Trailer Brake Lights/Turn Signals Not Working But Brake Lights Will Work when Truck Door Is Opened

Question:

I have a 2012 ford lariat, after I plugged in my towing trailer I had no brake lights or signal lights. When I open the drivers door the brake lights on the trailer came on bot no signal lights, what could the problem be? Could it be a fuse? what would be the number for the trailer fuse?

asked by: Earl I

Expert Reply:

So the brake lights start working when you open the door on the truck? That is a neat trick but not something you want your truck and trailer to do!

This sounds like a combination of things. It sounds like there is a short somewhere, possibly a bad connection between the truck and trailer side connectors, and possibly a ground issue as well.

The first step I recommend is to make sure the truck and trailer side connectors are as clean and free of corrosion as they can be. Corrosion can build up in the backs of the connectors where the wires attach as well as on the pins.

You can also inspect the wiring as much as you can on the truck and trailer and fix any pinched or damaged wires as needed.

On the trailer side make sure that all of the light grounds are attach to clean and corrosion free bare metal surfaces. A bad ground connection can cause all sorts of strange things to happen on a trailer. I have included some links that should help.

I do not believe it is a fuse because if it were the lights on the trailer would not work at all.

expert reply by: Michael H