Troubleshooting: 7 Pin Trailer Running Lights Not Working

The more pins the more that can go wrong. Then there are those trailers that don't follow the color-coded system. When you run into that situation it takes a lot of work to figure out how to make the right connections. Eventually, you will get the solution if you keep working on it.

These connectors are color-coded. It should only be a matter of matching the colors to make sure the lights work as they should. However, things don't always go so smoothly and you may need the help of an electrician to figure out the problem.

To learn more about troubleshooting your 7-pin connectors, just continue to read our article. If you can’t do it yourself, going to an expert is the best and easiest solution to try. Take a few minutes to see if the following information solves your problem before you pay for an expert’s help.

7 Pin Trailer Running Lights Not Working

7 Pin-Trailer-Running-Lights-Not-Working

One of the causes of this problem would be that either the brass connectors or the wiring have corroded in some way. This needs you to clean up the connectors and get rid of the corrosion, or you may have to cut and replace wires.

This may not be the only source for your problem. Figuring out what is wrong can be a bit confusing as sometimes the colors do not provide the power to the stated electrical device. Then when that happens you have to go on a search to find out how the 7-pin wires connect up and to which device they operate.

This statement is supported by the fact that the middle connection is often labeled as an AUX circuit when in reality, it is not a ground terminal but operates the back up lights. The key to fixing this problem is to test each terminal and then match it up to the function it really operates.

Or, you may have to replace one or both of the connectors to get all the functions, including the running lights, to work properly. There is no easy solution to this situation.

What Would Cause Trailer Lights Not To Work?


There is a myriad of sources for this situation. One would be that the ground wire is not connected properly or there is some rust and corrosion that has marred the connection. You have to make sure this wire is connected to an unpainted surface and there is no dirt in the way.

Then you should check the fuses and the light bulbs in each of the trailer lights. The fuses may have blown and have cut off the power supply. Or the bulbs may have burned out, due to many reasons, and they or the fuses simply need to be replaced.

Another source could rest with either the tow vehicle’s wiring or the trailer’s wiring. If either are bad, then you will have to re-wire the trailer or the vehicle to get proper function out of the trailer lights.

Or the wires have come loose, corroded, or frayed at some point. In any of those cases, replacing the wire or tightening the connection are the best options. Then the terminals may be loose or bad. The crimp-style terminals are known for coming loose or collecting corrosive materials.

Re-tightening the terminals, cleaning them up, or replacing them with a better type of terminal are your solutions here.

How Do You Troubleshoot a 7 Pin Trailer Wiring?


This will depend on the type of problem that occurs with the 7-pin connection. One of the simplest sources is when the connectors become bent or damaged. Once that happens the connection becomes loose and you may get a warning on your brake controller that the trailer is not connected.

You have two options to use when you find yourself in this situation. The first one would be temporary as all you have to do is bend the connectors back so that they fit tight. This doesn't take long and you will easily know when you have succeeded in bending them far enough.

The permanent solution would be to replace the plug. In this solution, you would have to cut off the old plug and then expose the wire ends. Then you simply have to connect those wire ends to the right connectors to make sure all your trailer’s electrical devices work properly.

If there are other problems, then you should get your multi-meter out and start testing for continuity, power, and so on. This is a step-by-step process and may take you a little time to find the source of your problem.

Once you find the problem, then you should know exactly what you need to do to fix it.

Is There a Fuse For Trailer Running Lights?


Yes, there is and it is usually located in our fuse box. The exact location of your fuse box will be determined by the make, model, and brand of your vehicle. Sometimes it is under the hood and other times it is near the floor on either the driver’s or passenger’s side of the car or truck.

However, before you start changing fuses, you need to check your wiring first. if there is a short in the system, you will keep blowing fuses. If you do replace your fuse and it blows right away or very quickly, then you know you have a short somewhere in the system.

When you start looking for the short, you have to follow the wire from the connector to the lamps. The color of this wire is usually brown so you can find it very easily. As you trace the wire, look for crimps, cracks, bends, frays, and pinches.

The presence of those elements tells you that there is a problem with the wire and not the fuse. Replacing the wire is your solution. Also, check for loose light bulbs, rusty bulb sockets to make sure those are not causing the fuse to blow.

Tightening the bulbs or cleaning the sockets will be your solution to those issues.

7 Pin Trailer Plug Hot Wire


If you are not an electrician, a 7-pin wire connector can be a bit intimidating. Until you get used to the color-coded system. The colors are listed to make troubleshooting a lot easier.

Once you learn what the colors are supposed to stand for and connect to, then your troubleshooting time should be simplified. For the hot wire or the hot battery lead, the color of the wire is black. This color is the same on both the SAE and the RV standard systems.

What you have to take special note of is the gauge or size of wire needed for each pin in the 7-pin system. For the turn signals, brake lights, reverse lights and running light wires pins, you will need 16 gauge wire.

However, for the ground, brake power, and battery hot lead pins, you will need 12 gauge wire. These two figures are the recommended minimums. So you can go larger but it is not wise to go lower.

The other important thing to take note of is that under the SAE standard system, the hot lead wire could also be red. The same goes for the RV standard system. For the latter system, the wire gauge minimum is lowered to include 12 gauge. You can use either 12 or 16 gauge wire when you re-wire this pin.

You can see why some people get confused and intimidated as some changes can be made without notice.

Trailer Running Lights Not Working 4 Pin


The same issues that stop 7-pin lights, etc., from working, will apply to the 4-pin system. The first place to check will be your ground wires. If they are not connected, loose, or attached to the wrong surface and have become corroded, then your running lights may not work.

Next, you would have to use a circuit tester to see if the trouble is on the trailer side of the vehicle side of the combination. Usually, if there is a problem with the signal on the pin closest to the ground wire pin, then the trouble is on the vehicle side.

If there is no problem with the signal, then the trouble is on the trailer side. This trouble could be a bad wire or connection. Beyond this, you may need a pro to help you diagnosis the trouble.

Some Final Words

If you are not comfortable working with electricity, get a professional to do the troubleshooting. Also, make sure they know their stuff before you do. 7-pin connectors can be confusing and hard to work with.

Once you have figured out the problem, the repair is not that hard. Especially if it is only corrosion, a loose wire, or a blown fuse.

Leave a Comment: