignition OFF, remove the test lamp and disconnect the harness connector at the K20 EngineControl Module, ignition ON.
2. Test for less than 1 V between the control circuit and ground.
   • If 1 V or greater, repair the short to voltage on the circuit.
   • Go to next step: If less than 1 V
3. Ignition OFF.
4. Test for less than 2 Ω in the control circuit end to end.
   • If 2 Ω or greater, repair the open/high resistance in the circuit.
   • If less than 2 Ω, replace the K20 Engine Control Module.

• If the test lamp is always ON

1. Ignition OFF, remove the test lamp and disconnect the harness connector at the K20 EngineControl Module.
2. Test for infinite resistance between the control circuit and ground.
   • If less than infinite resistance, repair the short to ground on the circuit.
   • If infinite resistance, replace the K20 Engine Control Module.

• Go to next step: If the test lamp turns ON and OFF

7. Verify a test lamp does not illuminate between the control circuit terminal 87 and ground.

• If the test lamp illuminates

Repair the short to voltage on the circuit.

• Go to next step: If the test lamp does not illuminate

8. Ignition OFF and all vehicle systems OFF, disconnect the harness connector at the Q2 A/C Compressor Clutch. It may take up to 2 min for all vehicle systems to power down.

9. Test for less than 10 Ω between the ground circuit terminal 1 and ground.

• If 10 Ω or greater

1. Ignition OFF.
2. Test for less than 2 Ω in the ground circuit end to end.
   1. If 2 Ω or greater, repair the open/high resistance in the circuit.
   2. If less than 2 Ω, repair the open/high resistance in the ground connection.

• Go to next step: If less than 10 Ω

10. Connect the harness connector at the Q2 A/C Compressor Clutch.

11. Ignition ON, connect a 10 A fused jumper wire between the B+ circuit terminal 30 and the control circuit terminal 87.

12. Verify the Q2 A/C Compressor Clutch is engaged.

• If the Q2 A/C Compressor Clutch does not engaged.

1. Ignition OFF, disconnect the harness connector at the Q2 A/C Compressor Clutch.
2. Test for less than 2 Ω in the control circuit end to end.
   • If 2 Ω or greater, repair the open/high resistance in the circuit.
   • If less than 2 Ω, test or replace the Q2 A/C Compressor Clutch.

• Go to next step: If the Q2 A/C Compressor Clutch is engaged

13. Test or replace the KR29 A/C Compressor Clutch Relay.

Component Testing

1. Ignition OFF, disconnect the KR29 A/C Compressor Clutch Relay.

2. Test for 60 - 180 Ω between terminals 85 and 86.

• If not between 60 - 180 Ω

Replace the KR29 A/C Compressor Clutch Relay.

• Go to next step: If between 60 - 180 Ω

3. Test for infinite resistance between the terminals listed below:

• 30 and 86
• 30 and 87
• 30 and 85
• 85 and 87

• If less than infinite resistance

Replace the KR29 A/C Compressor Clutch Relay.

• Go to next step: If infinite resistance

4. Install a 15 A fused jumper wire between terminal 86 and 12 V. Install a jumper wire between terminal 85 and ground.

5. Test for less than 2 Ω between terminals 30 and 87.

• If 2 Ω or greater

Replace the KR29 A/C Compressor Clutch Relay.

• Go to next step: If less than 2 Ω

6. All OK.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the repair.

• HVAC Component Replacement Reference
• Control Module References for engine control module replacement, programming and setup

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