Troubleshooting 5th Wheel Trailer Lights Not Working When Connected to 2017 GMC Sierra 3500

Updated 05/20/2024 | Published 04/07/2019

Question:

When we hook up our 2017 3500 Sierra Denali to our fifth wheel we are not getting any trailer lights. Not sure if this is a fuse issue and which one

asked by: Joe R

Expert Reply:

You can definitely start by checking the fuses in the fuse block located in the engine compartment of your 2017 GMC Sierra 3500. These should be the fuses in the 14 (left turn/stop), 15 (running lights), 16 (reverse lights), and 17 (right turn/stop) locations.

If those look good then the next step to take is to use a circuit tester like part # PTW2992 to make sure that all of the functions on the 7-Way of your Sierra are working properly (see attached photo).

Since you have a new pickup I would imagine the problem is with your trailer wiring (if your fuses look good). You can verify this by grounding the negative terminal on a 12V battery (like what is in your pickup) and then using the positive terminal to test each of the light functions on your trailer-side 7-Way (see attached photo).

If your fuses look good then perform the tests mentioned above and let me know what you find so I can help you troubleshoot further.

expert reply by: Jon G

click to enlarge

Dubeau R.

5/18/2024

I heard that it most likely is the brake control module. Any thoughts?

Jon G.

5/19/2024

@DubeauR The brake controller is it's own circuit so it wouldn't cause the lights to not work. With that being said, you're more than welcome to test it out still! Let me know what you find once you do!

Dubeau R.

5/20/2024

@JonG thanks, I’ll let you know