Troubleshooting Stop/Turn Circuit not Functioning on Trailer Wiring

Question:

I am stumped. I brought a brand new trailer and brought it home and checked the lights to discover that the LEFTdrivers side brake light was not working despite the fact that the taillight worked. First thing I did was replace the bulb and that did not fix my issue. Second thing I did was check the output from my vehicle with a circuit tester. I have a 4 pin connection and everything coming out from the vehicle was working correctly. Third thing I did was disconnect the light boxes from both sides and swapped them to see if the problem was the light. The problem persisted on the left side. Fourth thing I did actually brought me somewhere. I inspected the connection from the 4 pin that plugs into the vehicle and the other end that contains several harnesses. I discovered that if I swapped the harness that went down the left side with the harness that went down the right side that my issue now was that the RIGHT side brakelight was not working. I figured it was this connection so I brought a brand new cable and replaced the old one. However the issue persists where the LEFT side brakelight does not work. Any idea what I can try next?

asked by: Justin

Expert Reply:

Sounds like a weak ground for that assembly. This is a pretty common problem and if you clean up the ground wire of the assembly or run an additional ground wire you will fix it.

expert reply by: Jameson C