Agree. If it’s on both vehicles, it’s the trailer.

 

Trailer is the issue here. If a known good truck works on a different trailer than it is the trailers fault.

 

Mine had a blown fuse for running lights from start, don’t remember the number but check it’s a 30 amp fuse under the hood.

 

First, fuse #26 was good. I do trust my RV mechanic, but I will do some basic testing on the trailer. They tested and worked on the trailer for hours and said it was checked out had to be a truck setting issue. I did not watch him the whole time, so I don't know exactly what procedures were performed.

Y2KW57 Now that I have tried another truck I will try some diagnosis myself. I have zero experience with LED's, but I do assume if I apply 12v to the running light circuit they should light. I'll go from there. Trailer is new to me. Super sanitary wiring job you did, Bravo!

 

"They tested and worked on the trailer for hours"

It is doubtful that they would have invested all these hours, multiplied by the number of men working on it, if they did not themselves suspect that their work was the cause of the trailer lights not working. Whether they voice this suspicion to the customer... or not, so as not to undermine customer confidence in their competency, that is an awful lot of manhours, for a business whose principle commodity is labor, to just give away. I think they knew they might have messed up, even if they didn't know how they messed up.

LIFO is an accounting term for Last In First Out that applies equally well for electrical and mechanical diagnosis. The last thing changed prior to the fault discovery, should be the first thing examined after the fault is found.

The Tremor wasn't changed. The trailer was. That leaves the trailer as Suspect #1.


"but I do assume if I apply 12v to the running light circuit they should light"

As long as there is also a continuous ground return path to the same direct current power source that is supplying the 12V to the running light circuit... otherwise, they won't light.

The importance of a good ground cannot be over emphasized, so I'll reiterate this again, despite member Morbus already saying it earlier.

In my trailer harness connector wiring project above, I extended the Ford supplied ground circuit to two posts, bridged by a hand fabricated ground buss bar that electrically linked the posts, and further adding a ground jumper to the frame.









To summarize the suggestions illustrated by the combination of responses and illustrations,

1. Ensure that the designated function of circuits are located in the correct receptacles within the trailer end's connector.

2. Check the continuity and physical integrity of each circuit on and under the trailer

3. Take whatever time is needed to establish good solid grounds.

 

I guess I will add my $.02 here. OP, see post # 20.
In the last picture @Y2KW57 stated “more often than not it is the ground” is 100% on the mark.
When diagnosing trailer lighting systems, here is my order of business.
Bulbs good? Check
Fuses good? Check
Next step.......make sure all circuits are properly grounded On said trailer / make sure said trailer is properly grounded to tow vehicle before going any further.

Good luck

 

Any luck?

Jim / crewzer

 

Anyone figure it out?

 

I’m having this same issue, trailer is 100% fine…it has been tested. My lights are not set to auto, all fuses for the trailer module are good.

I have tested power at fuse box for fuse 99, trailer running lights…. It has 12v

I have 3.4v (pin3) at the trailer running lights 7 pin connector, but it needs 12v on the number 3pin… brown wire.

So I went to the trailer control module, beside emergency brake pedal. The brown wire is trailer running lights… it reads 3.4v also with lights in on position.

Something is wrong after the fuse box to the trailer control module.

Right now the Suspect is the module itself, but I’m not 100% sure

OP…. Did you find the problem with yours? We may have something in common.

 

In the dash's DIC, under the towing tab, does the truck recognize the trailer as being connected and report that the lights on the trailer are OK? Just a thought...

 

Yes, the truck dose recognize a trailer is connected and reports all lights are good.

 

So just an update on my no trailer running lights from my 2017 F350 Plat 6.2

All fuses ok

I cut the brown wire running into the trailer control module, and attached 12v to it from the battery for a test.... trailer lights now work.

So the trailer control module for trailer running light is not working, and probably was like that from the factory.

The truck is out of warranty. I'm thinking of running a tap fuse from the passengers footwell fuse box, and adding a toggle switch to turn the trailer running lights on and off. I have no upfitters.

My trailer is a 16' enclosed with all LED lighting so not much draw I'm guessing? I have adjustable pedals rated for 15amp and will never use them....maybe run off that? or should I find something with higher amp rating?

I have no moonroof, and that's rated at 30amp.... could I just add a fuse tap to that empty slot?

 

I would use the adjustable pedals.

I would really opt for a new TBC to correct the issue for all future trailers, but your route likely will cost you a few cents vs ? for a TBC.

 

I ended up using the power moonroof 30amp fuse socket in the passengers foot well. My platinum dose not have the moonroof option, but the fuse and power are present. Used a tap fuse, and fuse the led trailer running lights at 15amps.... installed a 20amp toggle switch to turn them on and off with accessory power. I very rarely pull a trailer.... so will be fine for me.

Cost me $12. for the switch and already had a few feet of 14g wire laying around

Works good, no visit to the dealer!

 

Did you figure out what it was. My 2024 is doing the same thing after adding the 12 way with the 7. Worked before but won't now.