How to Determine Wire Functions on a Tandem Axle Dump Trailer

Question:

I have a tandem dumping truck with a 7 pin and the trailer I am hooking it to only has 5 wires It has 1 yellow 1 green 1 Brown 1 black 1red I assume the Brown is tail/ running lights the green is right turn and the yellow is left turn They red goes to the brakes on both axles and the back was grounded to frame The problem I am having is when I wire the green yellow and Brown to truck I have right turn left turn and tail lights but where do I wire the red off of truck that is for brakes to brake lights on trailer

asked by: Marty R

Expert Reply:

Trailers in the US have combined turn signals and brake lights on the same circuit. So it is likely that the yellow wire is left turn/brake lights, green is right turn/brake lights and brown is running lights. That leaves black and red. One of them is the ground, likely black, and the other is for another function, possibly 12 volt power or for an electric reverse lockout if the trailer has hydraulic brakes.

The easiest way to determine wire function would be to run power using a separate 12 volt battery. Touch each wire to the positive terminal and see what lights up, turns on, etc.

I have included a link to our help article on trailer wiring that shows how various trailer connectors are wired. Wire color is not set in stone though so testing is the preferred method of determining wire function.

expert reply by: Michael H