Lights don't work if we plug the trailer in

[gemarsh]

Although I own a newer GMC model, I think the wiring may be the same. There's an inline fuse inside a rubber fuse enclosure, behind the 7 pin trailer socket accessible from below the tailgate, its quite easy to change. Its easier to reach and work below if you remove the spare tire first.

Can you get a picture of the fuse holder? Only one that I know of in that location is for the kicker. I can't find it on the wiring schematics in the upfitters manual.

 

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[gemarsh]

@Tommy, did you check fuse number 74 and 82 in the engine compartment fuse block?

gm uses a separate trailer fuse for each circuit for the trailer. I believe you say that, no lights are working on the trailer, I think you have a fuse to the module or module issue.

 

[gemarsh]

The fuse in question is below the tailgate and part of the wiring harness leading to trailer plug.

Leave it to gm to put a single fuse, somewhere undocumented, in the worse possible place to change in a parking lot covered in snow or mud at night with no trailer lights.

 

[Viking Truck]

Pinebaron is a pretty smart guy and useful contributor for sure but I'm not so sure about this one with the fuse back there. Sounds like it is for an aftermarket accessory.

 

[pinebaron]

Well guys i litterally cant find any fuses where the spare wheel is located. Any ideas in what direction it could be? Left right?

Like Viking said, just follow the wire from behind the trailer plug wiring harness bundle and trace if about 30cm behind and look for a small rubber enclosure covering a single fuse; unfortunately I can only relate to what I see under my 2024 HD2500 AT4X and can only presume something similar exists in yours.

 

[pinebaron]

Pinebaron is a pretty smart guy and useful contributor for sure but I'm not so sure about this one with the fuse back there. Sounds like it is for an aftermarket accessory.

I only know because I tapped into the same wire leading into the trailer plug (after the inline fuse) to provide power to rear fogs and its control circuit. Since I finished the rear fogs job several weeks ago, I zip tied several bundles of wires and the rubber fuse holder is in one of the bundles and not accessible to view any more. Power to this wire (leading to the trailer plug) is ignition switched and feeds rear lights to my 10x5 trailer. The reason I remember the inline fuse was, I was afraid it may blow with trailer connected and with rear fogs also switched on but it worked; the fuse I remember was no more than 10amp which I left it there, since it offered protection to my hand built control circuit, in case it ever failed due to design issues.

In your case, being a different GMC model, it may have a different fuse location, the fact your lights stopped working, clearly indicate a fuse somewhere has blown. Sorry I can't be of any more help from being so far.

 

[LegalBrief]

It sure why you are cutting anything, Curtis makes a trailer adapter that splits the signal, from there you can tap the wires or add an additional plug, to pull what you need. Nice and tight, not cutting, cutting makes the short your issue not GM. Super easy.

Amazon.com

 

[joerone]

I have a 2021 Sierra 3500 AT4 and my backup camera on the TT would not work. I check around and found out te
hat the new GMC's use a PWM (Digital) signal rather than analog, so I bought this adapter and all works fine now. Not sure if this will solve your problem or not, but wanted to mention it.

Furrion 7 Way Adapter

 

[Viking Truck]

@Tommy Did you get this figured out?

 

[bigrackHack]

FR87PR https://www.etrailer.com/Accessories-and-Parts/Furrion/FR87PR.html

This fixes it. I had the same issue.