Troubleshooting Trailer Lights Not Working When Tow Vehicle Headlights Turn On

Question:

I have installed new lights ST6RB and ST7RB on a small utility trailer. Replaced the old ones that had seen better days due to a lot of water, snow and road salt. Didnt work and I had no Jeep lights. Kept blowing my 15 amp fuse in the fuse box. Replaced the #1157 bulbs. Same problem. Discovered a bare wire short in the utility trailer wiring. Replaced the old 4 way trailer wiring with a new 4 way flat trailer connector and wiring assembly and connected it to the new lights. Still kept blowing 15 amp fuses and no lights anywhere. My next step was to replace my old Jeep tail light wiring harness with new part #118416. I am no longer blowing fuses and all lights and their functions now work on the trailer and Jeep EXCEPT as soon as I turn on my Jeep parking lights or headlights I have no working lights on the trailer. Nothing. But all lights on the Jeep still function without any issues. Turn off my parking lights/head lights and all trailer lights and Jeep lights function properly. Any suggestions as to what is causing this problem and recommendations on how to fix it?

asked by: Ron

Expert Reply:

Typically when turning on one of the light functions causes the other lights to cut out, it signals an issue with the ground connection on the trailer; the # ST6RB and # ST7RB lights both ground independently with their mounting hardware but there should be a ground wire on the trailer. I would start by checking the ground connection there and also cleaning off the trailer connectors to make sure there isn't any dirt or debris that's causing an issue.

The next thing I would check out is the actual wiring on the trailer, so you can make sure there aren't any cut or damaged wires there that might be contacting the frame and causing the short that's causing you to blow fuses on your JK.

If you can let me know what you find when you check the ground and the harness, I can give you a few other spots to look to see if there is anything else that could be causing this.

expert reply by: Erik B