How to Test Old Trailer Wiring to Find Out What Each Wire/Circuit Does

Updated 06/11/2018 | Published 06/07/2018

Question:

I have a white, red, green, pink and black wire on my trailer.....I just need to figure out which one controls what and cannot find anything. Green is supposed to be right Red may be left White ground? But no idea what black and pink are for...

asked by: Rachael L

Expert Reply:

You have discovered the "dirty little secret" of trailer wiring: wire colors are not a consistent and reliable way to figure out what each one does! But it is easy enough to check this simply by applying 12V power to each circuit and noting what function on the trailer comes on.

It is easiest to use your vehicle battery (or a well-charged spare battery) along with a set of jumper cables and two lengths of wire. This makes it simple to apply +12V power to each circuit on the trailer (one at a time, of course) to see what function activates. Then you can label them and complete your wiring.

Usually (but not always) a trailer ground is the white wire, so I would start off your testing with this, but of course you can trace the wire that actually connects to the trailer frame. The ground usually has a ring terminal on it. Once you know which wire is ground you can connect that to battery negative and then one by one apply the +12V wire to the leads on the trailer to see what lights up.

You can also refer to the article on wiring that I included for you; this shows the usual wiring formats for RVs, both on the vehicle side and the trailer side.

expert reply by: Adam R