Connecting a Trailer Plug: Step-by-step Guide

Are you looking to connect a trailer plug? Not sure which wires attach to which connector on your trailer plug? We get it! It can be confusing with all the different wire colors and connectors.

Complete with a detailed trailer wiring diagram for each type of trailer plug, this guide will show you how to connect a trailer plug wiring the right way, including the different types of trailer plugs and vehicle connections.

Different Types of Trailer Plugs & Wiring Diagrams

Trailer plugs come in a variety of shapes and sizes and are available from four to seven pins, but the basic purpose of each remains the same. By law, any vehicle towing a trailer must be connected to the tow vehicle's wiring system in order to provide power to the trailer tail lights, brake lights, turn signals, and any other necessary electrical systems.

There are several standards for trailer wires, and each has a corresponding trailer wiring diagram. Below you'll find the corresponding trailer wiring diagram for your plug, which should help you in identifying any wiring issues you may have with your trailer. Moreover, these standards are universal and apply to any trailer plugs.

4-Pin Connector Wiring Diagram

The 4-pin connector, also known as a 4-way connector, is the simplest scheme of trailer plugs. At the very least, all trailers need 4 functions, these being:__ brake lights, tail lights, and the left and right turn signals__.

The 4-pin trailer plug type has three pins and one socket - this socket is considered the 4th pin. Generally, there are two types of 4-pin connectors available:__ flat__ and round. You'll typically find this type of connector on a small camper, utility trailer, or boat.

The following wires are used in a 4-pin connector:

• The white wire is the ground wire - connected to the trailer frame.
• The brown wire delivers power to the marker lamps, such as the taillights, running lights, and side marker lights.
• The green wire delivers power to the rear right lamp for turning and stop indication.
• The yellow wire delivers power to the rear left lamp for turning and stop indication.

5-Pin Connector Wiring Diagram

A 5-pin connector's wiring diagram is very similar to 4-pin's wiring diagram, but it adds a connection (blue wire) for the electric braking system. If your trailer has brakes (surge brakes or hydraulic brakes), then it needs a 5-pin connector.

Note that not all trailers have reverse lights, so consider your trailer as you wire a 5-pin plug.

The following wires are used in a 5-pin connector:

• 1-4 wires (White, Brown, Yellow, & Green).
• The 5th is a __blue wire that powers __the electric brakes or hydraulic reverse disable.

6-Pin Connector Wiring Diagram

A 6-pin connector is often used with gooseneck trailers, as well as 5th-wheel, utility, and boat trailers. This type of trailer plug introduces two new functions, a wire for +12-volt auxiliary power and a wire for connecting trailer brakes. Ultimately, this connector allows for use with a brake controller.

The following wires are used in a 6-pin connector:

• 1-5 wires (White, Brown, Yellow, Green, & Blue).
• The 6th is a __red or black wire __for battery charging and other accessories.

7-Pin Connector Wiring Diagram

The 7-pin trailer plug is found on most recreational vehicles and is used on larger gooseneck, boat, 5th-wheel, and utility trailers. These plugs come in two variations, 7-pin round and 7-pin RV Blades - although these two look the same, the wiring connections and placement are different.

With a 7-pin trailer connector, it is okay to leave a pin or two unused and unconnected (should your trailer have a 5-pin or 6-pin plug).

The following wires are used in a 7-pin connector:

• 1-6 wires (White, Brown, Yellow, Green, Blue, & Red/Black).
• The 7th is a __purple wire __for backup lights (this can sometimes be another color).

Trailer Wiring Diagram & Connector Application

This trailer wiring chart is a typical guide. Wire colors may vary based on manufacturers. If you're unsure, use a circuit tester to check connections.

This color chart is universal for most trailer connectors:

• White wire = Ground wire
• Green wire = Right rear lamp
• Yellow wire = Left rear lamp
• Brown wire = Marker lamps
• Blue wire = Trailer brakes
• Red or Black wire = Trailer battery charging
• Purple wire (or another color) = Backup power system

Steps for Connecting A 7-Pin Trailer Plug

Now that you understand the different trailer lighting functions and auxiliary functions of each trailer connector, it's time to connect one.

The approach for you depends on your electrical needs and what trailer connector you have. To start, every trailer needs lights. Some trailers may also need side markers and running lights and others may need electricity for their brakes — to actuate electric brakes or disable hydraulic brakes when reversing.

For this step-by-step guide, we will be connecting a 7-pin trailer plug. These are the most commonly used trailer connectors.

Step 1: Prepare for wire installation

Start by making sure you have everything you need to connect your trailer plug:

• 7-pin Trailer Plug & Cord
• A Trailer Wiring Diagram
• Wire Strippers
• Phillips Head Screw Driver
• Flat Head Screwdriver

Step 2: Open the trailer plug

Unscrew the nut from the base of your new trailer plug and undo the clip (or unscrew the screws holding the plug together) to open the plug. In the meantime, slide the nut onto the trailer wiring cord.

If the trailer wiring cord has not been pre-stripped, you can go ahead and gently slice open the outer rubber shielding with your wire cutters at about 0.5 to 1 inch to expose the colored wires.

Step 3: Strip the colored wires

Some trailer wiring cords will come with the colored wires pre-stripped. If they are, you can skip this step.

Individually separate each wire so that you have some leverage to work with. Using your wire strippers, strip the wire shielding from each existing wire by a half inch.

With all colored wires stripped, you want to twist the ends of each wire to ensure the cable stranding does not separate.

Step 4: Insert the cord into the trailer plug and loosen plug head screws

After you've stripped back all your wires, take your trailer plug and slide the trailer wiring cord with the exposed wires through the end of the plug housing. Doing this step before connecting each wire will make your installation easier.

Once you have your wires at the end of the plug housing, take your flat head screwdriver and loosen all the screws around your plug assembly to make space for the colored wires.

Step 5: Connect colored wires to terminals

Some trailer plugs will either have a color or number system indicating which wire goes into which terminal. To ensure you avoid wiring problems, refer to your trailer service manual and plug installation instructions to see which number corresponds with which color.

Following the number or color code, place each colored wire into its corresponding terminal and tighten the screws. You may find it easier to connect the center wire first. Remember that this color can be different depending on your 7-pin plug.

Tip: To verify connections, you can use a circuit tester before crimping each colored wire into the terminals.

Step 5: Assemble plug over wires

Once all wires are connected, it's time to put the trailer plug housing back together.

Bring the plug housing back up the cord into its original position over the terminal assembly with the colored wires. Align the slot in the cover with the groove in the plug to ensure all colored wires in the cord connect to the correct terminals inside.

Now close up the plug. Some trailer plug housings will just click together while others need to be tightened with screws.

Screw the nut for the base of your trailer plug and your installation is complete!

Step 6: Test the plug

Your final step is to test your trailer plug. If your vehicle already has a 7-way connector, then simply plug the trailer-end connector into the vehicle-end connector.

Different Types of Vehicle Connections

Your trailer wiring system will either plug, clamp, or splice into your vehicle's already existing lighting.

Plug-In Style

Some vehicles may not be equipped with a standard trailer wiring connector, and instead, the vehicle manufacturer has "pre-wired" the vehicle with a special socket designed for installing wiring.

Here you can simply plug your trailer connector into the plug-in location. This can typically be found near the tail lights underneath the vehicle or behind the paneling in the back cargo area.

If you would like to expand to a different trailer connector (5-pin, 6-pin, or 7-pin trailer connector), you can connect a T-connector into your vehicle's existing wiring and then connect this to your trailer with a wiring adapter.

Clamp-On Style

Other wiring harnesses clamp onto your vehicle's existing wiring without causing feedback, power draw, or interference from your vehicle's wiring system.

With this style, you clamp the wiring harness's sensors to the appropriate vehicle wires and then run the hot lead (this will be the red or black wire for the trailer battery charging) through to your vehicle's battery.

Splice-In Style

Electrical converters splice into your vehicle's wiring system and provide a standard trailer wiring connector - this converts your vehicle's wiring system to be compatible with your trailer's wiring system.

After verifying your wire functions, you can connect the wires using one of 3 methods:

1. Solder: Soldering the wires together with a solder gun creates a strong, more reliable connection.
2. Crimp butt connectors: If you're not able to solder the wires together, you can simply heat shrink the butt connectors with a heat gun to create watertight seals.
3. T-Tap: One of the easiest and quickest ways for connecting wires is with a T-tap, also known as a quick splice. This forces a metal piece into two separate wires to connect the circuit. Note that although the easiest, this method is the least reliable.

Looking For More Information On Trailer Plugs & Wiring?

Haven't found what you're looking for? Take a look at our other articles on towing and trailer wiring:

• Replacing A Trailer Plug: Step-by-Step Guide
• Article (link to other articles on client website)
• Article (link to other articles on client website)
• Article (link to other articles on client website) etc.

Closing Thoughts

Although it seems like a lot of information and work, connecting a trailer plug is more simple than you'd think!

Always refer to your wiring diagram when wiring and connecting your trailer plug. It will save you the frustration of connecting the wrong wires to the wrong connectors.

Depending on which trailer you own and what lighting functions you'd like it to have, know that there are different types of trailer plugs and by using this guide, you'll be able to quickly identify which plug is the perfect fit for your specific tow vehicle and trailer.

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