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Testing Your Trailer Lights-DIY

We’ve all been in the situation where we are all packed and ready to head to the lake, we are in the process of hooking the boat up to the vehicle, we plug the trailer lights in and go to make sure they are working…..and nothing, or maybe half are working and the other half are not.

Fear not, in this post we will walk you through the basic steps on how to properly diagnose and check your trailer lights, and hopefully, salvage your boating trip.

Before we get started there are some tools and equipment we are going to need to test the lights properly.

12 Volt Source

-12V Battery Source-This could be a spare car battery you have laying around or even a battery from a drill

12 Volt Test Light

-A test light or Multi-Meter- I prefer a test light as it is a little quicker but not everyone will have one of these laying around (If you don’t you should for the price $10) a multi-meter will work just fine.

Jumper Leads

-A pair of jumper leads-To hook the battery to the trailer connector

-Basic hand tool- Screwdrivers, socket set, wrenches

-A second person-Definitely not critical but will speed the process up so you are not running back behind the boat all the time.

Getting Started

Let's disconnect the trailer from the tow vehicle as we want to isolate the problem. We will test the trailer connector on the tongue and won’t talk too much in regards to the tow vehicle issues at this point.

Lets first get an understanding of the trailer wiring colors before ripping everything apart. Trailer wiring colors are universal and SHOULD be the same trailer to trailer. If yours is different from below then someone at some point has made some changes and will make diagnosing the issue slightly harder.

You should have 4 wires on a trailer without brakes and 5 wires on a trailer with brakes:

White-Ground Wire

Brown-Running Lights/Stop Lights

Yellow-Left Turn Signal

Green-Right Turn Signal

Blue-Brake Solenoid (If Equipped)

Trailer Wiring Colors

Let's start by taking our jumper wires and hooking up the WHITE (ground) wire to the battery negative(-). Next, we will hook the BROWN (stop lights) wire to the positive side of the battery(+). Have a walk around the trailer and make note of which lights are on and which lights are off. EVERY light should be on at this point in a properly working system. This would be like hitting the brakes on the tow vehicle and turning on the running lights.

If none of the lights come on

When none of the lights come on it usually indicates a bad ground or you have hooked the battery up wrong. Assuming the battery is hooked up properly let's take a look at the ground wire. It is unlikely all the lights have failed at the same time, so follow the white wire from the back of the trailer connector to where it connects to the trailer. Ground current is supplied to the lights through the frame of the trailer, similar to a vehicle. Once you have found where the wire connects have a good look at, is it broken? Very rusty? Making poor contact? Most of the time we find the wire has broken and needs to be replaced or the point of contact to the trailer is extremely rusty and needs to be cleaned. If that’s the case perform the necessary repair and recheck all the lights.

White wire grounded to the trailer frame

Some lights work others do not

Still with our battery hooked up to the brown and white wire let’s take a look at which lights work and which do not. In the case of the smaller marker lights not working we will have to remove the light from the trailer and test the wiring with our test light or multi-meter. We should have the same color wires running to all the marker lights (brown & white) so it is just a matter of hooking your test light or meter leads up to those wires and checking for voltage-No voltage indicates we have a broken wire somewhere-Voltage on the meter or light indicates we have a bad marker light-time to replace.

If one or both of the tail lights fail to work then let's do the same thing with them. They will have an extra wire running out of them (more on that later) but right now we are focusing on the brown and white wire. Let's hook our meter or light up the wires and check the voltage and the scenario will be the same as above. Now some tail lights you can simply remove the cover and replace the bulb if we have good power going to it whereas others are completely sealed and the whole unit will need to be replaced.

Signal Lights Not Working

Ok, so we’ve done all the testing on the running lights. Let's move over and test the signal lights. We will do one at a time. Back up at the front of the trailer lets switch around our battery leads. We can leave our white wire hooked to battery ground, but we want to hook up the yellow wire now to battery positive. If we remember from the picture above the yellow wire is the left tail light. That is the left light when standing at the back of the trailer looking at it. After you got that properly hooked up lets head to the back of the trailer and check to see if it's working. If it is great! Move on to the next test. If it’s not working let's figure out why. We will do the same test as for the running lights, on the back of the tail light lets hook our leads up to the yellow and white wire’s and see what we get. No voltage indicates and bad wire or connection from the front connector and proper voltage (12V) indicates a bad tail light.

After we’ve made the necessary fix for the left light lets do the same test for the right light. The right light is the green wire, so let's go ahead and hook that up to battery positive and go back and check the function of the light. If it’s not working it will be the same test as above but on the green wire. Repair as necessary.

I like to double check all the lights one last time after any repairs, so just hook your jumper leads back up one at a time to the brown/yellow/green wires and do a quick walk around the trailer. After that, it is time to hook the vehicle back up and recheck all the lights, if some are not working we know the issue is going to be with the tow vehicle because we just eliminated any potential issues with the trailer side of the wiring. Common issues with tow vehicles lights not working could be a blown fuse or bad ground at the connector.

I also wanted to include this picture below of your trailers wiring just to give a better understanding of the system.

Trailer Wiring Schematic

Hopefully, at this point you have a better understanding of your trailers wiring and any future issues that pop up you can easily address. Have any questions or comments please leave them below and we will be quick to respond!

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