Hi all....

I have a 2017 F350 Plat 6.2

Bought a new trailer and guess what, the running lights are not working. Trailer was tested at the dealer...all good

I checked fuse 99 under the hood, was good but replaced anyway, still no good.

I put a multi-meter on the running light pin and get 3.4 ish volts, all others are ok.

The plug looks great, I'm in Ontario so the truck see's a bunch of salt. the second year I had it... I stuff dielectric grease from back and put some on the leads also. Maybe in one year salt screwed it up?

Maybe remove the plug for a better inspection.. any tips on that? or am I missing something.

 

Check the trailers plug. I have to bend the plugs on my camper to make better contact with the trucks plug in order for my running lights to work. My other lights work fine, it’s just the running lights I have an issue with from time to time. I’m assuming your other lights work on the trailer?

 

There's a plug up near the spare tire. Well known to get water intrusion and cause all kinds of weird stuff. I had it happen on a 2017. You may be able to clean and pack with dielectric grease. Mine was replaced under warranty by dealer.
For a few bucks, I also keep a trailer plug tester on hand.

 

Trailer side is fine. Its 100% truck side.

 

The connectors have been packed with dielectric grease after the second year of ownership. Trailer has never been hook up.... till this weekend, and found I had a problem.

I will have to go back and check for corrosion behind the 7 pin, and connectors I guess.

 

I’m not saying there’s anything wrong with the trailer. It’s the shape of the plugs when they mate up. Mine has a little slop specifically on the running light pin because of how it sits in the trucks connector. The trucks connectors are fixed so I put a little pressure on the trailers pin to make sure it has good contact with the truck.

do you have a 7 pin tester? If you have a test light, might be worth checking at the fuse to make sure you at least have power on both sides of the fuse.

 

Turn headlight switch to on instead of auto.
Trailer running lights are only active when headlights are on.

 

My lights are always on "auto". It seems that the fender marker lights are always on, even in broad daylight with headlights off. I wonder if they're part of DRL? Yet the tail lights are not. No complaint, just pointing out how mine seems to work. Last couple of trucks have been that way also.
But I agree, the tails usually are not on for running lights unless the headlights/parking lights are on.

 

If your trying to get running lights during daylight that is your issue. Plug in, turn light switch to on and I bet you have running lights. Just like those of us that have backup cameras on the trailer the light switch has to be turned to on for the camera to work. I used forscan to change my setting to use lo beams as DRLs and now my running lights work on auto. They didn't before the change.

 

The light switch has been tried in the on position.... I never bother with auto headlights.

The truck side 7 pin plug has been tested and shows 3.4 volts on running lights, all other pins are good. the running light pin is not the problem.

3.4 volt coming from the running lights is not right. I'm going to test the fuse box power to fuse 99 soon and then if that's good, work my way to the back...yes the fuse is good, already checked number 99

 

Sounds like you're closing in on it. Trying to diagnose electrical is not my favorite thing. Almost sounds like a weak ground but I would think that would make everything on the 7 pin weak, not the running lights only.

 

So from what I read... the 3.4 volt is always there, its to sense when a trailer is attached I think. so I'm no further ahead.

I have checked the fuse box with the meter.... its at 12 volts. pulled 7 pin plug and took apart. no corrosion. Test plug while apart.... still 3.4 v on running light pin.

Now I guess the only thing it could be, is a broken wire in that very long wiring harness? I don't think I will attempt to take it apart

Could it still be a separate plug somewhere in the rear that's corroded? or would that trailer running light wire run all the way to the front fuse box?

I'm getting some voltage so I don't think it broken.

Going to look at relays now.

 

Well relay good, took all 7 plugs apart at the rear spare tire area and checked... all look real good.

I'm stumped.... anyone think of something I have not done? before I drive it off a cliff.

 

Have you tried testing the voltage with a trailer plugged in? Even a different trailer?

Are you reading the voltage from the pin to a known solid ground, or are you reading off the ground in the plug?

I'd test the voltage with a known good trailer plugged in. Test from the pin to a known solid ground, then compare to the ground on the plug...

Like you said, the computer won't send the full voltage without a load on it...

Fwiw, there are plugs on the driver side frame rail where the hitch is that are for a 5th wheel plug in the bed. I believe those are tied into the bumper plug from the factory tory so you can get a good test location when a trailer is plugged in...

 

Trailer is new and was tested after my problem, I was with the dealer when the trailer was tested. I always test with trailer plugged in.

I think it may be my trailer control module, its beside the emergency brake.... I tested the brown wire coming out of that and have 3.4v also.

So I get 12v from fuse box and the 3.4v from that module.....so it has to be between those two.