It's factory tow wiring built into the harness.

 

The brown wire is the running lights. The yellow and the green are the turn/stop lights. The white is the ground.

I don't have a schematic of a 97 f150. But go to the fuse box up under the dash and also under the hood, and see if you can find some relays. See if you can find a trailer marker lights relay. You may or may not also find some trailer turn relays in another box under the hood. If you find any of these relays, pull them out and then do your voltage checks again.

 

There are no relays labeled in the manual for trailer or trailer lilghts. Should there be?

 

I have diagrams for a 96 and a 99. The problem, is they changed these trailer tow circuits a lot, almost every year, so I am afraid to use one of those diagrams to troubleshoot your truck.

But through the years, they always seemed to use a relay for the tow marker lights. I did look on Autozone's sight, and their diagrams are not very complete for your year, but they do show a marker light relay for towing.

Are you positive your wiring in the rear is hooked to some official trailer tow wiring? You should have at least one relay somewhere for it I would think.

Maybe someone has some info on a 97 and can write in. My goal for you to take the relays out, was to kill all power to the rear trailer tow wiring, and see if you still had voltage on the wires with no lights on. If you did still have voltage readings like you do now, you have a serious wiring problem with the tow wires, and this is not a normal failure for these circuits.

 

Still found no relay, but I'l try the same thing with the fuses. Maybe it will show something...

 

OK, I'm with you it has to be a relay or two causing this. Problem is no relays are marked for trailer in the owner's manual. There is a small relay box in the engine compartment that is unmarked, and not referenced in the owner's manual. It must be one or ore of those. If I could find the function of those relays I think this will be simple fix. Any ideas how to obtain that information from Ford or this forum? If not, I coudl just start pulling them. Thanks again.

 

1997 & 1998 Trailer Relays and Fuses

 

Yes I had just discovered it. The expedition is a little different but I think we can assume it will be the big relay. I'll test it tonight. I might have killed it in hopes that it would provide enough power to lift our hi lo camper when the battery was dead. I hear the hydraulic takes about 100 amps. Oops.

 

Thanks projectSHO89. That's the exact info I need that wasn't in my owner's manual. I'll think I'll be able to troubleshoot it with that info.

 

Update,

I found the relay. It is in a box that the manual doesn't reference, adjacent to the power distribution box under the hood. Picture below. This is a 1997 Expedition, and it looks different than the F-150. The charge relay is the big one. I validated that because I received no voltage out of the wiring harness when I removed it.

I then substituted the relay with an equivalent relay from the other box. Unfortunately while I received voltage this did not solve the problem. So I'm going to run on the theory that this is some sort of short or bad connection in the hitch harness. My next steps are to either replace the hitch harness or run a bypass direct from the batt. I'm leaning towards the bypass.

 

Thanks Franklin2 and projectSHO89. Problem was three bad relays. Surprised they cost almost $14 each, but working now!