Function of Black Wire on 4-Pole Trailer Wiring

Question:

The plug coming out my truck is green white brown and yellow. The one coming off my trailer is black brown yellow green. What is the black wire for?

asked by: Kevin

Expert Reply:

Usually a white wire on a truck or trailer harness is a ground wire. But it is certainly possible that the black wire on your trailer was substituted.

You can test the trailer with a spare 12V battery (or the one in the truck) to see what function is activated when you apply power. You can try using the black wire as ground when you test. Connect black wire to battery negative and connect one of the others to the battery positive. If the light function works then you know the black wire is ground.

You can also trace it to see if it terminates in a connection on the trailer frame; if so it is indeed a ground.

The linked article on trailer wiring may helpful to you.

expert reply by: Adam R