Troubleshooting Horse Trailer Brake Lights and Turn Signals

Updated 07/05/2017 | Published 07/02/2017

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Hook Probe Circuit Tester

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$14.40

Dielectric Grease for Electrical Connectors, 2 oz.

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$11.41

Question:

The rear lights on our 2004 kiefer built horse trailer were slightly flashing when left or right turn signal engaged, now, they dont work at all...testers indicate the everything getting power, but when we plug in the light units self contained , not individual bulbs and replace them in the trailer, everything goes dark. Running lights work fine...when one tail light is attached to connectors and allowed to hanginch it flashes, left or right, doesnt matter...place it back in its place, quits altogether.... Tow vehicle is a Ford F250 Diesel Super Duty. Would relay switches have anything to do with this problem...

asked by: Karen

Expert Reply:

The single most common cause for odd behavior from trailer lights is a weak or intermittent ground connection. This ground issue can be on the truck's main wiring harness ground, the trailer's main ground or at the individual light fixtures. Lights that use a mounting stud as their ground can have issues from paint, dirt or corrosion on the trailer that prevents a clean solid ground connection. Lights that use a separate ground wire may have a break in the wire or similarly have an issue due to paint, rust or grease at the grounding point. If you find and correct a bad ground, you can protect it with dielectric grease # 11755.

It is usually best to test the vehicle and trailer separately when you do this kind of troubleshooting.

If you have a circuit tester like # PTW2993 you can test the truck's connector for the correct signals while a helper activates the lighting functions on the truck. You can test the trailer on its own by applying 12V power direct from the truck battery (or a spare) to the connector on the trailer, one circuit at a time. These tests will help you isolate the issue to either the truck or trailer. The linked article on wiring shows the various circuit positions on a 7-way wiring system.

expert reply by: Adam R