Trailer Lights Only Work with Headlights Off on a Toyota Tundra

Updated 07/09/2015 | Published 07/07/2015

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Question:

The Trailer signals and 4-ways will only work when headlights are turned off.on my Toyota Tundra. The trucks signals work fine either way. Grounds cleaned on vehicle. No change.

asked by: Greg

Expert Reply:

There are a couple of things that might be causing this problem. First, to isolate where the problem is you can first use a circuit tester such as # PTW2993 to test the vehicle side trailer connector without the trailer connected.

Have someone sit in the cab of your Tundra and operate all the light functions with and without the headlights turned on while you test the trailer connector at the back of the truck. If everything is working fine when you perform these tests, then the problem is likely on the trailer.

But if it is not functioning correctly then the issue is on the vehicle side. If the trailer connector is factory installed, you might need to have a Toyota dealer look at it. If it is an aftermarket trailer connector check all the connections for loose, corroded, pinched or otherwise damaged wires.

If everything is working on the vehicle side then you can turn your attention to the trailer. On the trailer I am thinking that it could either be a ground issue or and overdraw situation. Ground problems normally show up when there is more draw on a system such as when you have the running lights on and then use another function like a turn signal or brake light.

Make sure that the vehicle side and trailer side connectors are clean and free of corrosion inside and out. The main connector grounds should be attached to clean and corrosion free bare metal surfaces.

The light grounds should also be attached to clean and corrosion free bare metal surfaces. If there is anything between a light ground and the metal trailer frame or if the trailer has an aluminum frame, tilt bed, or folding tongue, any of these things can interrupt the ground. Running jumper wires from the light grounds to the main trailer connector ground can help.

And if the trailer has a lot of lights it could be drawing more power than the tow package wiring can handle. To test this you can disconnect some of the lights that operate on the running light circuit. If everything then starts working then the lights were likely drawing too much power for the system. Switching to LED lights on the trailer would solve that problem.