Troubleshooting 2002 F-250 Towing Horse Trailer with Working Exterior Lights but No Interior Lights

Question:

Hello! I have a 2002 F250 V10 with what appears to be a factory 7 way plug. When I plug my horse trailer in, a 2001 Trails West, the tail lights and running lights work fine, as do the electric brakes, but none of the interior lights or exterior loading lights work. They all worked when I had my old 1990 F250, and worked fine when my friend pulled it with her Dodge, so Im thinking its my truck. Any idea which fuses I should be looking at? I guy at work mentioned testing relays, but I have no clue where to even start there. I dont have wiring diagrams or fuse or relay diagrams and my google searches havent been too helpful. Any help is appreciated, thanks!! Oh, I was going to replace the breakaway battery in the trailer too, its the original so its long over due Im sure. Trails West owners manual says its a 12v gel cell battery, is that the same as the lead batteries you sell?

asked by: Jamie H

Expert Reply:

It sounds like the interior lights on your trailer are powered through the 12 volt circuit on your Ford's 7-way. So, start by testing to make sure that pin is active. On a standard RV style 7-way, this is located at the 1 o'clock position. I've attached a diagram for visual reference. Using a tester like # PK12741 would be the easiest.

You could also simply have a case of corrosion on/in the connectors that is preventing the +12V auxiliary power from reaching the trailer. If this is the case, # PK11893 is a suitable replacement.

The exterior lights (tail/turn/stop) are powered through different circuits, which explains why they function normally.

I took a look at an online version of your owner's manual which indicates there is a 20 amp fuse in the 115 location for the trailer tow battery charge as well as a trailer tow battery charge relay in location 305. Check both of these locations and make sure they are present and in good working condition, replace if needed.

I recommend the Curt Push-To-Test Trailer Breakaway Kit with Built-In Battery Charger # C52044, which includes a 12 volt gel cell battery. Or, to replace only the battery, you can use # HM20008, which is sealed and DOT approved and will work well with your existing kit.

I've attached a couple videos and a more comprehensive trailer wiring article for you at the bottom of the page as well.