GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2008 🢒 Epica 🢒 DTC 🢒 DTC P0646 or P0647

Circuit Description

The air conditioning (A/C) compressor clutch relay is controlled by the engine control module (ECM). Ignition 1 voltage is supplied directly to the A/C compressor clutch relay. The 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, a DTC sets.

DTC Descriptors

This diagnostic procedure supports the following DTCs:

    • DTC P0646 Air Conditioning (A/C) Clutch Relay Control Circuit Low Voltage
    • DTC P0647 Air Conditioning (A/C) Clutch Relay Control Circuit High Voltage

Conditions for Running the DTC
    • The engine is running.
    • The ignition 1 voltage is more than 11 volts.
    • The A/C is requested ON.
    • The A/C compressor is running (P0647 only)
    • The DTC runs continuously once the above conditions are met.

Conditions for Setting the DTC

The ECM detects a low voltage on the A/C clutch relay control circuit for more than 25 seconds.

OR

The ECM detects a high voltage on the A/C clutch relay control circuit for more than 25 seconds.

Action Taken When the DTC Sets
    • The control module stores the DTC information into memory when the diagnostic runs and fails.
    • The malfunction indicator lamp (MIL) will not illuminate.
    • The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Failure Records.
    • The driver information center, if equipped, may display a message.

Conditions for Clearing the DTC
    • A current DTC Last Test Failed clears when the diagnostic runs and passes.
    • A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other non-emission related diagnostic.
    • Clear the DTC with a scan tool.

Diagnostic Aids
    • For HVAC system concerns, refer to HVAC Preliminary Checks .
    • If condition is intermittent refer to Testing for Intermittent Conditions and Poor Connections .
    • The following table illustrates the A/C clutch relay terminal locations in the underhood fuse block:

A/C Clutch Relay Terminal Identification

B+ - Switch

Control

Ignition 1 Voltage - Coil

A/C Compressor Clutch

Front of Vehicle

Test Description

The number below refers to the step number on the diagnostic table.

  1. Listen for a click when the A/C relay operates. When the A/C relay is commanded On with the scan tool, the ECM pulses the A/C relay ON and OFF several times. Repeat the command as necessary.

Step

Action

Yes

No

Schematic Reference: HVAC Schematics

Connector End View Reference: Component Connector End Views

1

Important: This diagnostic trouble code (DTC) will only set for a failure on the control side of the A/C Clutch Relay.

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

  1. Turn ON the ignition, with the engine OFF.
  2. Command the A/C Relay ON with a scan tool.

Does the A/C Relay turn ON and OFF?

Go to Step 3

Go to Step 4

3
  1. Observe the Freeze Frame/Failure Records for this DTC.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

Go to Step 4

Go to Diagnostic Aids

4

  1. Turn OFF the ignition.
  2. Disconnect the A/C clutch relay.
  3. Turn ON the ignition, with the engine OFF.
  4. Probe the ignition 1 voltage circuit of the A/C 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 9

5

  1. Connect a test lamp between the control circuit of the A/C clutch relay and the battery positive voltage circuit of the A/C clutch relay.
  2. Start the engine.
  3. Command the A/C Relay ON and OFF with a scan tool.

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

Go to Step 10

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 A/C clutch relay control circuit for the following:

    • An open
    • A high resistance
    • A short to voltage

Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 14

Go to Step 11

8

Test the A/C clutch relay control circuit 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 11

9
  1. Repair the open or the short to ground in the ignition 1 voltage circuit of the A/C clutch relay. Refer to Wiring Repairs .
  2. Replace the fuse as necessary.

Did you complete the repair?

Go to Step 14

--

10

Test for an intermittent and for a poor connection at the A/C 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

11

Test for an intermittent and for a poor connection at 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 14

Go to Step 13

12

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

Did you complete the replacement?

Go to Step 14

--

13

Replace the ECM.

Did you complete the replacement?

Go to Step 14

--

14
  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate he vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

Go to Step 2

Go to Step 15

15

Observe the capture info with a scan tool.

Are there any DTCs that have not been diagnosed?

Go to Diagnostic Trouble Code (DTC) List - Vehicle

System OK