Troubleshooting Trailer Interior Lights That Won't Work While Exterior Lights Do Work

Question:

2015 f350 dually. All interior lights wont work in trailer. Exterior lights work fine on trailer. Suggestions?

asked by: Helen G

Expert Reply:

The exterior lights on your trailer - stop, turn and tail - and the interior lights will usually be fed by different power sources. On an RV-style 7-way the stop/turn light signals are carried on the pins at the 3-o-clock and 9-o-clock positions and the tail/clearance light signals are carried on the pin at the 11-o-clock position.

The +12V auxiliary power circuit on the 7-way that would normally feed a trickle charge to maintain a battery mounted on the trailer is carried on the pin at the 1-o-clock position. Please refer to the linked photo that shows each circuit on a typical 7-way socket.

The issue with your trailer's interior lights depends on how they are wired. Is there a battery on the trailer that powers the interior lights? Or does the trailer have no battery and instead pull its power for interior lights from the 7-way circuit that is fed by the vehicle battery?

I suggest you find the point where all of the interior lights' wiring joins the trailer's, either at a trailer-mounted battery or to the 7-way's +12V auxiliary circuit. Your trailer could also have a fuse/power distribution box that might have a blown fuse for the interior lights.

You could also simply have a case of corrosion on/in the connectors that is preventing the +12V auxiliary power from reaching the trailer.

expert reply by: Adam R