Wire Functions of Trailer Stop, Turn and Tail Light ST-31RB and Causes of a Dim Light

Question:

i just bought a trailer light, has 2 wires!!! the incoming connecting wire has 3 wires: black red/yellow red/white how do i figure out how to wire this. i figure, black is ground, i gather ill ground that to the base or screw. but which of the wires is the running or brake??? when trying to connect one its really bright as if brake light is on, in otherwise doing opposite as it should: i hit the brake it gets dimmer, i let go of brake gets bright! now, when switching wires, at times its not even lighting at others, it just stays dim. it was working before i tried replacing with new light??? confused, lost, need to use my trailer... thanks

asked by: Lou

Expert Reply:

It sounds like there is a ground problem and that is what is causing the light to be dim or otherwise not function correctly. 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.

As for the wire functions light # ST31RB, like most trailer lights, has one wire for running lights and one wire for turn signal and brake lights. It grounds through the mounted hardware. The light black wire is for the running lights and the red wire for turn signal/brake lights.

expert reply by: Michael H