Troubleshooting Trailer Lights Not Functioning When Headlights are On

Question:

I pulled all the old wire out of my boat trailer and ran new wire from each light to a junction box which I connected this # H20046 to using the color code chart on your web page. When I have the vehicle lights OFF, the right turn, left turn, brake lights all work fine. When I turn on the vehicle lights the trailer clearance lights work, and the right turn works on the vehicle and trailer, but the left turn works on the vehicle but does not work on the trailer. Turn the vehicle lights off and then the left turn works on the trailer. This is 14 year old shorelandr trailer with Dry Launch SP7E taillights that have a single bulb with multiple elements, for running light and turn/brake light. Any suggestions on what to check? Thank you,

asked by: Ed K

Expert Reply:

It sounds like you have a bad ground connection. If all ground connections are not making good contact to bare metal, when the tail lights are turned on then the trailer lights will go out or not function properly.

You will need to check all of the ground connections especially where the tail lights are mounted to the trailer frame, and where the light brackets are mounted if they are bolted to the frame. If a plastic license plate holder is mounted between the left side taillight and the trailer frame this could be the problem.

Once you have all of ground connections checked, if the problem persists, you will need to check for a broken pinched or frayed wire on the trailer somewhere. If this is not the case then you will need to use a circuit tester, item # PTW2993 if needed, and make sure your wire connections have been properly made at the tail lights. You should also check the connectors for corrosion that can sometimes cause issues with the trailer lighting. If you look into the connector and see green corrosion you will need to clean out the connector.

expert reply by: Jameson C