How to Test Trailer Wiring to Determine Function Handled by Each Wire Color

Updated 09/19/2018 | Published 09/18/2018

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Question:

My trailer has 5 wires 2 yellow 1 brown 1green and 1white . My trailer has no brakes and my 5wire connector I’m trying to hook up has 1 white 1gvreen 1 brown and 1yellow how do I connect the 5wires to those 4 wires? Do both yellow on trailer go to the 1 yellow on piggy tail?

asked by: Mark

Expert Reply:

The wire colors and number of wires on your trailer don't quite match up to what is common but we can help you sort it all out.

The best way to start is to identify what function each trailer wire powers by connecting them one at a time to a well-charged battery.

First, find the wire that connects to the trailer frame since this is the ground. Usually a white wire is ground but it is always possible that alternate wire colors were installed at some point in the past. Whichever wire connects to the trailer frame will be ground.

You can use your vehicle battery (or a fully-charged spare battery if you have one around the garage) along with a set of jumper cables and two short lengths of wire for testing the trailer functions.

Connect the trailer ground wire to the jumper cable negative clip; connect one of the other wires to the positive clip and then connect the two other ends of the jumper cables to the appropriate posts on your testing battery. You should find that one lighting function on the trailer activates from this battery input power. Note the activated function on a piece of tape and apply a piece to each of the trailer wires as you test. Once you're done you will know what each wire does.

If both yellow wires on the trailer activate running lights then both of these should connect to the BROWN running light wire from the 4-pole harness. Usually a brown wire on a vehicle or trailer handles the running lights. Some trailer wiring setups use dual running light wires (one for each side of the trailer frame) and these are called a wishbone harness, like 4-pole # A35W42B. It is not standard but is possible that your trailer was fitted with dual yellow running light wires.

I linked an article on wiring to help you. This includes images of each type of trailer connector and what signal each of those pins/wires usually carries. If you have any trouble making the correct connections please let me know.