Troubleshooting 6-way Trailer Connector on 1999 Ford F350 Flatbed Truck

Question:

We have a 99 Ford F-350 super duty truck with an after market flat bed and six pin wiring harness. Our tail lights on it and the trailer are not working. When taking voltage measurements, the center pin has 12 volts going to it when the headlights are turned on. Is this a normal reading or not? We have two other vehicles with a 6 pin connector and do not get a 12 volt reading on the center pin.

asked by: Jim A

Expert Reply:

There are actually two different accepted methods of wiring a 6-way trailer connector. The center pin on a 6-way trailer connector usually carries the 12-volt accessory circuit that is used for any trailer mounted accessories that might need a power feed, although sometimes it can be wired so it carries the output voltage from the brake controller to the trailer brakes.

If you take a look at the FAQ article, I have linked you to, you can see a wiring diagram showing how a 6-way should be wired, and make sure the 6-way on your truck is wired correctly. You can use a circuit tester like part # PTW2993 to verify which circuit controls which function.

As far as why the taillights on your F350 are not working, I would recommend checking for a blown fuse. The fuse box should have a legend on the lid showing which fuse protects which circuit. The owner's manual should have a fuse diagram as well. If the fuse checks out okay, you will need to trace the wiring feeding the taillights looking for a loose or broken wire. Since both taillights on the truck are not working, a blown fuse is the most likely cause.

expert reply by: Mike L