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Troubleshooting Trailer Tail Light That Stays On With Trailer Uncoupled and Wiring Disconnected


Ive read a lot and it still stumps me. Its a enclosed utility trailer with a breakaway battery as well as a 12v aux vehicle battery for running interior lights etc. Trailer not plugged into tow vehicle, trailer unhooked off the ball. Right tail lights on but very dim. All lights work well when hooked up to tow vehicle and tested. I did notice that using a multi meter with the ground attached to the ground on the trailer side 7pin plug, and the positive hooked to the 12v prong I get the full 12v. All the rest dont give any voltage as they should when not hooked up to tow vehicle except, the right turn plug is reading about 4volt. I suspect that is why the right tail lights are dimly lit. The left turn plug is reading 0 volt and is not lit at the light. The only thing I did was adjust a side marker light that had a loose ground but it works well now. Any thoughts on why the right tail light on the trailer is getting 4volt? Thanks


Expert Reply:

Since your trailer lights are getting some power even when the trailer is not hitched and not connected to the tow vehicle wiring this tell us that one of the batteries on the trailer, either the auxiliary battery for the lights or the breakaway battery for the trailer's electric brakes, is somehow leaking power to the lights. Most likely there is a short on the power leads to one (or possibly even both) of the trailer batteries.

You can inspect all of the breakaway and auxiliary batteries' wiring from end to end, looking for any signs of damage such as worn or cracked insulation, or wiring that is touching sharp edges of the trailer frame or fixtures. At some point in your wiring setup battery voltage is being shorted to the trailer.

You may be able to repair this fault with electrical tape like # DW04367 (black) or # DW04368 (red) but odds are you will be better off replacing any suspect wiring with new, such as # 10-2-1 which includes a protective sheath. Or you can use wire loom like # 39035 and Loom Clamps # A0500 to protect and secure the new wire you install.

While you are checking your wiring make sure that all battery and light grounds are SOLIDLY affixed to clean bare metal surfaces that are free of corrosion, oxidation, paint and primer.

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