Passenger Side Running Lights On Boat Trailer Have No Power But Stop/Turn Lights Work

Question:

I just purchased a boat and the passenger side running lights do not turn on. The passenger side tail light turns on when brakes are applied and when the blinker is on but not when running lights are on. I cleaned the ground on each of the running lights and still found no voltage coming from the multimeter. What should I do next? Thanks!

asked by: Benson H

Expert Reply:

Running light signals on a trailer are carried by a different wire than the one that carries stop and turn signals. On a 4-pole flat wiring harness like Optronics # A35W42B the brown wires carry running light signals and the yellow and green wires carry left- and right-turn signals and brake lights, respectively. The linked article on trailer wiring shows this and other connector types and the function typically carried on each wire.

Checking your individual lights' grounds is a good start. A weak ground can let some lights work while others will not. And you can also check the main trailer ground where the wire from the connector attaches to the trailer frame to make sure it is solidly attached to a clean bare metal surface. Clean that spot of rust, if necessary, or simply move that main ground wire to a new clean spot on the trailer frame.

You can also try testing the trailer light directly with a 12V battery, without the trailer and vehicle connected. This will help you isolate the issue to the trailer or to the vehicle. You can use your car's 12V battery or a spare if you have one. Simply use some wire to connect the battery negative terminal to the ground pin on the trailer connector and then one by one connect the positive terminal to each of the circuit contacts. Again the linked article will help you here. If your running lights operate when tested in this manner then you know the issue is likely to be in the tow vehicle's wiring.

If the running lights do not work then you can check the condition of the metal contacts on the trailer's connector. Look for even slight whitish or greenish discoloration as this can indicate corrosion bad enough to block signal transmission. You can use a wire brush and/or spray-on contact cleaner to remove any corrosion or dirt. Once you've corrected the fault I suggest an application of dielectric grease # LT37534 to keep dirt and moisture out of the contacts.

expert reply by: Adam R