Wiring Guide for Installing Lights on Trailer with 5 Wires

Question:

I have 5 wires from my trailer but only 4 on my new lights how do I connect them up

asked by: MARCIN C

Expert Reply:

A trailer can have either 4-pole or 5-pole connectors and the number of actual wires can vary too.

For instance, the part you referenced # 16-4B-1, is 4-pole wiring like what is included in a part like # W787274 which is a trailer-side 4-pole setup that carries stop, turn and tail light signals. There are also 5-pole connectors like # HM47910 that add one additional circuit that can be used for trailer brakes or another function.

Then there are also wishbone style harnesses that have a second wire connected to the pin for the running light circuit. The second wire just simplifies hookup for both sides of the trailer. There are 4-pole wishbone harnesses like # A35W42B and 5-pole wishbone harnesses like # A40W5B, both of which have the second BROWN wire on the running light circuit's pin. The linked photo shows a 4-pole wishbone connector with two brown running light wires.

The best way to know what each wire on the trailer does is to test it by applying power from the vehicle battery or a well-charged spare if you have one. This will let you know how to connect each wire to the lights you wish to install. The linked article shows the way each type of trailer connector has its various circuits arranged.

If the linked article does not give you the information you need please tell me more about your lights and perhaps include a photo of your trailer wiring connector.