GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2006 🢒 XLR 🢒 DTC 🢒 DTC P0645, P0646, or P0647

Circuit Description

Ignition voltage is supplied directly to the A/C compressor clutch relay. The engine control module (ECM) controls the relay by grounding the 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. Each 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, this DTC will set.

DTC Descriptors

This diagnostic procedure supports the following DTCs:

    • DTC P0645 A/C Clutch Relay Control Circuit
    • DTC P0646 A/C Clutch Relay Control Circuit Low Voltage.
    • DTC P0647 A/C Clutch Relay Control Circuit High Voltage.

Conditions for Running the DTC
    • The ignition must be ON and the voltage must be between 9-18 volts.
    • The ECM driver is activated or deactivated.

Conditions for Setting the DTC
    • The ECM detects an improper voltage level on the output circuit that controls the A/C relay.
    • The condition is present for at least 5 seconds.

Action Taken When the DTC Sets
    • The ECM will not illuminate the malfunction indicator lamp (MIL).
    • The ECM will not store Freeze Frame or Failure Records.
    • The A/C compressor clutch is disabled for the ignition cycle.

Conditions for Clearing the DTC
    • The DTC will become history if the ECM no longer detects a failure.
    • The history DTC will clear after 40 fault-free ignition cycles.
    • 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. 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. Verifies that the engine control module is providing ground to the A/C compressor clutch relay. If light always on circuit shorted to ground.

    4.

  4. Tests for a short to voltage or an open.

    5.

  5. If the A/C fuse is open ensure to test the A/C compressor clutch supply voltage circuit for short to ground.

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

  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 HVAC Systems Automatic Special Functions, HVAC 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 3

3

  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 battery positive 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 4

Go to Step 8

4

  1. Connect a test lamp between the control circuit of the A/C compressor clutch relay and the battery positive 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 6

Go to Step 5

5

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

Did you find and correct the condition?

Go to Step 11

Go to Step 7

6

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 11

Go to Step 9

7

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

Did you find and correct the condition?

Go to Step 11

Go to Step 10

8

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

Did you complete the repair?

Go to Step 11

--

9

Replace the A/C compressor clutch relay.

Did you complete the replacement?

Go to Step 11

--

10

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

Did you complete the replacement?

Go to Step 11

--

11
  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