Four Pin Trailer Wiring Install – Wiring Diagram & Info

In Electric by Magnus SellénLeave a Comment

four pin trailer connectorA lot of people often struggle with the wiring for the trailer and even after several attempts they still seem not to get it right.

Trailer driving regulations stipulate that whenever you are driving your trailer the lights should be in harmony with the vehicles. This means when you apply brakes or turn on the signals it should be reflected on the trailer lights. This helps to communicate to other drivers where you are heading.

When you purchase a trailer the wiring system should be connected to the cars through a connector or a plug and socket. Modern cars use a converter for this. Some trailers still use the two-wire system. In the wiring system signals for braking and stopping are sent along one wire.

Three wire system

Using a converter you are can send signals for stop, turn and trail. The system is in place for many trailers and uses three wires for this. A converter is used to connect the wires.

Four wire system

In this system, you have four wires working together with the car’s wiring system through a connector. The wires are colored differently with the white being used for ground, green for right turn and brake, yellow for left turn and brake, and brown for the tail lights.

Installing your 4 pin trailer wiring system is simple if you follow the right procedure. The first thing you need to ensure is that your connector is working fine. If it is not, no amount of wiring will make the lights work. Ensure that your all wires are conducting electricity.

One way to check for faulty wires is by using a circuit tester. You plug this to each pin in the connector and this should easily troubleshoot the faulty wire. If the problem continues you may need to redo the wiring on your trailer.

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When purchasing trailer wires ensure that they are of the right thickness for added durability. A gauge of 16 is ideal. The 4 pin trailer uses yellow, brown, green and white wires.


Installing the 4-pin trailer wires

You will need to check the trailer manual for the correct wiring but typically the white wire is known as the ground wire while the Brown for tail lights. Yellow and green are for left and right turn and brakes.

Begin by cutting short the white wire and attaching it to the trailer frame. The rest of the wires are wired underneath.

Find a suitable entry point in the trailer for running the wires. This should be a position that protects them from damage. Hollow parts are recommended. It is optional to separate the wires and feed them through the trailer to the other side individually.

If you choose to separate the wires ensure you zip tie them. This has the added advantage of letting you add more wires to the frame.

four pin connector

Power & Ground

To ground and power the lights the white wire must be attached to the trailer. You do this by cutting the wire about ½ an inch back and attaching it to the trailers heat shrink tube. You will need to heat the surface with a heat gun then drill a hole into it. Attach the ground wire through a stainless steel screw.

Rear Lights

The brown wire is attached on one side to the rear lights and the market lights. Strip both ends and using a butt connector attach them on both ends.

Marker Lights

You will need butt connectors to connect one end from the marker lights to other ends.

The other wires are connected in the same way as the brown wire by connecting same color wires with the corresponding wires from the tail light. You can attach metal clips to fasten the wires into place and prevent them from hanging loosely.

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In case you notice that the trailer lights are still not working but the wiring is fine, check the trailer bulbs and ensure that they are not burnt.

Other trailer wiring systems

Most modern trailers are today connected to pulse width Modulation system (PWM). This system enables you to make multiple connections which are run through one wire. The system will vary the signal intensity hence control the lighting. The PWN are often divided into two – ST systems and STT systems.

In the ST systems, one wire will handle the tail lights and stop signals while another wire controls the left and right turn signals. In the STT systems, a single wire connects the stop, turn and tail lights.

For those cars, without a custom converter, they often use an electrical one. The electrical converter serves the purpose of enhancing compatibility between your simple trailer wiring with the complex car wiring.

You need to distinguish between the use of plugs and sockets when making connections. When we refer to the socket we talking about the side of the vehicle where we are making the connection while the plug is the trailer side. A boat trailer will use a four-way wire system, a boat 5 way, utility trailer 4-way system, camper trailer 7-way system and 5th wheel trailer 7-way wiring system. It is paramount that you get familiar with the wiring system from the manufacturers manual.

In these scenarios, it is highly likely that your trailer will have a different connector type from your vehicle. You can bridge the disconnect by purchasing an adapter. Most of the adaptors are plug and play but you may be required to ground some wires.


Wiring your four pin trailer can be challenging if you are doing it for the first time but it is a simple do-it-yourself task. It gets complicated when you have trailers with more wires and in this case, you will need an adapter to make the connections. The first step in wiring your trailer wires is to first ground the white wire.

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Run the rest of the wires through the trailer frame. Clip the wires to prevent them from dangling. Once you have installed the marker lights to provided bolts you can then install the tail lights. If your lights do not work after installation then the problem could be with the tail lights bulbs.

Hello I'm Magnus, the owner and the writer of this website. I have been working with cars since I was 16 and I'm specialized with in-depth Automotive diagnostics. Also been driving drifting for the last 6 years. I'm here to give you answers to all your automotive questions and I hope that you enjoy our content.

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