How to Determine the Functions of 4-Way Flat Trailer Connector when All Wires are the Same Color

Updated 05/17/2016 | Published 05/16/2016

Question:

I am restoring an old boat almost 50 years old and its trailer small boat, approx. 14 foot trailer. The problem I have is with the trailer wiring...all of the wires are the same color. I have tried simply plugging them in one at a time and have determined that the lights are all functional. I just cannot figure out which wire is the ground. The trailer wires are inside of the structure of the trailer and I cannot easily see one of them grounded which would solve the problem. Is there another way to determine the ground?

asked by: Shannon W

Expert Reply:

Yes, on a 4-Way flat trailer connector there is one covered pin and 3 exposed pins. The one covered pin is the ground. Next to it is the running light pin. Then the left turn signal/brake light pin. And the last pin is right turn signal and brake lights.

If any of the lights light up when you applied power to the ground then there is a short somewhere that will need to be fixed. That will be easy though since you can just attach a new wire to the ground wire at the back of the connector and attach it somewhere else on the trailer.

With a trailer that old you may want to consider replacing the wiring with something like # HM48240. This is a wishbone harness so it has 2 running light wires that come from the one running light pin. This is so you have a running light wire to run down each side of the trailer instead of having to cut and splice wiring.

expert reply by: Michael H