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P0645



DTC P0645

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle (Initial Inspection and Diagnostic Overview) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Initial Inspection and Diagnostic Overview) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Initial Inspection and Diagnostic Overview) provides an overview of each diagnostic category.



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




Diagnostic Fault Information






Circuit/System Description

Ignition voltage is supplied directly to the A/C compressor clutch relay. The engine control module (ECM) controls the relay by grounding the A/C clutch relay 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. The 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 ECM senses a voltage other than what is expected, a DTC will set.


Conditions for Running the DTC

* The ignition voltage is between 11.0-18.0 volts.
* The engine speed is less than 600 RPM.
* The ECM A/C compressor clutch relay control driver transitions from ON to OFF or from OFF to ON.



Conditions for Setting the DTCs

The ECM detects an open on the control circuit of the A/C compressor clutch relay when commanded off with the engine in crank or run status.


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 (Electrical Diagrams)
Connector End View Reference

Component Connector End Views (Connector Views)
Description and Operation

* Air Delivery Description and Operation (Air Delivery Description and Operation)
* Air Temperature Description and Operation (Air Temperature Description and Operation)

Electrical Information Reference

* Circuit Testing (Component Tests and General Diagnostics)
* Connector Repairs (Component Tests and General Diagnostics)
* Testing for Intermittent Conditions and Poor Connections (Component Tests and General Diagnostics)
* Wiring Repairs (Component Tests and General Diagnostics)

Scan Tool Reference

Control Module References (Programming and Relearning) for scan tool information


Circuit/System Verification

1. Ignition ON with the engine OFF, command the A/C Relay ON and OFF with a scan tool.
2. Feel/listen for the A/C relay to click. Observe the scan tool A/C Relay Circuit Status function turns on and off.



Circuit/System Testing

1. Ignition OFF, disconnect the A/C CLTCH relay.
2. Ignition ON, verify that a test lamp does not illuminate between the relay coil control circuit terminal 86 and ground.

• If the test lamp illuminates, test the control circuit for a short to voltage.

3. Verify that a test lamp illuminates between the relay coil B+ circuit terminal 85 and ground.

• If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal and ignition circuit fuse is open, test or replace the relay.

4. Connect a test lamp between the relay coil B+ circuit terminal 85 and the relay coil control circuit terminal 86.
5. Command the A/C relay ON and OFF with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.

• 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.

6. If all circuits test normal, test or replace the A/C CLTCH relay.



Component Testing

1. Ignition OFF, disconnect the A/C CLTCH relay.
2. Test for 60-180 ohms of resistance between terminals 85 and 86.

• If the resistance is not within the specified range, replace the relay.

3. Test for infinite resistance between the following terminals:

* 30 and 86
* 30 and 87
* 30 and 85
* 85 and 87


• If not the specified value, replace the relay.

4. Install a 30-amp fused jumper wire between relay terminal 85 and 12 volts. Install a jumper wire between relay terminal 86 and ground. Test for less than 2 ohms of resistance between terminals 30 and 87.

• If greater than the specified range, replace the relay.




Repair Instructions

Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.

* Relay Replacement (Attached to Wire Harness) (Relay Replacement (Attached to Wire Harness))Relay Replacement (Within an Electrical Center) (Relay Replacement (Within an Electrical Center))
* Heater and Air Conditioning Control Replacement (Service and Repair)
* Control Module References (Programming and Relearning) for ECM replacement, setup, and programming


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