For 1990-2009 cars

### 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 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 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.

### DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0645 Air Conditioning (A/C) Clutch Relay Control Circuit

### Conditions for Running the DTC

• | The ignition voltage is between 9-18 volts.

• | The ECM driver is activated for at least 0.5 second.

### Conditions for Setting the DTC

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

### Action Taken When the DTC Sets

• | The ECM will not illuminate the malfunction indicator lamp (MIL).

• | The ECM stores the failure records.

• | The A/C compressor clutch is disabled.

### Conditions for Clearing the DTC

• | The diagnostic trouble code (DTC) will become history if the ECM 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. This step tests for voltage at the coil side of the A/C compressor clutch relay. A 15-amp fuse supplies power to the coil side of the A/C compressor clutch relay.

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

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

5. Perform the programming procedure for the ECM.

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. Turn OFF the ignition.
2. Start the engine.
3. Place the blower motor switch in the OFF position.

Does the A/C compressor operate?

Go to Air Conditioning Compressor Malfunction

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 and 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 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
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. Did you complete the replacement? | Go to Step 14 | --
13 | Replace the ECM. 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