Wiring diagram needed - trailer lights always on

[Chopper_Man]

Hey all, looking for a wiring diagram for the trailer harness with the 5th wheel prep. Having a fault with my running light circuit and bleed-thru. Running lights stay on even with truck off.

Thanks!

-Brian

Edited February 6, 2025 by Chopper_Man

[Another JR]

If you are trying to verify pin assignments, your owners manual has that information. 

[Major Tom]

Could be the anit-theft (Trailer) detection system.  May be working as designed.  look at the last paragraph on the below page.

 

https://www.gmc.com/support/vehicle/entertainment/apps/trailering-app#:~:text=If the trailer's wiring harness,to help discourage trailer theft.

[Chopper_Man]

Thank to both. Looking for a full diagram, not just pinout. I also don't believe is security related as it's not flashing. Great thought tho!

 

I have a constant 12v to the lamp circuit. There are also 2 codes associated to it. B3885 and B3886, left and right tail lamp circuit short to battery. I have the 12v even when pulling the fuse.

 

I have access to AllData at work, their diagrams only show from fuse block to trailer control module. I don't just want to throw a tcm at it without further troubleshooting.

[Crazy Canuck]

Don't have the wiring diagram but my '21, '22 and first '24 all did this.  Although I have towed with my new one, I have yet to have a trailer tooked up when it's not running.  I read somewhere that it is the trailer module but never had any of them looked at before I got rid of them.

[AndrewF]

Do the online service manuals that you have to subscribe/download also have the wiring diagrams?

[bruceb58]

On 2/4/2025 at 4:05 PM, Chopper_Man said:

I have a constant 12v to the lamp circuit. There are also 2 codes associated to it. B3885 and B3886, left and right tail lamp circuit short to battery. I have the 12v even when pulling the fuse.

I have a factory manual for my 24 and those  particular codes don't show up when I did a search of the manual. Your 2022 may be different than an 2024.

 

In the 2024, the trailer lamp control module controls trailer lighting on some models. It receives commands over a serial bus from the BCM. Other models do not use it.

 

If your particular model does not use the trailer control module, it looks like the parkinglamp relay controls the running lights. If that's the case, it's relay  KR53 in your engine wiring harness junction box. Since you are getting codes, you probably have the trailer light control module.

 

Hope something here helps. As I said, this is from a 2024. Yours may be different.

 

 

Edited February 7, 2025 by bruceb58

[Chopper_Man]

10 hours ago, AndrewF said:

Do the online service manuals that you have to subscribe/download also have the wiring diagrams?

 

They end at the trailer control module. The diagram Bruce supplied above is missing in AllData.

[Chopper_Man]

6 hours ago, bruceb58 said:

I have a factory manual for my 24 and those  particular codes don't show up when I did a search of the manual. Your 2022 may be different than an 2024.

 

In the 2024, the trailer lamp control module controls trailer lighting on some models. It receives commands over a serial bus from the BCM. Other models do not use it.

 

If your particular model does not use the trailer control module, it looks like the parkinglamp relay controls the running lights. If that's the case, it's relay  KR53 in your engine wiring harness junction box. Since you are getting codes, you probably have the trailer light control module.

 

Hope something here helps. As I said, this is from a 2024. Yours may be different.

 

 

 

This is absolutely perfect, thank you. I have no trailer relays, fuse feeds the trailer light module directly. Based on the above, it can really only be a shorted light module or a canbus control issue. Rolled the dice and ordered a lighting module, was only $80. Should be here in a few days.

[EddDe]

On 2/4/2025 at 1:49 PM, Chopper_Man said:

Hey all, looking for a wiring diagram for the trailer harness with the 5th wheel prep. Having a fault with my running light circuit and bleed-thru. Running lights stay on even with truck off.

Thanks!

-Brian

Do you have solar panels on the trailer? I read somewhere that a diode needed to be installed on the trailer light plug to eliminate the 12v "feedback" to truck from the solar system.

[Chopper_Man]

3 hours ago, EddDe said:

Do you have solar panels on the trailer? I read somewhere that a diode needed to be installed on the trailer light plug to eliminate the 12v "feedback" to truck from the solar system.

 

Ruled out anything with the trailers (I have multiple). I have a constant 12v on the tail lamp circuit with nothing plugged in and the truck off. Thanks for the reply!

[Chopper_Man]

Just a quick followup. It was the trailer lighting module. Part number 13554026. Took about 20 minutes to install. Drop the spare tire, undo the connector, and 2 10mm nuts. Working perfectly now.

[bruceb58]

23 hours ago, Chopper_Man said:

Just a quick followup. It was the trailer lighting module. Part number 13554026. Took about 20 minutes to install. Drop the spare tire, undo the connector, and 2 10mm nuts. Working perfectly now.

Did you ask the dealer if this was covered under warranty? 

[Chopper_Man]

19 minutes ago, bruceb58 said:

Did you ask the dealer if this was covered under warranty? 

I'm under my extended warranty, $100 deductible. Part cost me $80. Didn't bother with the dealership.