Troubleshooting Trailer Running Light Circuit on Trailer Towed By 2010 Ford F150

Question:

Hello, my 2010 F150 trailer harness was working ok until a few weeks ago. my running lights seem to get no power on that leg of the harness. turn signals and break lights work ok and when I test trailer on another vehicle all lights work. my question is, is there a separate fuse for the running lights on this vehicle??

asked by: Dean

Expert Reply:

First, use a circuit tester like part # 40376 and test the running light pin on your 7-way, which is the pin in the 11:00 position. If you're not getting power on that pin, make sure that the connector isn't damaged or corroded. If it is, clean or replace as needed. If it needs to be replaced, I'd recommend part # HM40975.

If you're not getting power on the running light pin, and the connector is in good shape, you're most likely dealing with a bad relay or fuse located in the fuse box in the engine compartment. Check fuse number 31, and replace if needed. If the fuse is okay, relay number 6 could be bad. You can't look at a relay and tell its bad, but you can temporarily replace it using an adjacent relay to see if that fixes the problem. Number 3 or 9 would work , as they're the blower motor relay and the trailer tow battery charger, respectively. If you find the relay to be bad, you can obtain a replacement from your Ford dealer or a local auto parts store.

If you find a bad fuse, you'll want to find out what casued the fuse to blow before plugging your trailer back in. Closely examine the running lamp circuit, looking for pinched wires or wire with worn or cracked insulation. If you don't find and repair the short, you'll continue blowing fuses.

expert reply by: Mike L