SOLVED - 12V Battery feed circuit seems to be tied to the Tail Lamps circuit

Ranger473 · September 2, 2023

EDIT: See end of thread for resolution.

I have a new to me 2020 GMC 3500HD SLT. When I hook up to a trailer (this occurs with 3 different trailers and all three trailers work normally on at least 5 other vehicles) the tail/running lights come on and stay on. It does not matter if the truck is running, accessory mode only, or turned off or what position the light switch is in.

When I connect the trailer plug to the truck, all of the Tail/Running lights come on and stay on. The ONLY way to turn off the trailer Tail/Running lights is to disconnect from the truck.

It seems like the Battery Feed circuit has been wired to the Tail Lamps circuit. If they had been switched somewhere, I should see the #4 pin (Battery Feed) light up when tail lights are turned on but I never get a signal from the #4 p.

The truck Trailering app tells me I have a problem with the Tail/Running lights.
I do not get an error for the Battery Feed on the Trailering App.
When I test the 7-pin bumper plug, the #2 pin (Tail Lamps) is ALWAYS hot.
When I test the #4 Pin (Battery Feed), I am unable to get any power to it including when I turn the lights on. And my trailer battery does not recharge when driving.
Left turn, right turn/brake, Electric Brakes, Reverse Lamp, and Ground all work normally.
When I test the 4-pin plug and 7-pin plug located in the bed, I get the same results. The Tail Lamp circuit is always hot and I can never get any power to the Battery Feed pin (#4).
I pulled the Bumper plug and it does not look like it has been altered. All of the wiring bundles under the truck also look like OEM work with no splicing bundles, etc.

My questions are: Is there another junction / electrical box where the trailer wiring circuits could have been modified or switched?

Is the problem in/with the K68 Trailer Lighting Control Module?

Edited September 14, 2023 by Ranger473
Problem solved

64BAwagon · September 2, 2023

Are there any obvious modifications to the wiring under the hood or in the back of the truck leading up to the plug ? My first guess is that someone has modified it and made an incorrect connection. I doubt youre going to figure it out any other way.

Ranger473 · September 3, 2023

Thanks for your reply!

I assumed that I would spot the modification with a good inspection under the truck - I certainly have dealt with people modifying lighting circuits on both trucks and trailers over the years, as, it sounds like, have you. I have never had a problem spotting the modifications or splices or even pins being changed on the plug.

But no, all of the wiring bundles look like they are OEM with the PET Tape. The back of the plug is not modified and the heavy 6 or 8 (I think) gauge wire for the 12V Aux goes to the #4 pin (Battery Feed) But never has any power.

The #2 pin is fed by a lighter 12 gauge wire but it is ALWAYS hot. I assume that the Battery Feed should only be hot anytime the "key" is on or the truck is running but in my case, the Tail Lamps pin is always hot.

And, because the bumper plug and the bed plug are both wired the same way, the change is upstream from that and that wire bundle is certainly OEM with no ragged tape or bulges indicating a splice or modification.

64BAwagon · September 3, 2023

My next step would be to disconnect the harness from the receptacles and test the harness for correct operation. A lot of people with RV's like to jumper the plug to get their trailer lamps to stay on for whatever reason while parked.

I havent dug into my 24 much so far but a #6 or #8 wire sounds heavier than what GM used to use for the 12v+ circuit. If I recall it was not much more than a 12g wire before.

Edited September 3, 2023 by 64BAwagon

Ranger473 · September 3, 2023

Thank you for your reply and ideas - I have disassembled the bumper plug and tested the wire ends with the same result that I get from the assembled plug.

Regarding the wire gauge - I am estimating and could easily be wrong about the gauge. The wire going into the #4 pin (Battery Feed) is significantly heavier than all the rest which aligns with 12V Battery Feed in my mind. It is the Red/Green wire in the photo.

I made the "TOP" marking as a cue for the correct position when I re-assemble and to help me keep track of which pin is which while testing...

UWSkier · September 4, 2023

I have this exact same issue with my 2024. The running lights on the trailer on from the second I plug into the truck to the second I disconnect, and I get the warning about trailer lamps being inoperative. My truck goes in for this issue in a couple weeks. Service advisor believes the BCM just needs a reset. We'll see.

gemarsh · September 4, 2023

1 hour ago, UWSkier said:

I have this exact same issue with my 2024. The running lights on the trailer on from the second I plug into the truck to the second I disconnect, and I get the warning about trailer lamps being inoperative. My truck goes in for this issue in a couple weeks. Service advisor believes the BCM just needs a reset. We'll see.

Keep this topic updated on the bcm reset.

I don't see where his problem with not having 12v+ at the 7way, goes through the bcm looking at the schematic. I also don't see where he has said he checked the 30amp fuse for the 7way 12+. He might have 2 separate issues.

64BAwagon · September 4, 2023

Keep in mind the 24 has different electrical architecture than the 2020 @gemarsh

@Ranger473 Man, youre sure not making this easy .... LOL

My next step would be to follow that wire loom towards the underhood area checking for signs of alterations. I dont have any data on the 2020 underhood electrical centers.

Edited September 4, 2023 by 64BAwagon

UWSkier · September 5, 2023

8 hours ago, gemarsh said:

Keep this topic updated on the bcm reset.

I don't see where his problem with not having 12v+ at the 7way, goes through the bcm looking at the schematic. I also don't see where he has said he checked the 30amp fuse for the 7way 12+. He might have 2 separate issues.

I will. And as it turns out, I have the same problem with no 12v on the 12v prong on my setup either. Upon further examination, I have a blown 30a fuse on the trailer 12v circuit.

@Ranger473 do you happen to know if the previous owner of your truck towed a camper? If so, did he use a Furrion pigtail adapter to make his backup cameras work? That's the only thing I have used on my truck that I figure could have caused an issue. I am rewiring the trailer so I won't need the pigtail anymore.

But even if I swap a 30a fuse from another spot onto my 12v, I still have constant power to the running lights...

So it, in my case, is two separate issues perhaps.

64BAwagon · September 5, 2023

Thats a really good point @UWSkier since the rear cameras on many RV's are fed from the tail lamp circuit. Maybe the P.O. modified it so they could have the camera on all the time ?

UWSkier · September 5, 2023

4 hours ago, 64BAwagon said:

Thats a really good point @UWSkier since the rear cameras on many RV's are fed from the tail lamp circuit. Maybe the P.O. modified it so they could have the camera on all the time ?

Likely used a 7 way adapter to supply a constant 12v instead of PWM to the running lights.

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

Ranger473 · September 5, 2023

Thanks for all the replies and head-scratching! I truly appreciate it! Responses below:

@gemarsh I don't see where his problem with not having 12v+ at the 7way, goes through the bcm looking at the schematic. I also don't see where he has said he checked the 30amp fuse for the 7way 12+. He might have 2 separate issues.

I failed to mention that I had checked my fuses... Both the Trailer Parking Lamps #41 and Trailer Battery Battery #63 appear to be good.

@64BAwagon My next step would be to follow that wire loom towards the underhood area checking for signs of alterations.

I dropped the spare tire and followed it as far as I was able - it disappeared into the space between the bed and frame. It looks like it goes to what I think is the TRAILER LIGHTING CONTROL MODULE. The information I have is that becomes a dealer-level item to test and possibly Flash???

@UWSkier do you happen to know if the previous owner of your truck towed a camper? If so, did he use a Furrion pigtail adapter to make his backup cameras work?

I don't know if it was used to tow a camper. The bed does not have any scars indicating a 5th wheel hitch was installed and there was not a dedicated Trailer set up in the trailering app. They could have deleted it but they didn't delete any of their mobile phones or data so it leads me to think pulling a trailer was not a prime purpose. It does look like someone spilled a bunch of red paint in the bed and then tried to hide it with a DYI bed spray... Red paint has dripped down under the bed in various areas. I have assumed it was a work truck but I really don't know.

I understand what you are saying regarding the cameras - My 2016 F250 had to have the lights turned on to power the camera on my RV. The camera is always powered with this GMC in the current configuration. But if the wiring was modified for that purpose I am unable to find where it was done. Also, why disable the battery feed? It seems like it would require modifying the truck and trailer to ensure the camera is always powered. But who knows...

Also good info to know regarding the adadpter from eTrailer. The camera works well know but when I get this issue resolved I might then need the adapter. Thanks! @UWSkier

Ranger473 · September 5, 2023

I will keep this thread updated as I learn more and when I resolve the problem.

64BAwagon · September 5, 2023

On past models of GM trucks the 12v+ feed for the trailer was a ring terminal connected to a post in the underhood electrical center. I am not sure where that connection is on this new truck and it may be different between the 2020 and the 2024.

Does someone here have access to the information on where the feed for the 12v+ power to the plug originates ? I really feel like @UWSkier has hit on something with the rear camera power feed. Of course I could be wrong but it seems entirely plausible and a very likely way to explain this.

If you pull the fuse for the trailer feed does that change the parking light function at the plug ? It could be modified with a simple resistor jumper under the hood. The 2024 manual shows the trailer park lamps as fuse 41 in the underhood center. I would be checking under there.

Edited September 5, 2023 by 64BAwagon

gemarsh · September 6, 2023

The 7way 12+ is a straight shot from the underhood fuse block if it's a diesel.

If its a gas burner the wiring schematic does something weird. It has a plug somewhere that ties the aux battery into the wire. Supplied by F3 30amp fuse.

I have been concerned with the 7way 12+, ignoring the park lights being hot at all times issue.

Can you swap the relay KR53 with a neighboring relay? Maybe the relay is fused together. If there isn't a matching relay (doubtful), just pull it and see if your trailer park lights die.