In the article below, we will explain how to hook up 5-wire flat trailer harness.
4-way flat wire harnesses are typical in trailer electrical connections, but their 5-wire counterparts have an added advantage.
They feature an extra wire to run one additional feature in your trailer.
6-pin and 7-pin wire harnesses also exist, but the 5-wire type covers all the basics.
Let’s look at the functions of these connectors/wires and how to hook up the 5-wire type to your trailer.
Table of Contents
- Trailer Wire Color Coding
- Recommended Wire Gauge for 5-Wire Flat Harnesses
- How To Hook Up a 5-Wire Flat Harness Trailer
- Wrap Up
Trailer Wire Color Coding
The most basic 4-wire trailer harness has the following color code.
A car trailer towing electrical cable
| Pin Color | Function and Description | |
| Pin 1 – White | Ground (connects to the trailer’s frame to ground all trailer electrical functions) | |
| Pin 2 – Brown | Power wire for tail lamps, side markers, and running lights | |
| Pin 3 – Yellow | Multi-function wire for the rear left side tail lamp (left turn signal and brake lights) | |
| Pin 4 – Green | Multi-function wire for the rear right side tail lamp (right turn signal and brake lights) | |
5-pin trailer harnesses have an additional blue wire for powering the braking system. Some trailers have electric brakes, and this wire supplies electrical current to this system.
But it can also be for hydraulic brake disengagement. In some trailers with no electric brakes, the pin can provide an electrical link for the reverse lights. However, this function is uncommon and usually comes into play when using the 7-wire harness.
The 6-wire trailer harness adds a red or black wire that provides 12-volt power from the car battery. This +12V auxiliary power pin enables trailer battery charging or the running of 12V accessories.
Trailer coupling with the harness connected
Lastly, the 7-pin trailer harness has an additional purple wire for powering backup lamps in tail lights or hydraulic brake disengagement.
Recommended Wire Gauge for 5-Wire Flat Harnesses
The wires transmitting these electrical signals should be at least 18-gauge, but the ground wire should be slightly thicker (16-gauge).
However, we recommend using 16-gauge for the rest because they are sturdier and provide a broader surface area for making splice connections.
But there is an exception. The braking system is relatively more power-hungry than the rest, meaning you should use a thicker wire. A 14-gauge wire can work, but a 12-gauge is better.
And if upgrading to a 6- or 7-wire, use a 12-gauge cable for the auxiliary power line.
4- and 5-pin trailer wiring usually have flat connectors, but 6- and 7-way trailer connector sockets are round.
How To Hook Up a 5-Wire Flat Harness Trailer
We’ll split this process into trailer and car sides.
Trailer Side
Step 1: Prepare the Trailer for Wiring
Once you’ve bought the 5-wire flat trailer harness, check if you have the complete trailer kit to utilize the five electrical connections.
We recommend wiring a trailer with electric brakes to put the blue wire to good use. Also, ensure all the lights are okay. If they need replacements, buy them.
Step 2: Connect the Ground Wire
The ground wire color is white. Connect it to the trailer’s frame on a bare metal section using a ring terminal and self-tapping screw.
This setup allows you to ground each electrical component to the chassis instead of running multiple, lengthy negative wires. Plus, it minimizes the risk of having a single component’s ground issue affecting the others.
A person repairing trailer light wiring using a pair of pliers
To ensure the effectiveness of the ground wire, apply dielectric grease to prevent build-up or corrosion at the connection. And the connector should be at least two feet from the tongue.
Step 3: Route the Wires
Next, install the wires along the trailer’s frame while avoiding chafing on other parts. Use any hollow sections in the frame to keep the harness safe from the elements. If there is no access to the cavity, drill tiny holes and use grommets to keep moisture out.
The alternative is to use zip ties or wire clips to secure the harness to the frame and prevent loose hanging.
You can route the wires on one side or split them to have brown, yellow, and blue on the left and brown, green, and blue on the right.
Step 4: Link the Wires
Each rear light should have three wires. Connect one to the brown wire for powering the tail lamp and running light.
The other should link to the green or yellow wire (depending on the side) for the brake and turn signal lights, while the last cable goes to the trailer chassis for grounding.
The individual wires in a trailer harness (note the color coding)
Use a wire stripper, crimping tool, and waterproof connectors (or regular ones with heat-shrink tubing) to make these electrical connections.
You can use a traditional 5-pin trailer wiring or a wishbone (Y-harness). The latter is better because it splits the brown power line into two after the tongue, simplifying left and right tail light wiring.
Car Side
With the trailer side ready, do the vehicle wiring using these steps.
Step 1: Prepare the Car
Some vehicles have 5-pin connectors that simplify the installation. However, if your car has a 4-way connector, install a 5-pin adapter. And if it doesn’t have any connector, install a 5-way type. Alternatively, you can proceed to step two to make the connections.
Step 2: Connect the Ground Wire
All cars have their battery’s negative terminals connected to the chassis. Since the trailer will use power from your car, you have to transfer its ground connection to your vehicle’s frame.
Find a clean, bare metal surface on your car’s chassis, then connect the white wire to that section using a ring terminal and self-tapping screw. Apply dielectric grease on this surface to prevent corrosion.
Step 3: Connect the Wires
Remember to carry out these wiring steps only if your vehicle has no adapter. And if it has a 4-way adapter, you only have to take care of the blue wire. Let’s start with this fifth wire.
After installing the 5-pin adapter, the blue won’t have a power connection. If using it for the trailer’s reverse lights, link it to your car’s backup light circuit.
A person repairing damaged trailer wires
But if using it to power the braking system, we recommend running a separate wire directly to the brake controller.
We also recommend installing separate wires to the battery for the tail lights. Linking them directly to your vehicle’s tail light circuit can overload the cables, making them burn.
How To Hook Up 5-Wire Flat Trailer Harness: Blue Wire Notes
Be careful when using this fifth wire because it can have different functions depending on the trailer.
If not for the braking system, you can use this cable to disengage the hydraulic brakes when reversing. In such a case, ensure you label the wire and connect it to the reverse circuit on the car and trailer.
But it is better to use the purple wire in a 7-way connector.
An SUV towing a trailer with a motorboat onboard
Wrap Up
The most vital factor to consider when using a 5-way connector is to ensure the trailer and car wires have matching functions.
4-wire trailer harnesses offer basic functionality, so the blue wire makes the difference when switching to five cables. Ensure you label the cable’s purpose and connect it to the correct circuit in your vehicle.
You can learn more about troubleshooting trailer wire harnesses in this article and comment below if you need further clarification.
