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Ways to Install New 4-Pole Trailer Connector

Question:

I have to replace the connector on my trailer which is a four way flat. The wires coming off trailer are white, and a yellow/brown, and green/brown. How do they tie in with the new connector which has a white, brown, green , and yellow wire?

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Expert Reply:

Four flat trailer wiring, also called 4-pole wiring, uses four wires and pins configured as shown in the linked photo of a trailer side 4-pole connector.

The one pin that is different from the other three (on both trailer and vehicle side connectors) will be the ground pin and this usually has a WHITE wire that will ground to a clean bare metal spot the trailer frame with a ring terminal.

The next pin is for the running light circuit and this typically uses a brown wire to activate the tail lights on both sides of the trailer. The next pin is for the LEFT combined stop/turn signals and this is usually assigned to a yellow wire. Finally the 4th pin carries the same combined stop/turn signals for the right-hand lights.

On occasion a trailer can be fitted with a 4-pin plug that actually has 5 wires. This is called a wishbone harness and it uses dual running light wires connected to the one pin to simplify connections. An example is part # A20WB. This is still a 4-pole setup despite the 5th wire.

If you have only three wires on the trailer then something is missing. I suggest you trace your trailer harness to see where all wires are routed. This will help you find the 4th wire and also let you confirm which function each wire connects to. Although there are wiring conventions, such as a running light circuit using a brown wire, there is never a guarantee that all trailers will follow these conventions.

You may care to bypass all the existing wiring and install a complete lighting/wiring kit like # TL29BK or an LED version such as # TLL9RK.

If not you can use a junction box like # 3375601100 along with 4-pole bonded wire # 16-4B-1 and ring terminals # DW05702-1 and butt connectors # DW05744-10 to connect all the existing lights to a new 4-pole such as # W787264.

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