No running lights on trailer

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Mar 6, 2025 | 01:20 PM

Mkgo53

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1st Gear

Joined: Mar 2025

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No running lights on trailer

2021 F250 super duty 7.3
problem I’m having is no running lights on trailer.
All other trailer functions work as they should. Checked #10 fuse and it’s good. Not sure if I’m missing a fuse somewhere for trailer tow or what? Also already tried another truck and trailer functions are working. Test light shows prong on trailer plug is dead for running lights. Is there a relay I don’t know about or am I in for a broken wire hunt?

Mar 7, 2025 | 08:56 AM

Log Hog Kust'em

Tuned

Joined: Oct 2009

Posts: 418

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From: South Texas

In 50+ years of pulling trailers, I have found most of the light problems can be found with a broken ground, especially if all of the trailer lights are out. Do not assume the trailer hitch will provide an adequate ground. did you change anything since the last time the lights worked?

Mar 7, 2025 | 07:56 PM

~erik~

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Joined: Jun 2017

Posts: 361

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From: Clayton, Georgia

Found with our Airstream that if we wanted the running lights and the camera at the rear of the trailer to work we had to move the light switch from "auto" left one click to "on". No problems after that. If you want to test the lights separate from the truck unplug the seven way plug from the truck. Hold the plug with the locating tab up. Find the connectors at the 11-o'clock and 1-o'clock positions and jumper them together with a fuse, a paperclip, or a small piece of wire. That will connect the trailer's B+ battery power directly to the running lights.

Mar 9, 2025 | 10:33 AM

Mike Bartlett

Tuned

Joined: May 2010

Posts: 495

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From: Pleasanton, CA

I agree with the “check grounds” on truck and trailer. When I got my 2019 and hooked my boat the lights on the trailer wouldn’t work. After messing around for awhile I used my jumper cables, one side, found a good ground spot on the trailer and truck then made the connection. Everything started working fine. I had an existing ground wire through my trailer wiring but size wise it wasn’t sufficient for the 2019. I replaced with larger gauge wire no problems since.

Good luck,

Mike

Mar 10, 2025 | 04:18 AM

ejb69

More Turbo

Joined: Jan 2016

Posts: 684

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From: Green Bay Wi

Like others are saying bad grounding can cause weird things to happen.

Mar 10, 2025 | 06:34 AM

99powerstrokedF250

Logistics Pro

Joined: Jan 2010

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There is a very good chance this is a ground on the trailer. Have you confirmed whether it is your truck, either with a plug tester OR another trailer? I have multiple so if I have a problem I check with a different trailer.

There is a dedicated ground wire in the plug, so the hitch is more of a 'backup' ground.

I chased an issue with my flatbed for a LONG time - occasionally it wouldn't have brakes but all lights would work, then the lights were very dim but the brakes worked fine. I replaced the plug with a GOOD one (not the cheap $10 amazon ones but one that was fully sealed back to a junction box), and then I had brakes 100% of the time, but the lights were then ALWAYS dim if they worked at all. I wanted to replace the incandescents with LED anyway, so I broke all the bolts for the old housings, ground down to raw metal for new grounds, wired everything up...and lights work 100% of the time and flash as fast as my truck lights now.

I suggest checking and maybe redoing all your lights' grounds (if they aren't wired back to the plug directly). There's a good chance that will fix your issue.

Mar 10, 2025 | 07:08 AM

projectSHO89

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Joined: Jan 2004

Posts: 19,762

Likes: 1,079

From: St Louis

Quote:

Checked #10 fuse and it’s good.

Put your test light on the fuse's test points and verify it goes HOT when the park/running lights are turned on. There is a (non-serviceable) relay upstream of the fuse that has to work in order for the power to get to the rear trailer connector. The relay is built into the BJB and is controlled by circuitry in the BCM. If the fuse does not go HOT, the BCM needs to be scanned to see if it has faulted the TT PARK circuit and shut it off.

This does not sound like a ground problem since it was stated that "Test light shows prong on trailer plug is dead for running lights." and "All other trailer functions work as they should."