How to Wire Boat Trailer Lights

Tom Wiesen

Keep trailer taillights, turn signals, brake lights, and running lights functioning properly for everyone’s safety. This tutorial teaches you how to troubleshoot issues

Learn how to wire boat trailer lights and repair your boat trailer lights in this easy to follow tutorial and troubleshooting guide.

As a boat owner it is essential to keep your trailer in safe operating condition. The trailer lights play a huge role in safety for brake lights, turn signals, and taillights for nighttime operation.

Over time as a result of corrosion or broken wires, a boat trailer’s lighting system will need attention. Luckily, if you understand the color-coding of the wires, it’s easy to troubleshoot for a repair, or do a complete re-wire.

Keep trailer taillights, turn signals, brake lights, and running lights functioning properly for everyone’s safety. This tutorial teaches you how to troubleshoot issues
Keep trailer taillights, turn signals, brake lights, and running lights functioning properly for everyone’s safety. This tutorial teaches you how to troubleshoot issues

Parts of a Boat Trailer Lighting System

Starting at the tongue of the trailer, you have the plug that feeds the wiring harness.

There are flat plugs that come in 4-pin or 5-pin versions. There are also 7-pin round plugs. It is common to use an adapter to go from the 7-pin round to a flat plug.

Next you have the wire boat trailer lights that runs the length of the trailer. You’ll be making splices in the wire, so you’ll want to use heat shrink tubing to make your splices water tight and secure. Ideally, the best wire connections are soldered first, then protected with heat-shrink tubing to make the connection waterproof.

The last parts are the running lights and taillights. The running lights run down the sides of the trailer for nighttime visibility. All of the lights that face the sides are amber, and any lights that face the rear are red.

The light fixtures on a boat trailer need to be sealed or “submersible” lights.

Otherwise they will corrode in very little time. Sealed LED lights are considered the best.

This brand new trailer has a 5-pin flat plug and has the standard colored wires. White for ground. Brown for running and taillights. Green for right turn signal and brake light. Yellow for left turn signal and brake light. And blue for trailer brakes
This brand new trailer has a 5-pin flat plug and has the standard colored wires. White for ground. Brown for running and taillights. Green for right turn signal and brake light. Yellow for left turn signal and brake light. And blue for trailer brakes

Color-Coding Made Easy

All trailers are wired using a universal color-code system. The brown wires are the running lights and taillights. These are the lights along the side and at the rear. They are always on as long as your lights are turned on. There is a brown wire boat trailer lights on both the left and right sides of the trailer.

The Yellow wire controls the left turn signal and brake light. The green wire controls the right side turn signal and brake light.

The white wire is the ground. The ground wire will be secured by drilling into the tongue of the trailer frame and tapping with a screw to hold the ground wire securely to bare metal to create a solid ground.

Lastly the blue wire is used for the trailer brakes if applicable.

The wiring harness first feeds through the tongue of the trailer. Where it splits into left and right sides is a point where the wires may be spliced. Look here for integrity of the electrical connections
The wiring harness first feeds through the tongue of the trailer. Where it splits into left and right sides is a point where the wires may be spliced. Look here for integrity of the electrical connections

Repairing and Troubleshooting Trailer Lights Often times a running light will cease to function or a taillight goes out. Did all of the running lights go out on the same side as the taillight? If so, your issue is toward the front of the trailer.

If all of the running lights and tail light went out on the same side of the trailer, go toward the front where the trailer forks into the left and right sides. Oftentimes there are splices here. Carefully inspect the connection of the common brown wire that ties all of the running lights and taillights.

Similarly, if a turn signal went out on one side, check the connection to the yellow or green wire where the trailer splits into the left and right sides. Also check the power connection at the fixture for integrity. Lastly, clean the plug terminals on both the trailer side and the vehicle side with sandpaper or a wire brush.

If all of the running lights are functioning, but a taillight is out, then the issue is close to the taillight. If a single running light or taillight is not working, look at the individual connection to the brown wire.

Secondly, keep in mind that every light fixture has its own ground to the boat trailer frame. Make sure the ground post or screw is in clean contact to bare metal. Again, a quick use of sandpaper can clean the connection, and solve the issue.

If the light fixture uses a standard incandescent bulb, look closely to make sure the filaments are in tact. In order to work correctly a fixture only requires a functioning bulb, a clean ground, a solid connection to the power, and a clean socket where the bulb screws in.

If all of the lights went out on the trailer, take a close look at the ground wire and make sure it is secure, and it is attached to clean metal for a good electrical connection. Also clean the plug terminals for the ground.

Every running light and the taillights are powered by the brown wire. Each fixture must also have a clean and secure ground
Every running light and the taillights are powered by the brown wire. Each fixture must also have a clean and secure ground

Full Rewiring of Boat Trailer Lights

If the lighting system on your boat trailer is old and there are several non-functioning lights, it’s likely time for a complete rewire.

First, choose which plug best suits your needs, a flat plug, or a round plug. The color-coding is the same for both. Then purchase a kit that includes the wiring harness and sealed taillights. The best long-term choice is going to be sealed LED lights.

Be sure to choose sealed or “submersible” taillights. Protect wires with flexible conduit
Be sure to choose sealed or “submersible” taillights. Protect wires with flexible conduit

First, you will remove the old wires, but you will use them to pull the new wires through the tubes if that’s the style of trailer construction you have. Otherwise, if the structural framing is open, it’s largely a matter of clipping the old zip ties.

This aluminum boat trailer is constructed with I-beams, so the wires are zip tied to the inside where they’re protected
This aluminum boat trailer is constructed with I-beams, so the wires are zip tied to the inside where they’re protected

To feed the new wiring harness through the tongue, first clip off the old plug, and attach the new wiring harness to it with electrical tape. Feed it from near the coupler, through the tongue, and to the point where the trailer splits into left and right sides. If there’s a rubber grommet to protect the wires where it enters the tongue, slide it over the wires prior to feeding through the tube of the tongue.

Leave the wire length to the plug long enough to reach the vehicle plug, plus sufficient slack for turning.

Next, remove the old running lights, and clip the wires. At the point where the trailer splits into left and right sides, feed the yellow and brown down the left side, and the green and brown down the right side.

Use the old wires to pull the new wires through the tube. If the framing is open, position the wiring along the frame using zip ties. Leave slack in the wire to accommodate the side running lights.

Starting toward the front, with each running light, separate the brown power wire from the signal/ brake wire. Splice in the running light fixture to the power. Use solder and heat shrink tubing for the best long-term splice. Make certain the ground contact for each fixture is on clean metal and firmly secured.

There is a splice on the brown wire at each running light. Solder the connection for durability. Then use heat shrink tubing to make the splice waterproof
There is a splice on the brown wire at each running light. Solder the connection for durability. Then use heat shrink tubing to make the splice waterproof

Repeat this process for each running light. Once the running lights are all wired, secure the wiring harness using zip ties if you have an open structure frame on your trailer.

Next determine the left taillight from the right by looking at the wire colors. Make sure the grounding point is clean bare metal. Secure the taillights and splice in the taillight wire, and the turn signal wire.

Lastly, install the ground wire on the tongue by crimping a ring connector on the end of the wire. Drill a hole for a metal screw and sand the metal bare for a good solid connection.

Screw the ground to the metal trailer frame and check it is tight. Give the ground wire a bit of slack so it isn’t under constant tension. Otherwise it will eventually snap.

Note: if you have trailer brakes, connect the blue wire to the brakes.

Now that the wiring is complete, go back through with zip ties and make it as neat and secure as possible.

Finally use flexible plastic conduit from the plug and into the tongue of the trailer to bundle the wires and protect them from chaffing.

When the trailer light wiring installation is complete it should look clean with all wires tucked out of the way
When the trailer light wiring installation is complete it should look clean with all wires tucked out of the way

Conclusion

Wiring a boat trailer is a straight-forward task, but requires crawling around on the ground. If disassembling old corroded parts and assessing electrical connections is not for you, consider using Trailers and Transport’s repair service in our shop, or our mobile boat trailer repair service can come to you.

We handle all aspects of boat trailer maintenance from wiring, to wheel bearings, to brakes and paint.

Call Trailers and Transport today at (678) 680-4423 or email us at service@trailersandtransport.com and let us help you with your trailer needs.

Trailer Sales, Rentals, Repairs, and Transport. We are your trusted source for trailers and we serve all of the Lake Lanier communities.

Troubleshooting Guide For Trailer Lights

ISSUE CHECK

All the lights ceased functioning. Check the main ground connection on the tongue.

Clean the plug connection for the ground.

All the running lights are out and Check the splice where trailer splits to taillight on one side. the left and right.

A single taillight or running light is out. Check the power and ground connections at the fixture.

Check the bulb and clean the socket with sandpaper.

A single turn signal is out. Check the power connection at the fixture.

Clean the connection at the plug.

Check the bulb. Clean the socket.

The taillights and running lights are Check the connection at the plug.

out on both sides, but turn signals work.

Trailers and Transport is the go to source for boat transport, boat trailer sales, boat trailer rentals, and boat trailer repairs and maintenance
Trailers and Transport is the go to source for boat transport, boat trailer sales, boat trailer rentals, and boat trailer repairs and maintenance