Four Pin Trailer Wiring Install (Wiring Diagram & Info)

Trailer Wiring Install

Many people often have difficulties with the wiring of their trailer, and even after several attempts they still don’t seem to get it right.

The regulations for driving trailers stipulate that when driving the trailer the lighting must harmonize with the vehicles. This means that when you apply the brakes or switch on the signals, the lights of the trailer should mirror these signals. This helps you to communicate with other drivers wherever you are going.

When you buy a trailer, the on-board power supply should be connected to the vehicles via a plug or socket. Modern cars use a converter for this. Some trailers still use the two-wire system. In the onboard network, signals for braking and stopping are sent via one wire.

Three-Wire System

With a converter, you can send signals for stop, turn, and trail. The system is present on many trailers and uses three wires. The wires are connected to a converter.

Four-Wire System

With this system, you have four wires that work together with the vehicle’s electrical system via a plug connector. The wires are colored differently, using white for ground, green for right turns and brakes, yellow for left turns and brakes, and brown for the tail lights.

Installing your 4-pin trailer wiring system is easy if you follow the correct procedure. The first thing you need to do is make sure that your connector works properly. If it is not, no amount of wiring will get the lights to work. Make sure that all cables are electrically conductive.

One way to check for faulty wires is to use a circuit tester. This is connected to each pin of the connector and should make it easier to find the faulty wire. If the problem persists, you may need to rewire your trailer.

When buying wires for trailers, make sure they are the right thickness to increase durability. A thickness of 16 is ideal. The 4-pin trailer uses yellow, brown, green, and white wires.

Installing the 4-Pin Trailer Wires

You must check the trailer manual to see if the wiring is correct, but normally the white wire is called the ground wire, while the brown wire is used for tail lights. Yellow and green are for left and right turns and braking.

Start by cutting the white wire and attaching it to the trailer frame. The rest of the wires are wired from below.

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

If you decide to separate the wires, make sure you connect them with cable ties. This has the added advantage that you can add more wires to the frame.

4-pin trailer wiring diagram

Four Pins Connectors

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Power & Ground

The white wire must be attached to the trailer for the grounding and power supply of the lights. This is done by cutting the wire about half an inch back and attaching it to the shrink hose of the trailer. You must heat the surface with a heat gun and then drill a hole in the hose. Attach the ground wire with a stainless steel screw.

Rear Lights

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

Marker Lights

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

The other wires are connected in the same way as the brown wire, by connecting the wires of the same color to the corresponding wires from the tail light. You can attach metal clips to secure the wires in place and prevent them from hanging loose.

If you find that the trailer lights still do not work, but the wiring is OK, check the trailer lights, and make sure they are not burnt out.

Other trailer wiring systems

Most modern trailers today are connected to a Pulse Width Modulation (PWM) system. This system allows you to make multiple connections that are routed through a single line. The system varies the signal intensity and thus controls the lighting. PWMs are often divided into two systems: ST systems and STT systems.

In ST systems, one wire controls the tail lights and brake lights and another wire controls the left and right turn signals. In STT systems, a single wire connects the brake lights, indicators, and tail lights.

An electrical converter is often used for those vehicles that do not have a customized converter. The purpose of the electrical converter is to improve the compatibility between your simple trailer wiring and the complex vehicle wiring.

When making connections, you must distinguish between the use of plugs and sockets. When we refer to the socket, we are talking about the side of the vehicle where we make the connection, while the plug is the trailer side. For a boat trailer, we use a four-way cable system; for a boat, we use a five-way system; for a utility trailer, we use a four-way system; for a caravan trailer, we use a seven-way system; and for a trailer with five wheels, we use a seven-way cable system. It is extremely important that you familiarize yourself with the wiring system by referring to the manufacturer’s manual.

In these scenarios, it is very likely that your trailer has a different type of connector than your vehicle. You can bridge the gap by purchasing an adapter. Most adapters are plug-and-play adapters, but you may need to ground some wires.


Wiring your four-pin trailer can be a challenge when you first do it, but it’s a simple do-it-yourself task. It gets complicated when you have trailers with more cables, and in this case, you need an adapter to make the connections. The first step in wiring your trailer cables is to ground the white cable first.

Feed the rest of the wires through the trailer frame. Disconnect the wires to prevent them from dangling. After you have attached the marker lights to the bolts provided, you can install the rear lights. If your lights do not work after installation, the problem may be due to the rear light bulbs.

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Written by:

Magnus is the owner and main author of MechanicBase. He has been working as a mechanic for over 10 years, and the majority of them specialized in advanced diagnostics and troubleshooting. Certified Automotive Diagnostic Technician.