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

Updated 09/01/2024 | Published 10/30/2018

Question:

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

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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.

expert reply by:
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Adam R
Ed S. profile picture

Ed S.

9/1/2024

This same thing same right tail light staying dimly on is happening to my dump trailer. Emergency brake power source is from on board 12v battery for dump hydraulic pump. The tail light stays lit even when positive battery cable is disconnected. Only when negative cable is removed will the tail light go off. It is not connected to tow vehicle. The tail light is LED. I replaced the broken tail light about a year ago and shortly after discovered the light staying on. I’ve examined much of the wiring which is difficult since it’s in wiring harness loom. Maybe bad light? How could it be on at all with positive disconnected?

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