Running Lights go out when auto lights come on…

CoryP · June 28

I have a 2024 GMC Sierra 3500 Denali. When pulling my fifth wheel, the auto lights come on and the trailer running lights stop working. I have had the trailer checked multiple times- a short was found in the box once, but even after fixing the short, the lights worked fine until the auto lights kicked on and it stopped working again. The trailer works with an older truck. I have replaced all trailer related fuses and I’m wondering if the new truck is just so smart it is shutting those down because they pull a little too much amperage. Anyone else have this specific problem?

64BAwagon · June 29

I would take it to the dealer and ask them to scan it and then run it through a diagnostic. It sounds like a module issue on the truck but thats just a WAG until theres some diagnostics done.

You could also try cycling the truck through the different scenarios while monitoring the plug on the truck. I have a simple tester for the 7 way that plugs in and has LED's that indicate power to the plug, its not sophisticated but its a quick and easy way to guide you where to start checking.

The 24 has a lot of electronics.

Another JR · July 1

Are the assignments of the 7 pins on your older truck and the trailer the same as the pin assignments on the new truck?

64BAwagon · August 3

Apparently it works now ?

hankins1 · November 17

On 6/28/2024 at 3:40 PM, CoryP said:

I have a 2024 GMC Sierra 3500 Denali. When pulling my fifth wheel, the auto lights come on and the trailer running lights stop working. I have had the trailer checked multiple times- a short was found in the box once, but even after fixing the short, the lights worked fine until the auto lights kicked on and it stopped working again. The trailer works with an older truck. I have replaced all trailer related fuses and I’m wondering if the new truck is just so smart it is shutting those down because they pull a little too much amperage. Anyone else have this specific problem?

I think I have the same issue, but my running lights will actually stay on untill I start driving, a few seconds after I start driving they shut off, stop and blinkers still work, I can shut the truck off open door shut door and turn the truck back on and they will work again untill a few seconds of driving and then off again.