GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2009 🢒 Lucerne 🢒 HVAC 🢒 HVAC - Automatic 🢒 DTC P0645

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 Relay Coil Ignition

2

2

--

--

A/C Relay Switch B+

2

2

--

--

A/C Relay Control

P0645

P0645

P0645

--

A/C Clutch Control

2

2

1

--

A/C Compressor Clutch Ground

--

2

2

--

1. A/C Clutch Always Engaged

2. A/C Clutch Inoperative

Circuit/System Description

When the A/C switch is pressed, the HVAC control assembly sends a serial data message to the engine control module (ECM) for an A/C request. This input will request the ECM to ground the A/C compressor clutch relay control circuit, which will switch the A/C CLTCH relay. 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.
    • The ECM driver transitions from ON to OFF or from OFF to ON.

Conditions for Setting the DTC
    • A short to ground on the A/C compressor clutch relay control circuit
    • A short to voltage on the A/C compressor clutch relay control circuit
    • An open circuit on the A/C compressor clutch relay control circuit or relay
    • An internally shorted or excessively low resistance A/C clutch relay coil

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

Component Connector End Views

Description and Operation

Automatic 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
  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.
    3. ⇒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.
    4. ⇒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.
    5. ⇒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 2 and ground.
    7. ⇒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.
    9. ⇒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.
    11. ⇒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.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Relay Replacement
    •  Control Module References for ECM replacement, setup, and programming