Troubleshooting Trailer Connector On 2005 Ford F-150 With Factory Trailer Harness

Updated 03/14/2016 | Published 03/10/2016

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Question:

05 f150 with combo 4 and 7 pin connectors. Have not been able to check 7 pin but 4 pin is giving dim turn signals and bright running lights. And apparently no brake or 4 way flashers. Ford installed. Bad wiring job on vehicle? Trailer harness And lights good on other 4 pin vehicles. Any ideas appreciated. Bought vehicle used with harness already on vehicle. I am thinking truck wiring at tail light has crossed running lights with brake,turn signal. Thank you.

asked by: Tim B

Expert Reply:

The best way to check the harness on your 2005 F-150 out is to use a circuit tester like part # 3808.

Have someone sit in the truck and run through the functions while you check the pins on the 4-way and 7-way pins. Then do the same and check the wires leading into the back of the harness to see if you are getting signal there. Keep working your way forward to find where the culprit is. You may find there is corrosion or a wire may be pinched or shorted out.

If you find the connector you have is the problem we do offer a replacement connector, part # HM40975. This simply plugs into the vehicle wiring behind the rear bumper by the spare tire and gives you a 4-way and a 7-way trailer connector.

When working with electrical connections I recommend using dielectric grease like part # 11755. This keeps dirt and moisture out of the electrical connections and helps prevent corrosion.

expert reply by: Jeffrey L