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

[Ranger473]

@64BAwagon Good idea about pulling the fuses.  I pulled BOTH the Tail Lamp fuse and the Battery Feed fuse---the #2 Tail Lamps pin is still hot.  With both fuses pulled.

 

@gemarsh I swapped the relays as you suggested and no change.  The #2 Tail Lamps pin is still hot with the relay out or with a substitute relay in place.

 

FYI - It is a diesel and the Battery Feed cable is a direct shot off the side of the fuse block, as you said.  And it is connected, the connection looks clean, and no corrosion.

 

The #2 pin is getting power (12.7V) from someplace other than fuse 41 (Tail Lamps) or Battery Feed (fuse 63? - I don't have the fuse block diagram right here).

 

I am thinking the problem may be in the Trailer Lighting Control Module located between the frame and the bed just forward of the spare tire.  I have seen a YouTube video where weird lighting was the result of a corroded circuit board.  The unit is sealed and needs to be tested by the dealer (my understanding).

 

I have an appointment with the dealer next Tuesday.  I made the appointment in person and provided a detailed explanation in writing of what needs to be corrected.  I did not think I would be able to explain over the phone.  I also am going to ask to speak with the tech when I drop it off to ensure he/she is very clear on what needs to be corrected.

I am still open to ideas - I would love to identify and fix the problem with the community help I am getting here.

In any case, many thanks to everyone for the ideas and discussion!!

[64BAwagon]

I would also print out the things you have found and hand it to the service writer. That way it wont get mixed up between the tech and the writer. They can hand it off to the tech with no misunderstandings

Good luck, please let us know . 

[Ranger473]

Yep, I provided the service writer a one-page summary with other support info.  I have another packet for the tech and I am going to ask to speak with the tech when I drop it off.

Thanks again for your time, interest, and ideas!  I will keep this updated.

[gemarsh]

On 9/2/2023 at 8:35 AM, Ranger473 said:

 

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.
 

1. The truck Trailering app tells me I have a problem with the Tail/Running lights.
2. I do not get an error for the Battery Feed on the Trailering App.
3. When I test the 7-pin bumper plug, the #2 pin (Tail Lamps) is ALWAYS hot.
4. 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.
5. Left turn, right turn/brake, Electric Brakes, Reverse Lamp, and Ground all work normally.
6. 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).
7. 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?

 

 

 

Hope you read this before going to dealer and spending your money.

 

Trailer module.

 

Looks like the 7-way was plugged in upside-down at some time. 12+ feeding parking lights and won't charge trailer batteries. 

 

 

[64BAwagon]

I cant help but think theres way more to that story about the dealer, especially the anecdotal story about the guy with the Duramax trans. 

Seems like an easy fix though. 

[gemarsh]

7 minutes ago, 64BAwagon said:

I cant help but think theres way more to that story about the dealer, especially the anecdotal story about the guy with the Duramax trans. 

Seems like an easy fix though. 

True.

 

My thoughts are, that the owner told the dealer the 7way cord was put in upside down. By the admission of the owner in the video, I immediately thought...that's not a warranty issue. If this is the case of honesty, I agree with the dealer.

 

ETA... I believe my dealer would run this scenario thru warranty for me. Just to make me happy. But in the long run, I think warranty work increases the price of new vehicles for everyone.

Edited September 10, 2023 by gemarsh
Added info

[UWSkier]

That's definitely my issue too.  I never plugged anything in upside down, but have the issue where I'm getting voltage on the running light pin on the truck even with the relay and fuse for that circuit removed.  I go in for this NEXT week so hoping the dealer can turn this around quickly without having to keep my truck.

[Ranger473]

Hey @gemarsh, thanks - that sure looks like my problem.  I don't recall exactly where but I was under the impression the dealer had to "pair" the module to the truck but that didn't happen in this video.  I am going to try the same thing and cancel my dealer appt.  I'll update this thread once completed.

[Ranger473]

The Part number shown in the video has been superseded.  And the price looks pretty good too at GMPARTSCENTER.net (when compared to the price mentioned in the video).  It is due to arrive 14 Sept 

[64BAwagon]

There was talk of the module being changed back to the one used in the 23 models due to the 24 not playing nice when it sees higher voltage due to solar on some RV trailers that keeps the batteries at a higher V than the module wants to see. 

[Ranger473]

SOLVED

 

I replaced the Trailer Lamp Contol Module ($84.16 plus shipping and tax). Now the tail lights come on only when the lights truck lights are turned on.

 

I replaced the 30A JCase Fuse, #63 TRAILER BATTERY, and now have 12V at the #4 BATTERY FEED pin all the time.

This is the first time I have pulled and inspected a JCase-type fuse.  I had pulled it before, inspected it, and thought it looked good.  However, after replacing the trailer control module, I still did not have 12V to the Battery Feed pin.  I pulled the fuse, popped the transparent cover off, and then I could see that the fuse had burned.  I replaced the fuse and I have power to the trailer batteries.

 

FYI - I cut open the old module but everything looked fine.  I thought perhaps it might have gotten corroded or something but it was clean and dry inside so something on the circuit board failed.

 

Thanks to everyone who contributed!

[gemarsh]

That is awesome. Glad you figured it out, without having a dealer fee. Appreciate the update.