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Troubleshooting Malfunctioning Trailer Lights On a 4 x 6 Utility Trailer


I recently purchased an older 4x6 utility trailer that the owner had recently put new lights on himself. He said that they worked fine on his vehicle, but I am not sure that I really believe the guy totally. I also bought the #118376 wiring harness for my 2003 Dodge Grand Caravan. This is the first trailer Ive pulled with my van, so I am not sure which part is having the problem, and how to fix it. The passenger side seems to be working just fine. The brake, running, and turn signal lights seem to be operating as expected. However, with the passenger side, I do not get anything from the tail light. However, the side amber running light does work when the lights are on. The other part of this is I thought maybe the previous owner miswired the passenger tail light. So, I switched the wires on the tail light, and that made the turn signal work when the lights were off. However, when the lights were turned on, it stopped working again. I took a look at the troubleshooting video on your website, but I am still not sure which part is most likely affected on my lighting setup. Thanks in advance for your help!

asked by: Chris H

Expert Reply:

T-One harnesses are almost impossible to wire incorrectly, so I would say it is most likely somewhere in the trailer wiring. Whichever side, driver or passenger, is having trouble, start at that tail light and work your way towards the vehicle.

If the light has a ground wire, make sure it has a good clean ground connection, and make sure it is touching metal and not paint. Some lights are grounded by the bolts or screws that attach the light to the frame, so make sure those are clean and solid as well.

I would also check the bulbs and make sure they are not burned out.

Finally, follow the wires up the frame to make sure they are not frayed, or nicked. Since the amber running light works, but the tail light does not, I would suspect your problem is somewhere in between those two lights.

If you have one side that works, and one that does not, use the good side as a map and make sure the bad side mirrors the good.

I will include a wiring diagram for trailers that shows which wire goes where, as well as some helpful videos.

expert reply by: Patrick B

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