Troubleshooting Lack of Trailer Brake Lights on Trailer

Question:

Hi, I have a 2005 GMC Sierra 2500HD. I have checked all fuses, including the CHMSL fuse and everything is fine. Cruise control, 3 stop lights and all lights on truck work fine. L/R turn signals work fine both on truck and trailer. I replaced the flasher unit and did not fix issue. If I apply brakes I see the voltage on the TR L/R fuses [on drive side fuse block] but I do not see same brake voltage on trailer connector. What else could it be? Ps: Very frustrated

asked by: Dan M

Expert Reply:

It sounds like there are a few possibilities and you've already narrowed many other possibilities by the work you've already done.

Using a circuit tester like # PTW2993, check the trailer connector wiring coming from your truck when the trailer is not connected. Test each function. If there is an issue there, you have isolated the truck as the issue and can troubleshoot the vehicle wiring. Disconnect the US Car connection behind the 7-way and test the pins for function there. You may want to add some dielectric grease # 11755 to any connection point after cleaning them and ensuring you don't have any bent pins.

If the functions work well there, I recommend connecting the trailer and testing the functions again right behind the connector. If everything works well there, you have confirmed the connection itself is good. Next, I would check the trailer ground wire(s) to make sure they have a clean metal to metal connection free of rust and corrosion. Ground issues are often at fault when only one function isn't working properly. Check the tail lights too as the mounts sometimes serve as grounds. You can use a jumper wire from a confirmed ground wire to test.

Lastly you can trace the brake wires to confirm function and replace where a short is found.

expert reply by: Jason S