Trailer Left Turn and Tail Light Not Working on a Trailer Towed by a 2006 Ford F-150

Published 10/20/2011

Question:

I own a 2006 F150 and all of the truck lights work correctly. When I plug in 4pin into truck or into 4pin adapter for 7pin into truck the LH turn and blinker will not work. So I hooked a direct ground from truck ground and I got LH tail to work. Still can not get turn to work. I took light off trailer and tested on battery, everything great. Now I rewired trialer with solider connections and still will not work. I checked 4pin and 7 pin on truck. I get no voltage from truck on LH turn. Now what???

asked by: Anthony I

Expert Reply:

What I think may be happening is there is the absence of a relay that will not allow certain functions to work on the trailer lights if the load on the system is great enough. I would consult the owners manual, find the location of any relays/fuses associated with the trailer wiring, and make sure they are in good working order or add the ones that may be missing.

Another issue could be a faulty ground somewhere since your ground test did get it to partially work. You will want to check the main ground on the trailer and make sure it is attached to a bare metal surface on the trailer frame. The trailer tail lights should also be attached to a clean bare metal surface.

Also make sure the truck and trailer side connectors are clean and free of corrosion. Corrosion can build up in the connectors and in the wiring behind the connectors. You can also check the main ground on the truck side trailer wiring to make sure it is properly attached.

A loose or broken wire that you cannot see may also be causing the problem. You would need to test the wiring a various points front to back on the truck and trailer to find a loose or broken wire.

expert reply by: Michael H