My older brother runs his own contracting company, and primarily uses his 05 F250 to pull his 25 foot enclosed trailer around along with skid loaders etc. The interior lighting for the trailer is run using the constant 12v power terminal of the 7 pin RV plug that his truck came with. This terminal has no power switched or constant. All the fuses are good, and the connector itself is in good shape, I also chased the wire back to the cab and it was fine. Looking over the wiring diagram in his chilton its 12v source is a trailer/tow relay, I am unable, however, to locate this relay in either fuse panel. Using the diagrams in the chilton and the owners manual proved useless, also switching the horn relay with all the others did not fix it or locate any bad relays. Anyone have any ideas what the heck is going on?

 

It probably is a fuse, actually I think there is a couple that go to it. I used a little light bulb tester back there are just started checking fuses under the dash and under the hood.........took about 20 minutes.

 

The fuse an relay are under the hood on the drivers side.
the owners manual shows the location.
we've had them blow when just hooking up a trailer with defective wiring .

Rich

 

The wiring on the trailer is a possibility, but all the fuses were tested, I found one bad one and replaced it but still no powah. The relays were also tested by switching them all out with a known good one.

 

This terminal is only "hot" when the engine is running.

Soooooo, is it running?

Pop

 

Tried that too. There is no way for that terminal to only be hot when the engine is running however, as long as the key is in the on position, the engine might as well be running.

 

there are two fuses.. one controls the relay itself and one that protects the wire to the trailer.. if you pull the relay do you get power at the two terminals?

inside the cab.

107 20A** Trailer tow battery charge

301 Full ISO relay Trailer tow battery charge

I don't see any labeling for the fuse to control the relay itself, but there must be one.. anyhow with the key on, there should be power at 2 of the relay terminals.. power off, only one.

sam