Troubleshooting Turn Signals Not Working with Running Lights On on a Utility Trailer

Updated 02/13/2012 | Published 02/10/2012

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Trailer License Plate Mount Bracket, Heavy Duty Zinc Steel

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$7.81

Hook Probe Circuit Tester

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Question:

I was given a used trailer recently. When I connect it to my truck, the running lights, brake lights and turn signals work on both sides when the trucks headlights are off. When I turn my trucks headlights on, the trailers running lights are on, but the turn signals wont work. I rechecked and moved my ground, but to no avail. I figured it had a weak ground. The wiring on the truck side is new and is a T-harness. What do you think is causing the issue?

asked by: Mark

Expert Reply:

First, check the trailer wiring harness on your tow vehicle with a circuit tester, like part # PTW2993, and make sure all of the functions are working correctly without the trailer connected. Make sure to test with the running lights on and off to be sure. If it is not working correctly, you will most likely have to replace the taillight converter. If it is working correctly, turn your attention to the trailer.

It does sound like a weak ground, or a short circuit issue, because when you are applying the most amperage to the system the problem is showing up. I would start out at the trailer lights themselves. Check each bulb for corrosion or improper installation.

Next, check the ground at each light. Make sure the lights mounting studs are contacting bare metal on the trailer frame, if there is not a separate ground wire for each light, because many trailer lights are grounded through the mounting studs.

Next, check the license plate mounting bracket. If it is plastic it could be causing a weak ground. I recommend replacing it with a steel mounting bracket, # LP15SB, if it is plastic.

Next, make sure that the light mounting brackets are connected directly to the trailer frame. If they are not welded to the frame corrosion could be causing a weak ground. Clean the connection between the bolted on brackets and the trailer frame.

Finally, check the wiring on the trailer for possible pinched or rubbed wires that could be contacting the trailer frame just enough that a short circuit is causing an issue when higher amperage is applied. I once had a utility trailer that had a weak short on the running light circuit from a staple that was piercing a wire and contacting metal. When the running lights were on they would work but when I stepped on the brake or activated a turn signal the left side light would quit working completely.