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All Trailer Lights Work on Re-Wired Boat Trailer Except for Tail/Marker Lights

Question:

Just worked up lights on my boat trailer and everything works except for the tail/marker lights. I dont understand why. I always use Saunders and shrink tube when I wire lights together to make sure they stay water proof. I cut all that off and used the factory wire nuts to eliminate any error I may have made and still nothing. I dont understand it cant be a ground wire because if it was I would have nothing right?

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Expert Reply:

The problem you are describing could be a grounding issue.

The tail/marker lights on your trailer remain on at all times the tow vehicle lights are on. Since they do not blink like turn signals or come on only occasionally and for short periods like a brake light, they will pull more current. It is possible that the existing ground connection is good enough to support the turn and brake light functions but is not solid enough to handle constant-draw demand from the tail and marker lamps.

I suggest you re-check your ground wire to ensure it is connected to a clean bare metal surface. You can try sanding off any paint/corrosion from the grounding point. If your trailer is aluminum (not a great conductor) you can try running a ground jumper wire directly up to the trailer connector. If your marker and taillights use a mounting stud for their ground (as opposed to a separate ground wire) you can try sanding off any paint/corrosion from their mounting locations to ensure the ground at each individual light fixture is sound. The linked article includes troubleshooting tips that may help.

You can try connecting another trailer to see if the problem remains and you can also use a circuit tester like # PTW2993 to test the contacts at the tow vehicle's trailer connector. With the trailer disconnected, you can test the pins for the taillights (typically the pin at the 11-o-clock position on a 7-way - see photo) to see if the proper signal is present at the vehicle. If not, you will need to check your vehicle wiring harness. A video on the tester is included.

Check the condition of the electrical contacts on both the vehicle and trailer connectors for any signs of even slight greenish or whitish discoloration, either of which can indicate corrosion that could easily cause the problem.

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