4-Pin Trailer Wiring Install - Diagram & Guide

Most light duty trailers will use 4-pin wiring. This is the standard harness for utility and boat trailers. All lights will work correctly if the connection is correct. Ground wire is the other pin. We have some helpful tips to help you install 4-pin trailer wiring.

You will need to match the wire colors with the frame in order for the wiring to be installed. Some steps will be performed from the trailer and some from the vehicle. Don’t forget to test the connection before you hit the road. 

Let's take a closer look at the wiring. You will also find simple instructions to install the wiring. 

4-Way Trailing Wiring Color Codes

Before starting to understand each color, it is important that you are familiar with its meaning. Although codes may vary between manufacturers, they all use similar generic options. You should still consult the instruction manual before you proceed.

These colors are for 4-pin trailer wiring diagrams:

• White: Ground wire
• Brown: Tail/running lights
• Left turn/brake lamp yellow
• Left turn signal/brake light

The minimum size recommended for the 4-way connector is 18 gauge wire. For the lights, this wire should be used. The minimum 16-gauge ground wire should be used.

4-pin Trailer Wiring Diagram

The following picture shows how to wire a 4-pin Trailer. But sometimes the Car is wired in a different way. It is important that you always use a Multimeter to check the wiring.

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How to Install 4-Pin Trailer Wiring

1. Collect Supplies

Make sure that you have everything you need before you start. To ensure that everything is covered, we recommend purchasing a trailer lighting set.

You may only need to replace some of the wires and connectors. Before you start, make sure you have an inventory. It is important to know what needs to be replaced, and what should remain. 

2. Connect ground

You must connect the ground wire to an unpainted, clear portion of the frame. For the connection, you will need a self tapping screw and a metal ring. Choose a ground location that doesn’t contain any contamination, corrosion or paint. Clean it first if you have to. 

One ground wire runs through the whole wiring harness. This wire is responsible for grounding all lights and features in the harness.

3. Run Wires

The remaining wires can be run across the trailer's frame to reach the lights. Watch that the wires don’t chafe or get caught along with the components of the trailer or you could run into problems.

Run the wires parallel to the trailer frame. You can also drill through the frame to install grommets if needed. You can also use wire clips and zip ties to help with loose wires. 

4. Installation of vehicle side wires

Once the trailer is connected, it’s time to focus on the vehicle. If you have a 4-way connector already installed in your car, then it is ready for action. To tow, simply plug your trailer in the car.

If there is no 4-way connector on your vehicle, it doesn’t take a lot of work to add one. The type of connector you use will depend on your vehicle’s make and model. 

5. Attach Trailer to Vehicle

Now you can connect the wires. You can remove the insulation using your crimper. The wires can be connected securely using a heat gun or butt connector. 

A Y (wishbone-) harness can be used to split the running lights into two wires. One side runs on the driver’s side of the trailer, while the other is on the passenger side. 

You will use the same techniques for grounding the vehicle as the trailer. You must connect the ground wire directly to the frame. You can add dielectric grease to help prevent corrosion. 

You will need to attach the wiring harness to your vehicle's lights. You don’t want to route the power wire from the electrical harness to the battery of your car or your lights will remain on all the time.

6. Try Connection

After everything is done, it's time to check the connections. Let a friend assist you in activating all the lights, while you look from behind. 

Before you drive the trailer, check that the lights are on. If you don't, your safety and security could be at risk. 

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Price of installing 4-Pin trailer wiring

The cost of trailer wiring materials can run between $150 and $300. It will be cheaper to hire someone else to do the installation.

It might only take two hours to install the trailer wiring if you are proficient in basic mechanics. If you can get a friend to help, the process will go much smoother and quicker.’

4-Pin Trailer Wiring FAQs