The Ultimate Guide to Wiring A Boat Trailer

When you first get a boat trailer, everything should be in 100% working order, but, over time, many trailers end up having wiring problems. If you buy an older, used trailer, you will likely end up with wiring problems at some point. Older trailers are known for having these issues, and you don't want to be left without boat trailer lights.

You've likely encountered these issues yourself if you're looking up how to rewire a boat trailer. Thankfully, this issue isn't too difficult to rectify, so long as you follow all of the steps that we have listed below.

First Things To Check

Before you start to try and wire a boat trailer, you should first try to figure out what aspects of your trailer are working properly and which parts are not. While unlikely, you may only have some loose wires in your trailer, which can easily be fixed by just reconnecting them.

You can do this by checking all of the trailer connectors and following the trailer wiring diagram of your boat trailer. To do so, follow the wiring of each of your trailer lights. If you have figured out that the wiring to your brake lights and turn signals is all in working order, then it is time to look elsewhere.

Wiring a Boat Trailer: Step-by-step

Below, we show you exactly how to wire boat trailer lights.

1. Get The Right Wire Harness

This first step will depend on the state of your wiring harness and whether you feel it needs to be replaced or not, but if you plan on rewiring your entire trailer and replacing it, then you will need to purchase the right type for your boat trailer.

There are two main types of wiring harnesses, which are named by the electrical plugs that they fit into. Older trailers will generally be equipped with flat-four plugs, while newer trailers will usually be equipped with flat-five plugs. This can be a little confusing, as flat-four plugs have five colored wires while flat-five plugs have six colored wires!

The difference in the number of wires is because the extra wire on a flat-five plug is used to deactivate the surge actuator in reverse on boat trailers with disc brakes.

2. Remove The Old Wires And Trailer Lights

Once you have found the appropriate wiring harness, it would likely be a good idea to look at your trailer's wiring diagram to figure out how and where everything is connected.

You will need to unscrew or unbolt the old trailer lights and running lights from the frame of your trailer and then disconnect or cut the wires that connect to the lights. If your wiring leads inside the frame of your trailer, then you can use your old wires to pull the new wires through, so don't completely remove the old wiring just yet if this is the case.

You can then begin unclipping the wire from your trailer frame, moving from the back of the trailer to the front. Once you have unclipped everything, you can pull the rest of the old wiring out near the tongue of the trailer, which is at the front.

3. Place The New Wiring

Once you have removed the old wiring and threaded the new wiring through the enclosed part of your trailer, you can lay out the new wires.

Take another look at your wiring diagram as a reference, and then lay out the yellow and brown wires on the left side of your trailer (driver's side). The green wire and the other brown wire should be placed on the right side of your trailer (passenger side). You can leave the white ground wire at the tongue of the trailer for now.

If you have a flat-five plug, then the blue wire can be connected to the solenoid on the surge actuator.

Ensure that both lengths of wire are equal to one another on both sides of the trailer, and also make sure that there is enough slack near your trailer's tongue so that you can plug the wiring into your vehicle.

4. Connect And Mount The Ground Wire

Remove the plastic coating on the end of the white wire by stripping it, and then remove your old ground bolt or coupler bolt that was used for the old white ground wire.

You can then wrap the copper wiring from the stripped end of the white wire around this bolt and then tighten it up again. You should make sure to tuck the white wire underneath your boat trailer coupler so that it does not end up being damaged by the elements.

5. Install The Boat Trailer Running Lights

This next step will differ slightly depending on whether you use your boat in salt water or fresh water.

If you only use your boat in fresh water, then pinch-wire connectors should be used to connect the positive 12-volt pigtail from the running lights to the rest of the main wires. You should also hook the clearance lights up in the same way.

If you use your boat in salt water, use crimp connectors, as these will not rust as easily, then use heat shrink tubing for extra protection from the elements.

6. Connect The Brake Lights

The wires that go to your brake light, turn signals, and marker lights (running lights) each serve different purposes.

The brown wires should be connected to the marker lights, which should have either brown or black wires. Then, starting on the left side, the yellow wire from your wire harness should be connected to the yellow trailer wire. This can sometimes be a red wire, so if this is the case, connect yellow wire to red wire.

You can then do the same on the right side by connecting the green wire to the other green or red wire. If you just plan on using your boat in fresh water, then pinch wire connections will be fine, but if you want to use your boat in salt water at some point, then you will need to use crimp connectors and heat shrink tubing again.

Once you have completed this step and reinstalled or replaced your boat trailer lights, then you should test them to see if the new wiring is in working order. If not, you should start the boat trailer light wiring process again to see where you went wrong.

7. Secure Your Boat Trailer Wiring

Once your boat trailer rewire is successful, you can secure all the wiring so that it does not come loose or become damaged.

Some trailers come with hoops that you can thread the wiring through, but if this is not the case, then you should use galvanized wire clips to make sure your wiring is secure.

Make sure the wiring is secured tightly, but not too tightly; otherwise, your wires could become unplugged if you take a turn too sharply. After the wires have been secured, check one last time that they are in working order.

If everything is working, then you have successfully rewired a boat trailer!

FAQS

What color is the ground wire on a boat trailer?

The ground wire on a boat trailer should always be white. While many of the other wires may vary in color, the ground wire of a trailer will always be white.

How much does it cost to rewire a trailer?

If you followed this guide and are still having trouble getting your boat trailer to work, or you don't like to get your hands dirty, you may want to consider getting a professional to do it.

This will normally cost you around $50 to $200, depending on the size of your trailer and the labor cost.

Does a boat trailer ground through the ball?

Yes, the ground is on the hitch ball on many trailers.

Final Thoughts

Wiring a boat trailer is not all that difficult, provided you have the tools and means to do so. This guide should provide you with all of the information that you need, but if you still have issues, taking your trailer back to where you purchased it to get it rewired should help you rectify any issues.

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