Troubleshooting Wiring New Trailer Lights That Lose Stop/Turn Functions When Running Lights are On

Question:

I have new trailer tail lights with red and black wires. I wired the black wire on each light to my brown running light wire and the red wires to the yellow and green turn signal wires. Now the turn signals and brake lights work fine when I don’t have the running lights turned on. When I turn the lights on at night everything lights up fine but my brake lights and turn signals do nothing? Please help.

asked by: Shane

Expert Reply:

It sounds like you have wired your lights correctly but that they are not grounded correctly. Often with two wire tail lights, the ground is in the mounting hardware, so you will want to make sure those connections are bare metal to metal. The increase in power when you turn on your running lights may be overwhelming the turn signal ground connection and that is why the other function isn't working properly. I recommend cleaning your ground connections and then testing your functions again.

Let me know if we need to continue to troubleshoot further. If your new lights are LEDs there may not be enough of a power draw for that brake and turn function and using load resistors like # DI34ZR may be required. You can test that by using an incandescent circuit tester like # PTW2993 on that wire as the incandescent bulb will draw more power and may help that light to function correctly.

expert reply by: Jason S