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Replacing the 7-Way/4-Pole Trailer Connector on 2010 F-350 Super Duty-King Ranch


I am looking for guidance on replacing the tow package wiring harness on my 2010 F-350 King Ranch Dually. When connected I have no running lights and Ive checked the fuses and they all seem to be intact. I did a light test on my 7-pin and I have no power except at the 12 oclock pin. I cant find any videos on YouTube or any information on a Google search to complete this repair. I also need help ing the correct wiring harness for that make and model vehicle.


Expert Reply:

Your F350 should have come with the factory tow package. You'll need to figure out if the connector you currently have is damaged, or if the problem might be further down the line. Follow the wiring for the existing trailer connector until you find the round US car connector. Unplug that connection. On the vehicle side of the connection, use a circuit tester like part # 40376 on the connector as a helper activates each taillight function. If you're getting signal for the running lamp, right and left turn and brake lamps, you've confirmed that the existing trailer connector is faulty and should be replaced. The # HM40975 would work great, it's the connector that Ford, Chevy and Ram are currently using as the OEM trailer connector. The US car connector will plug directly into the rear of the # HM40975.

If you aren't getting power on the running lamp circuit at the connector, check the 30 amp fuse in the # 42 location and the relay in the # 55 location in the power distribution box under the hood. Replace the fuse if it's blown. You can't tell by sight if a relay is bad, but you can swap it out for an adjacent relay, and see if that fixes the problem.

Getting the connector functional is only half the battle. If there's a blown fuse or a bad relay, chances are that there's a short in the trailer wiring that caused the fuse to blow. Since the problem centered on the running light circuit, that's where you'll want to check. Beginning at the trailer connector, trace the running light circuit (usually a brown wire) from the connector back to the trailer lamps. Look for pinched wires, loose connections or cracked or worn wire insulation that would allow the circuit to short out on the trailer frame. Repair the short as needed.

I'll link you to our trailer wiring and trailer wiring troubleshooting FAQ articles, which I hope you'll find helpful.

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expert reply by:
Mike L

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