How to Determine Function Carried on Each Wire on Trailer

Question:

Im trying to figure my trailer wiring. I have 5 wires. Thick white Thick red Geen Brown Thin red Can you help me understand what is what? Thanks

asked by: Gordie T

Expert Reply:

Thank you for the photo of the wiring on your trailer.

Wire colors alone are not a reliable way to figure out what signal the wire carries but heavier gauge wires are often used for the trailer's main ground (usually a white wire), for 12V auxiliary power for the trailer or for the brake circuit on the trailer (if it has electric brakes) for which usually a blue wire is used.

You can determine what each wire does either by tracing them back to the lights or other things they connect to or you can apply 12V power from a battery to see what function on the trailer gets activated. Although wire colors do not necessarily tell you what function each carries their positions within a 7-way plug like # A7WCB do matter since the function for any given wire needs to be carried on a specific pin on the plug. If you refer to the linked photo and article you'll see which function is usually assigned to each pin on the plug.

First, make sure which wire grounds to the trailer frame; this should be the white wire. Connect this to the battery negative terminal. Then one by one apply the battery + to each of the other wires to see what function becomes active. After you do through this process you'll know what each wire does and where to connect it.

expert reply by: Adam R