Troubleshooting Trailer Wiring Issue on 2006 Ford F350

Published 01/29/2016

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Question:

When I haul my trailers with my 2006 F350, the right lights work perfectly but the left doesnt work at all. I have tested the lights with another truck and they work fine. It makes me think that there is either a fuse missing or a bad fuse. It also tells me to check my trailer when I try to use my integrated brake controller. I havent been able to find a fuse diagram or a fuse list. What fuses control the trailer lights and how can I test them?

asked by: Crosby R

Helpful Expert Reply:

First, use a circuit tester like # PTW2993 on your trailer connector as a helper activates each lighting function and operates the manual brake switch on the brake controller. This helps isolate the problem to the trailer or the tow vehicle. If you're not getting signal on the left turn signal pin, access the wiring harness feeding the trailer connector, and probe the wires until you determine which one feeds the left turn circuit. If you get signal on the wiring harness behind the connector, but not on the connector itself, the 7-way connector is damaged or corroded on the inside, not letting the signal trough to the trailer. If necessary, you can replace the connector with part # HM40975, which Ford is now using as the OEM connector on new F Series trucks.

The trailer turn signal fuses are in position # 9 in the fuse box under the hood. The relay that controls the trailer left turn signals is in position # 202 in the underhood fuse box.

Given the fact that you're having problems with two unrelated systems, the trailer turn signals and the brake controller output, I'm leaning toward the trailer connector being the problem.

expert reply by: Mike L