Troubleshooting Function of Additional Trailer Side Marker/Signal Lights

Question:

Hi, I recently rewired my trailer and added dual-function three-wire lights on the fenders running and signal and used the converter 118158 to separate the signals so those don’t come on under braking. The brake wire from the converter is just capped and taped up. I’m having a problem when I signal where the tail lights on my truck alternate left and right and the wrong fender signal on the trailer flashes. The rear signals on the trailer still combined with brake work mostly as intended. I double checked all of the wiring, and it’s as it should be, but it seems like there’s some backfeeding. Do I need to install diodes? If so, where? Thanks!

asked by: Zach L

Expert Reply:

The problem you're having with your new wiring could be related to the ground, especially since it sounds like you may have added more lights than were originally on the trailer. It often happens that weird behavior from trailer lights results from a loose, corroded or insufficient ground connection.

First make sure that each individual light's ground is made to clean bare metal. Some lights like # TLL12RK use a wired ground (white wire with ring terminal) while others like # ST52AA ground through their mounting post. Either way, the ground must be made to clean bare metal that is free of paint, primer, rust, grease, etc.

If making these new ground connections does not correct the issue then check the main trailer ground where it connects to the trailer frame. Sand/clean that spot to expose clean bare metal and make your new ground connection.

You can refer to the linked articles on trailer wiring for additional information.

expert reply by: Adam R