GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2007 🢒 Lucerne 🢒 DTC 🢒 DTC P0645

Circuit Description

Ignition voltage is supplied directly to the A/C compressor clutch relay. The powertrain control module (PCM) controls the relay by grounding the control circuit through an internal solid state device called a driver. The primary function of the driver is to supply the ground for the component being controlled. Each driver has a fault line, which is monitored by the PCM. When the PCM is commanding a component ON, the voltage of the control circuit should be near 0 volts. When the PCM is commanding the control circuit to a component OFF, the voltage potential of the circuit should be near battery voltage.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0645 Air Conditioning Clutch Relay Control Circuit

Conditions for Running the DTC
    • The ignition voltage is between 9-18 volts.
    • The PCM driver is activated for at least 0.5 second.

Conditions for Setting the DTC

The PCM detects an improper voltage level on the output circuit that controls the A/C relay.

Action Taken When the DTC Sets
    • The PCM will not illuminate the malfunction indicator lamp (MIL).
    • The PCM stores the failure records.
    • The A/C compressor clutch is disabled.

Conditions for Clearing the DTC
    • The DTC will become history if the PCM no longer detects a failure.
    • The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
    • The DTC can be cleared with a scan tool.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. Listen for an audible click when the A/C compressor clutch relay operates. Command both the ON and OFF states. Repeat the commands, as necessary.

    2.

  2. This step tests for voltage at the coil side of the A/C compressor clutch relay. The 10-amp fuse supplies power to the coil side of the A/C compressor clutch relay.

    3.

  3. This step verifies that the PCM is providing ground to the A/C compressor clutch relay.

    4.

  4. This step tests if ground is constantly being applied to the A/C compressor clutch relay.

    5.

  5. This step tests for a short to voltage or an open.

    6.

  6. Perform the programming procedure for the PCM.

Step

Action

Yes

No

Schematic Reference: HVAC Schematics

Connector End View Reference: HVAC Connector End Views

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

Important: The ambient air temperature must be above 3°C (38°F).

  1. Turn OFF the ignition.
  2. Inspect the A/C compressor for free rotation operation.
  3. Start the engine.
  4. Place the HVAC control module in the OFF position.

Does the A/C compressor operate?

Go to HVAC Compressor Clutch Does Not Disengage

Go to Step 3

3

  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. With a scan tool, command the A/C Relay ON and OFF in the Special Functions, Engine Output Controls list.

Does the A/C Relay turn ON and OFF with each command?

Go to Testing for Intermittent Conditions and Poor Connections

Go to Step 4

4

  1. Turn OFF the ignition.
  2. Disconnect the A/C compressor clutch relay.
  3. Turn ON the ignition, with the engine OFF.
  4. Probe the ignition 3 voltage circuit of the A/C compressor clutch relay with a test lamp that is connected to a good ground.

Does the test lamp illuminate?

Go to Step 5

Go to Step 11

5

  1. Connect a test lamp between the control circuit of the A/C compressor clutch relay and the ignition 3 voltage circuit of the A/C compressor clutch relay.
  2. With a scan tool, command the A/C Relay ON and OFF .

Does the test lamp turn ON and OFF with each command?

Go to Step 9

Go to Step 6

6

Does the test lamp remain illuminated with each command?

Go to Step 8

Go to Step 7

7

Test the control circuit of the A/C compressor clutch relay for a short to voltage or an open. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 14

Go to Step 10

8

Test the control circuit of the A/C compressor clutch relay for a short to ground. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 14

Go to Step 10

9

Inspect for poor connections at the A/C compressor clutch relay. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Step 14

Go to Step 12

10

Inspect for poor connections at the harness connector of the powertrain control module (PCM). Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Step 14

Go to Step 13

11

Repair the ignition 3 voltage circuit of the A/C compressor clutch relay. Refer to Wiring Repairs .

Did you complete the repair?

Go to Step 14

--

12

Replace the A/C compressor clutch relay. Refer to Relay Replacement .

Did you complete the replacement?

Go to Step 14

--

13

Replace the PCM. Refer to Control Module References for replacement, setup, and programming.

Did you complete the replacement?

Go to Step 14

--

14
  1. Use the scan tool in order to clear the DTCs .
  2. Operate the vehicle within the Conditions for Running the DTC, as specified in the supporting text.

Does the DTC reset?

Go to Step 2

System OK