Troubleshooting Trailer Light Malfunctions - Running Lights Not Working But Everything Else Is  

Question:

Expert Reply:

If the turn signals are working properly on your boat, but the running lights do not work, then this is most likely a ground issue. The turn signals and brake signals do not require a constant power like what is needed for the running lights. So this is usually when grounding issues become apparent.

Since everything is new on your boat trailer, I would check to see if the ground wire on the trailer is grounded to an appropriate metal surface. Boat trailers are commonly made of aluminum, and this is not the best metal to supply a correct ground. You may need to run a jumper wire to a better grounding metal surface.

Boat trailers are often equipped with fold away tongues, this may also be causing the ground problem. To resolve this you may also need to route a jumper ground wire.

It is also possible that a newly installed license plate holder may be the culprit. Trailer lights usually ground through the mounting hardware, and if a plastic license plate holder is interrupting that signal it could also be causing a grounding issue.

I also recommend checking the ground and the connectors on the vehicle trailer wiring connector that you are using to tow your boat trailer. Again if the problem is only when the running lights are activated, and those require the most power, the issue is related to an incorrect ground. You will need to make sure that the trailer connection points are not corroded, bent, or damaged. Also make sure that the vehicle ground for the connector is properly grounded.

I have included several helpful articles that show traditional trailer wiring diagrams, explain trouble shooting trailer wiring, and connection points.

expert reply by: Rachael H

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