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Tips for Installing 4-Pin Trailer Wiring

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Matt is a professional painter who owns and operates his own painting business, specializing in interior and exterior house painting.

4-pin trailer wiring with diagram

4-pin trailer wiring with diagram

Installing 4-Pin Trailer Wiring

The original wiring on my boat trailer was damaged and installed incorrectly by the previous owner. Some of the lights were out, but the light bulbs were fine. Rather than inspecting the old wires that were already corroded and falling apart, I bought new 4-way trailer wiring and rewired everything myself.

Installing 4-pin wiring on a trailer is an easy project that takes a few hours or less. I had no choice but to install the wiring while my boat was on the trailer, and having to work underneath my boat made the job a little more difficult, but if your trailer is empty, installation is much easier.

Test the 4-Pin Connector on Your Vehicle

Whether you're troubleshooting loss of power to lights on your trailer, or installing new wiring, test the connector at the back of your vehicle first to make sure it's working correctly. If your harness isn't working, a disconnected ground wire underneath your vehicle may have caused the power to go out, or the wires may be faulty and need replacement.

On my truck, not only was the trailer wiring bad, but the wiring on my truck was faulty too. Initially, I bought a cheap $4 connection tester from Harbor Freight that indicated the wires were working, but I found out later that the tester was obviously wrong. Avoid these cheap connection testers.

The best way to make sure the harness wiring on your vehicle is working properly is to test each pin on the connector plug with a circuit tester, or have a friend connect their trailer plug to your vehicle. If you're troubleshooting bad wires, the circuit tester helps determine if the problem exists within the vehicle wiring, or the trailer. In my case, I had to get new wiring installed on my truck.

4-pin connector

4-pin connector

Trailer Wiring Kit and Supplies

Buy a 4-pin wiring kit with wires at least twenty feet in length, so it's long enough for your trailer. The most common wire thickness for a trailer is 16 gauge, but I bought thicker wiring for added durability. Depending on the size of your trailer and the number of lights, you might want to check on the Internet for the recommended gauge for your specific trailer.

Tools and Supplies List:

  • 4-pin wiring kit (green, yellow, brown, white wires)
  • Crimping tool combo with a cutter and stripper
  • Waterproof butt connector with heat shrink ends
  • Heat gun
  • Zip ties
  • Plastic wire tubing
  • Terminal connector
  • Power drill and small drill bit

Some trailer wiring kits include the connectors and supplies needed for installation, but some don't. You'll still need to buy a heat gun and crimping tool separately. Most kits include metal clips for attaching the wires to the frame of your trailer.

You can buy the tools and supplies for this project for under $100. If you don't want to buy a crimping tool, you can use pliers to crimp the wire connections together, but using a crimping tool combo with a wire stripper and cutter comes in handy, and these tools are relatively inexpensive.

The zip ties help keep the wires together so they don't separate or hang below the frame. The plastic tubing is optional, but it helps protect the wiring from abrasion. The kit I purchased didn't include connectors so I purchased butt connectors with heat shrink tubes on the ends. The wires are inserted into the connector, crimped, and the heat shrink ends melted with a heat gun to conceal the wires.

4-pin trailer wiring diagram

4-pin trailer wiring diagram

Install the Wiring

Referring to a wiring diagram for a trailer comes in handy during installation. The white wire is your ground wire, which must be cut short and attached to the front of your trailer frame with a screw to power your lights. The remaining wires get installed underneath. The yellow wire powers your left turn signal, with green powering the right turn signal. The brown wire is for the side marker lights and rear brake lights.

Position Your Wiring

If possible, feed the wires through the hollow opening of your trailer frame in the front where the trailer attaches to the ball hitch on your vehicle. This helps protect them, and it looks better than wrapping them around the front of your trailer.

I ran my wires through the hollow part of the frame and down one side of my trailer, with spliced wires running across the frame to the two lights on the other side. You can also separate the wires up front, feeding green down one side and yellow on the other. Make sure the connector plug up front has enough slack to reach the plug on your vehicle without being too tight or too long.

Keep separated wires together by zip-tying them where needed. This also makes it a lot easier to feed the wires through the frame. Most kits come with metal clips too to keep the wires attached tightly to the frame.

Attach the Ground Wire to the Trailer

The white wire must be attached to the trailer in order to ground and power the lights. Cut the white wire short enough for attachment to the front of the trailer near the connector plug. One to two feet is good.

Strip the wire end back about a 1/2-inch and insert it into a terminal with a heat shrink tube. Crimp the connection and melt the heat shrink tube with a heat gun. Drill a small pilot hole into the frame and attach your ground wire with a stainless steel screw. If the screw is loose, use a washer to keep it in place.

Connect the Brown Wiring

The brown wire needs to connect to both side marker lights, as well as your rear lights. You'll need to cut the main brown wire near the marker light wire, strip the ends back, twist both together, and insert the twisted wires into one end of a butt connector. I used butt connectors with heat shrink tubing on the end.

Measure how many feet across to the other marker light and cut off a spare piece of brown wire to the measured length. Connect one end of the spare piece to the other marker light wire, using another butt connector. Twist the other end together with the main wire that was cut and insert them into the other end of the butt connector. So now there should be two twisted brown wires at each end of the butt connector to complete the connection. Crimp the connector and heat shrink it. Repeat the same process in the back of the trailer, connecting the remaining brown wire to the brown wiring coming from the rear lights.

Connect the Remaining Wires

Simply connect your yellow wire with the yellow wire coming from the left tail light, doing the same with the green wire and right tail light. Fasten the metal clips to the frame to prevent loose wires from hanging down. If you wired everything correctly, the trailer lights should be fully functional. If the trailer lights are old, double-check the bulbs.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

Question: My wires are white, green, brown, and yellow. But in the lights, I have yellow, black and white. Where do they hook up, or which color to which color?

Answer: Please refer to the trailer wiring drawing in the article that illustrates how the wires should be connected. The white wire coming from your new harness is your ground wire that needs to be fastened to the trailer up front to get your ground. It doesn't connect to any lights. The white wire is usually very short in length. All of the lights on the left side use the brown and yellow wiring and the right side is green and brown. Your old trailer wiring was either installed wrong, or the color of the wiring has faded and worn off.

© 2018 Matt G.