How To Check Ground On Trailer Wiring: The Right Way

Checking the ground on your trailer wiring is a critical part of ensuring the safety of your vehicle and the security and functionality of your trailer lights. A properly grounded trailer light is essential for daytime and nighttime visibility and provides an extra layer of protection to other drivers on the road. This article will give step-by-step instructions on effectively checking your trailer’s ground wire, connector, and lights.

Whether you have a boat trailer, utility trailer, or travel trailer, the concept of checking for the ground is the same. All those types of trailers use a 12-volt battery system and use a positive and negative (or ground) power source to run lights and turn signals.

12 volt lights draw power from a battery through two connections known as positive and negative terminals. The positive terminal is connected to the power source, while the negative terminal connects to the ground, usually your trailer frame. When current flows between these two points, it powers up the LED or incandescent bulbs of the light fixture.

Any disruption or break in that voltage will cause the lights not to work. And since the ground wire is exposed to the trailer frame, it is more common for it to corrode and cause a break in that current flow.

Table of Contents

Ground Wire

First, let’s make sure that we are checking and testing the ground wire. Your 12 volt battery is usually red for positive and black for negative (or ground terminal).

For the wiring of your trailer, the colors do change. When it comes to ground wire colors, two options are most common: white wire and green wire. White wires are used when the neutral conductor is a grounded conductor. At the same time, green wires are typically used in general-purpose outlets and other types where grounding is necessary.

When you see a brown wire, this is the power (positive) side of your tail or running lights.

Vehicle Connector

Since we are talking about a boat trailer, utility trailer, or travel trailer, you get your power source from a tow vehicle and a connector. Likely, you have a 4 pin or 7 pin trailer plug as a connector.

7 pin connector

Before you start checking ground throughout the trailer frame, I suggest testing the power source at your trailer or truck for voltage. This is done easily by checking the connector with a multimeter to see that you are getting at least 12 volts. To test the connector, get your multimeter and probe the black leads on the ground, and the red leads on the power source of the connector. If you show at least 12 volts, let’s continue testing the trailer.

Trailer Frame

Now that we know we have power coming from the truck, we can investigate the ground on the trailer itself. The neat thing about the ground on a trailer is that it is mounted to the trailer frame. That makes the entire frame the negative terminal of the battery. This is very helpful when wiring lights, as all you need to do is run one wire to the power source and just a short wire from the ground to the closest part of the metal frame.

Although this is an advantage, there is a disadvantage to this. And that trailer frame is exposed to the elements and corrosion, and the bad connection is more common than the ground wire.

Dirty ground screw connection

Before getting the circuit tester and testing for ground, do a visual inspection of all the ground wire connections. Check the screws that mount the ground wire to the trailer frame. See if any signs of dirt or corrosion may not cause a good metal-to-metal connection.

If any connections show any corrosion, unscrew that connection and clean them with sandpaper or a wire brush. It doesn’t take much rust to cause a bad connection and an issue with your ground.

Also, check the condition of the ground wire itself. Since the wires are also exposed to the elements, they can become damaged by the weather or rodents. Make sure there are no signs of breaks within the wire.

Blown Fuse

Another thing to check is the fuses. Like any electrical connection in your truck, you will likely have multiple fuses for things like your turn signal and running lights.

Trailer fuses help protect the wiring harness of your towing vehicle from potential damage. If too much current runs through the wires, a fuse will blow, cutting off the power and preventing further damage. Fuses come in numerous sizes and styles depending on the trailer’s amperage demand. The most common type is a blade-style fuse that plugs into one end of the electrical harness near where it connects to your vehicle’s battery or power supply.

Circuit Tester

Now that you have determined that you have voltage at the vehicle, good ground connections, and all fuses working, let’s get out the circuit tester or multimeter to test the ground.

As we reviewed above, the entire trailer frame is ground. So when we start testing, we can use that principle to our advantage. How? Since a multimeter displays the voltage difference from the positive to the negative side of the battery, we can easily check the ground is working.

First, connect the red probe (from your multimeter) to the positive side of your battery or the incoming source to your trailer. If you have an extended lead wire, I suggest you use it here, as you will walk around the trailer with your multimeter.

Touch any exposed metal on the trailer frame with your black probe from your multimeter. If you have ground, it will show the voltage on your meter. Here is a tip, the probe won’t work through paint, so you need to connect to bare metal on your trailer frame.

Ground screws exposed to easily test

Now, using this method, you can check any light or wire to see if it is getting ground. Go to each end connection or wire and see if the voltage is showing on your multimeter. If you have at least 12 volts, your ground works correctly.

If you see low voltage, anything under 12 volts, you could have a broken wire or connection, which will need to be fixed.

Final Thoughts

Is your trailer wiring not working, and do you need to check the ground? It’s no difficult task, but it’s essential to double-check that everything is in order. With this article as your guide, you can ensure successful results when testing your trailer wiring ground.