Troubleshooting Boat Trailer Wiring

Trailer light problems are the reason I am losing my hair. While troubleshooting boat trailer wiring problems, I often pull my hair out trying to figure out what went wrong. I have wired many trailers in my time; however, after putting everything together on my latest adventure, it didn’t take long for the trailer lights to start to blink and eventually randomly go out. Here are some tips and ideas I use while troubleshooting boat trailer wiring problems.

Troubleshooting Boat Trailer Wiring

First things first, I am done with the bulb style lights. I think I have tried all the designs over the years and still haven’t found a favorite. Over many boating years,  I have replaced a lot of bulbs. It doesn’t ‘matter what precautions are taken, they still stop working. Since moving to LED (Light Emitting Diodes) style lights I have not replaced any due to light failure.

On the other hand, I have had trouble with trailer wiring harnesses. Some of the unknown manufacturers use a very fine wire, somewhere in the 22 to 28-gauge range, and it is very hard to work with and prone to failure. Manufacturers like Wesbar and other known names, use 16 to 18-gauge wire which is easier to work with.

When it comes to the troubleshooting of trailer wiring problem, I suggest a couple of handy tools to make working on boat trailer light troubles easier. First, a tow vehicle check connector. These come in the most used configurations of flat four and round seven styles. These both have LEDs to check for power coming from the tow vehicle to the trailer. They have one LED for each circuit which helps rule out the vehicle as a problem very quickly.


Troubleshooting Boat Trailer Wiring


The second tool is a long jumper wire to go from trailer tongue to the rear of the trailer. This wire and a good quality volt/ohm meter can handle just about any other trailer light problem.

Now we have our tools, we can get to the job at hand. Before starting, I suggest checking all the lights on the trailer and determine what works and what does not work. I find a thorough evaluation tends to help and makes the job a bit easier because all issues are known. Multiple issues may point in the direction of a single fix.

There are four wires on a boat trailer, yellow (left brake/turn), green (right brake/turn), brown (tail/running) and white (ground). There are two brown wires, one for each side of the trailer. If the problem can be isolated to a single signaling issue, knowing which wires to work with narrows the time tremendously.

Grounds are a very common issue in trailer wiring. If all lights are flickering or failing, it’s generally a ground issue. When installing or repairing the white ground wire be sure it has good contact with the trailer frame. An easy way to check it is to connect the trailer and tow vehicle lights without the trailer on the tow vehicle ball. If the lights work, the ground is good.

My latest problem on my reservoir rig trailer is all the lights started blinking at once. This advanced to all the lights randomly going out completely for a few seconds. Troubleshooting this one took me a little while to find. When I first wired this trailer, I kept the trailer wiring running through the trailer tongue. This created two problems. The first was this trailer has the old tilt trailer mechanism. Although I don’t use it, it still moves a bit when being towed. The ground was lost between the tongue and the tilt part of the trailer. A 10-gauge jumper wire across the tilt mechanism took care of this first problem, the blinking lighting issue. Secondly, in the related failure of the lights simply going out, the wiring harness evidently got caught in the tilt mechanism as it was moving and pinched the harness. I am going to replace the harness and run a half-inch PVC pipe up through the tilt mechanism to the hitch. This will protect the wiring and keep from having this same problem again.

After troubleshooting boat trailer wiring problems and fixing them correctly it is reassuring the trailer lights will come on and stay on keeping us and our important cargo (the boat) safe as we travel to and from the lakes and river.


Check out more articles on boat trailer maintenance like boat trailer tires by Kris in the UB Garage blog

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