Trailer Left Turn Signal/Brake Light Only Lights Up when Running Lights are On

Question:

Hello and thank you in advance for your help. When I turn the lights on in my truck, all of the trailer lights come one but the left signal light on the trailer contantly has the brake light on. When I activate the left turn signal nothing changes. It doesnt flash and the brake light stays on. When the lights are turned off in the truck the problem light goes out, but does not function with braking nor turn signal. The right trailer light functions as it should. I traced the wiring and could not find any loose ends or shotty wiring. My truck is a toyota Tundra and has a 7 way plug and I use an adapter plug to the 4-way flat trailer plug. Do you think this is a wiring problem or something possibly wrong with the 7-4 adapter? Thanks again.

asked by: Trevor

Expert Reply:

It sounds like the left trailer light might be connected to the wrong circuit since it comes on with the running lights and does not blink with the turn signal activated. Does it light up when you step on the brake pedal? If it does not then it is likely connected to the wrong circuit. If it does light up then there is likely a bad ground.

Ground problems can cause all sorts of weirdness to occur. Make sure the truck and trailer side connectors, including the adapter, are clean and free of corrosion inside where the wires attach and outside on the pins. All of the light grounds should be attached to clean and corrosion free bare metal surfaces.

I have also included a link on testing wiring that should help.

expert reply by: Michael H