I have a 2005 F-150 and am experiencing what I believe to be a bad ground on the 7 pin trailer plug. I have ruled out the trailer as a possible issue.

When I first hook up to my trailer none of the light on the trailer work until I go over some bumps in the road. It appears that the ball is my only ground and until the hitch and ball “seat” I have no trailer lights. Then everything works fine until the next time that I disconnect.

I’m going to crawl under the truck to take a peak but want to know if there is a ground wire (white) that comes off of the trailer harness on the truck and connects to the frame that may require cleaning?
Does anyone know the approximate location of this ground if there is one?
Thanks

 

The 7 pin definitely feeds a ground, yes

Here's just one diagram with 2 seconds of Google
https://www.ajtnt.com/Info/7-Way-Diagram

Probe the ground at your truck and see if it works while testing one of the other pins, too....signals or tail would be something you could leave on

Why is it not a possibility that the trailer ground wire is poor and it's THE TRAILER eventually grounding thru the ball?

I'm always suspect of trailer wiring, and I go thru and eliminate any ScotchLoks, the worst connector ever

 

When I test the trailer lights with a 12V source they work and when I hook them up to my neighbors truck they work. I wasn't grounding through the ball in either scenario.

Thanks for the diagram, but I'm looking for where the wiring harness for the trailer plug grounds to the truck.

 

Your problem is in the trailer harness connection. The 7 way plug and the harness attached get filled with moisture.

 

That's a good lead, thanks. Is your advise to change out the 7 way plug on the truck and the harness (3' section of "plug & play") at the rear of the truck?

 

You can cut the tape covering the wires and inspect the wire ends and terminals on the back and inside the plug.... but on a 14 year old truck, a new plug is not a bad idea..

 

In the past several years every error I've received is from poor connection at the male & female plug or dirt in the trailer connector (those slots are deep). All these problems would come & go with bumpy roads

Regardless, I'd start by testing all the pins on the truck. If they're all working ELECTRICALLY, then your problem is MECHANICAL (physical). Also the connections are kinda like a fancy spade connector and if the metal tabs spread (or collapse, depending upon truck or trailer) connectivity becomes weak or intermittent

OP I'm not saying you're wrong but I feel like basic diagnostics haven't been performed and you might be chasing your tail.

If you want an answer to your EXACT question, I don't know BUT I'd start by climbing under there and chasing the wires back. If one runs to a ring terminal on your frame, that's your answer (I'd do this on my '07 but it's parked in the snow and I didn't have time to pull it into the shop today).

Failing that, I'd just Google for a wiring diagram and find the factory color for their ground (historically Ford stays close to industry standards but likes to add a stripe of their own choice). But then I'd probably backprobe that wire and look for continuity (and resistance) to the frame.

If you're POSITIVE you've got the right wire you can cut and strip and ground to the frame yourself, nothing wrong with redundant grounds. But I doubt the factory wiring will need to be hacked here

 

Thanks to everyone for your help. It turned out to be a loose and dirty ground of the trailer harness on the truck side. I cleaned the connection and installed a new bolt on the ground strap and the lights are working well.