If you’ve ever had a light (or all the lights) go out on your trailer, you know that trouble-shooting it can be really intimidating. You’re smart enough to know that the problem could be the truck wiring, a bad trailer wire, or bad light. But how do you figure out which one of these it is?
To answer this question, I got together with my friend, Liz Miner, who is an electrical engineer and a lifelong rider. She explained the anatomy of trailer wiring and walked me through how to isolate a problem. I discovered that it’s not as difficult as I thought it would be. It’s worth knowing the basics because it might be something simple you can fix on your own and, even if it isn’t, pinpointing the problem can save your trailer professional diagnostic time (and impress them with your knowledge!).
It helps me to think of electricity like water flowing through a pipe. In your truck, electricity flows from the battery to a fuse box and from there to lights on the truck as well as the connector at the back of your truck (also known as the wiring harness) and from there to the various lights on your trailer. Pretty simple, right? Unfortunately there is a little wrinkle. Unlike water flowing through a pipe, electricity has to flow in a circle, or “circuit.” It can’t just dead-end at a light. Think of it as flowing from the battery, dropping off some power to a light, and continuing back to the battery. If it there is no return pathway, the light won’t work. The return pathway is called “ground.” Remember this.
When you plug the trailer into your truck, electricity is directed to various lights or other functions on the trailer. Each pin on the connector (plug) supplies a different light or function. These include:
- Running lights (the ones that are always on if the truck lights are on)
- Right turn signal and brake signal (flashes when you turn the blinker on and is steady red when you apply the brakes)
- Left turn signal and brake signal (ditto above)
- Trailer brake controller
- On some trailers, back-up lights
- On some trailers, accessories such as tack room lights
Here is an orientation to the pins. I have a 7-way so that is what I am showing. If you have a different type of connector, it’s okay; the basic concepts are the same (you should be able to find a diagram of the pins online; try here). Notice that the TRUCK connector is a mirror image of the TRAILER connector, which if you think about how they fit together, makes sense. Everything is oriented to the raised ridge on the connectors. On our truck the ridge is on the top, but yours could be on the side or even the bottom.
Let’s take the running lights (side markers), for example. Electricity flows from the battery to the truck’s brown pin in the diagram above, to the trailer plug’s brown pin, then along a brown wire to the running lights, then along a white wire back to the trailer plug’s white pin (ground) to the truck’s white pin, and back to the battery.
All the pins in the connectors are for electricity going towards the trailer (what I am calling “supply”) except the white pin. In every case, the electricity flows back via the white pin, also called “ground.” If you study the diagram below, you will see that each colored wire supplies an individual item (light or brakes), yet the white wire (ground) goes to all of them. The white “ground” wire completes the circuit for all the colored wires – it’s the pathway back to the battery.
To troubleshoot problems with the wiring, you’ll need a few things. One is an electrical tester that looks like this, which you can buy at your local hardware store. You should also have on hand some sandpaper (60 to 100 grit is fine) and/or a wire brush, and a long piece of insulated wire – long enough to stretch from the tongue of your trailer to the farthest light, and a screwdriver or two.
I am going show you how to trouble-shoot your wiring in two formats: in a video, which is probably the best way to learn it. Below that I also go over everything in text. This might be harder to follow, but if you print it out and keep it in your trailer, it might come in handy as a step-by-step guide.
Step One: Check the truck wiring. The first step is to make sure the truck connector (the one on your bumper) has electricity coming to all the pins. To do this, start the truck and turn the lights on. Set your meter to 50V~.
Lift the cap on the truck connector and place the black lead on your tester on the GROUND pin. Then place the red lead on the TAIL LIGHTS pin.
The needle on the meter should go up (don’t worry about how much). If it does, there is electricity flowing to this pin. If the truck’s lights are on but the needle on your meter does not move, there is no electricity coming to this pin.
Then check the other pins. For these, the function must be activated for it to have power. For example, when testing the right turn signal pin, turn on the right turn signal in the truck. Because it flashes, electricity will flow in an on-off pattern so the needle on your tester will go up and down. This same pin serves the right brake light so when you step on the brakes, power should flow continuously to this pin. [If you don’t have a friend to help, you can place a stick or board between the brake pedal and the seat to keep the brake pedal depressed while you check the pin.] Test all the other pins in the same way.
If none of the pins have electricity, it could be a problem with the ground wire, or possibly a fuse problem. If some work and some don’t, it could be a fuse or a supply wire problem. Check the truck fuses (look in your owner’s manual) and if they are all good, there is a problem with the wiring. Take it to a professional for repair.
Step Two: Ensure a good connection between truck and trailer. Assuming that everything checked out fine in Step One, the problem is not the truck. Make sure the trailer plug has a good connection with the truck. If none of the trailer lights work, it could be as simple as a bad connection and the most likely culprit is dirt or rust – usually on the plug side, but possibly on the truck side. If you previously put dielectric grease on your plug (which is a good idea to repel water and prevent rust), the grease can pick up grit if the plug falls onto the ground. Clean out the pin slots and if there is rust, remove as much as you can with sandpaper or a wire brush. Make sure the plug is fully seated onto the truck connector with nothing in its way. If this solves the problem, you are all set!
Step Three: Checking the trailer wiring. If none of the lights on your trailer are working, then you most likely have a bad ground wire (the white wire to the white pin in the diagrams above). If you know where this is you could try to find a spot where it has broken or been chewed by mice and fix it, but this can be tricky. I would probably just take it to a shop.
Step Three, Part A: Check the light. If just one light is not working, the first (and easiest) thing is to make sure the light is good. Remove the light from the holder or unscrew it from the trailer and disconnect the wire(s).
Set your meter to X1k ohms (the ohm symbol is Ω). To make sure the meter is working, touch the black and red leads together. The needle on the meter should move. If it does not, replace the battery in the meter.
What is up with this? Electricity is flowing from the meter’s battery to the red lead, then to the black lead (since they are touching), and back to the battery, completing a circuit. You need this to work so you can use it to test the continuity of the light or of a wire. “Continuity” means that it is not broken, allowing electricity to flow through it.
The back of the light should have 2 wires or pins that connect to corresponding wires or pins in the light holder; one for the supply wire and one for ground wire. Take the light and touch one lead (either one) from your meter to one wire/pin, and the other lead to the other wire/pin.
If the needle on the meter goes up, it means that the battery in your meter is sending electricity out to the light and it is coming back to the meter in a complete circuit; therefore the light is good. If the needle doesn’t move, the light is burned out, preventing the circuit from being completed. Replacing the light should solve the problem.
Step Three, Part B: Check the light’s connection. If the light is good, clean the connectors really well. Get any grit out, take sandpaper or a wire brush and polish the metal up. Also do this on the connectors in the light holder. Put the light back in. If it works, the problem was just a bad connection – this is the most common problem so do NOT skip this step.
Step Three, Part C: Check the trailer wiring, with trailer plugged into truck. If the light still does not work, the problem is a wire. It could be the supply wire coming from the plug to the light, or the portion of the ground wire going from the light back to the plug. The easiest way to check is with your truck connected, but you can also do it with no truck (if you are doing it with no truck, skip to Step Three, Part D).
With the trailer plugged into the truck, start the truck and activate the light you are working on (e.g. for a turn signal, turn it on in the truck). For running lights, the truck headlights usually must be turned on. Remove the light and set it aside. Set your electrical meter to 50V~. Place the red lead on the supply wire (colored wire) and the black lead on the ground (white wire or metal pin).
If the needle on the meter goes up, electricity is flowing, indicating that the wires are all intact. In this case, I’d go back to a poor light connection as the problem. If the needle does not move, you have found the problem – one of the wires between the trailer plug and the light holder is broken. You do not know if it is the supply wire or the ground wire. You can determine this in the next step.
Step Three, Part D: Check the individual trailer wires, with trailer NOT plugged into truck. In this step you will check the continuity of the wires between the trailer plug and the light holder to determine if the supply wire and the ground wire are intact.
Take a piece of insulated wire long enough to run from the front of the trailer to the light in question, and strip about 1/2″ of insulation from each end. Tuck one end into the pin socket in the trailer plug corresponding to the light that is not working. In this example, I am testing the running lights.
Run this wire (which I will call the “blue test wire” since mine is blue) all the way back to the part of the trailer where the light you are testing is. Set your meter to X1k Ω (ohms). Touch one meter lead to the blue wire. Touch the other meter lead to the wire that supplies the light (colored wire).
If the meter needle does not move, then the supply wire between the trailer plug and light holder is broken somewhere. If the meter needle does move, this wire is intact (because electricity is flowing from the battery in your meter through all the wires in a big circle) and the next step is to check the light’s ground wire.
Move the “blue test wire” to the ground pin in the trailer plug. Then, back at the light, touch one lead of your meter to the blue test wire and the other meter lead to the ground connector (or white wire) on the light holder.
If the meter needle does not move, then the ground wire is broken somewhere. If the needle moves, the ground wire is intact.
Step Four: Fix the problem. This depends on what the problem is. As I said above, you can easily fix a bad connection or replace a bad bulb. But if your diagnostics reveal a broken wire, that’s a different story. You can try to inspect it along its length to try to find the break or kink, but most wires are in conduit (tubing) and run through your trailer’s nooks and crannies, so they can be really tricky to find. You can certainly inspect the portion of the wire that you can see and give it a gentle tug to see if it’s loose. But unless you find an obvious problem, it’s time to take it to a professional.
Happy trails, and please leave comments below, especially if you have corrections or clarifications for any of the information presented.
Monica
