Test Av: Air Conditioning Relay Control Circuit: Testing

WARNING: This page is about a different car, the 2003 Ford Pickup, 2003 Ford F550 Super Duty, 2003 Ford F450 Super Duty, 2003 Ford Excursion, and 2003 Ford Cab & Chassis. However, it is still accessible from the selected car via links, so may be relevant.

CAUTION: The Powertrain Control Module (PCM) harness connectors must be properly seated and the connector latch properly attached to eliminate possible driveability concerns or a no-start condition. Installing PCM connectors on an angle may cause a bad connection, misdiagnosis and damaged components. Install the connector on where the lever pivots and seats itself. Apply light pressure to get the connector into position on the PCM lever to fully seat the connector.

NOTE: When the PCM engine harness connector is disconnected additional DTCs will be set. All DTCs must be cleared before returning the vehicle to service.

1) Preliminary Diagnosis For DTC P0645

Perform visual inspection. Connect the scan tool. Retrieve and record all DTCs. Clear all DTCs. Perform the KOEO ON-DEMAND SELF-TEST under QUICK TEST. Is DTC P0645 set? If yes, go to next step. If no, unable to duplicate condition. Check for loose connections, damaged or corroded pins. Wiggle the harness to attempt to recreate the fault. Clear all DTCs and return the vehicle to service.

2) DTC P0645

Possible causes are:
- A/C control circuit shorted to voltage.
- A/C control circuit open.
- A/C control circuit shorted to ground.
- Faulty A/C relay circuit.
- Faulty A/C relay.
- Faulty PCM.
Key on, engine off. Using scan tool, access ACCS PID. Select A/C control switch "OFF" and then "ON". Is ACCS PID indicating "OFF" when selected and "ON" when selected? If yes, go to next step. If no, see appropriate AUTOMATIC or MANUAL A/C-HEATER SYSTEMS article to diagnose the symptom (A/C request does not operate).

3) Check A/C Relay Circuit

Key off. Remove the A/C relay. Install a test light between auxiliary relay box A/C relay socket VPWR circuit terminal No. 2 and ground. See Figure. Key on. Does the test light illuminate? If yes, go to next step. If no, repair open in circuit. Restore the vehicle and retest the system.

4) Check A/C Control Circuit

Install a test light between auxiliary relay box A/C relay socket terminals No. 1 and 2. Start engine. Select A/C on and then off. Does the test light illuminate with A/C on and then off when A/C is turned off? If yes, install a new A/C relay. Restore the vehicle. Clear the DTCs and retest the system. If no, go to next step.

5) Check A/C Control Circuit For A Short To Voltage

Engine off, key off. Disconnect the PCM body harness connector C1381A. Key on. Measure the voltage between PCM body harness connector C1381A terminal No. 2 and ground. See CONTROL MODULE CONNECTOR IDENTIFICATION . Is the voltage less than 0.2 volt? If yes, go to next step. If no, repair short to voltage in the relay control circuit. Restore the vehicle. Clear the DTCs and RETEST the system.

6) Check A/C Relay Control Circuit For A Short To Ground

Measure the resistance between PCM body harness connector C1381A terminal No. 2 and ground. Is the resistance greater than 10,000 ohms? If yes, go to next step. If no, repair short to ground in relay control circuit. Restore the vehicle. Clear DTCs and retest the system.

7) Check A/C Relay Control Circuit For An Open

Measure the resistance between PCM body harness connector C1381A terminal No. 2 and auxiliary relay box A/C relay connector socket terminal No. 1. Is the resistance less than 0.5 ohm? If yes, install a new PCM. Restore the vehicle and retest the system. If no, repair open on circuit. Restore the vehicle and retest the system.