The purpose of this article is to help you understand how the lights on your boat trailer work and how to fix them when they don’t. I’ll also describe an organized approach to troubleshooting trailer light problems and describe some simple tools to help get you back on the road. To keep things simple and straightforward, I’m going to focus on a basic 4-wire system. Bigger more complicated trailer’s wiring work by the same principles as smaller trailers. And we’re not going to look at the wiring in your towing vehicle. Every vehicle manufacturer seems to use a different system of relays, fuses, and connections so any discu ion of that is beyond the scope of this article.
Basic Electrical Circuits
To understand how trailer lights work, let’s look at basic electrical circuits and make sure we understand what is required to make things work. All electrical circuits form a circle or a loop. See figure 1. There will be a source of power, a load, and two wires connecting the source to the load. This is always true! Your jumper cables have two wires, your trouble light has two wires, the computer you’re sitting in front of has two wires. So does a flashlight. Electrical power travels down the first wire, through the load, and then back to the source through the second wire. Sometimes it’s hard to see where that second wire is. In your boat trailer, it’s the frame of the trailer.
If the connection is broken at any point in the circuit, then the load won’t get any power. This is called an “Open Circuit”. If there is a way for the power to get from one side of the source to the other side of the source without going through the load, this is called a “Short Circuit”. When you have a short circuit you can expect to blow a fuse or circuit breaker. Open circuits won’t blow a fuse.
Trailer Circuits
The wiring in your boat trailer has to follow the same rules described in Basic Circuits. If it didn’t, it wouldn’t work! One of the main sources of confusion in trailer wiring is that one side of the circuit uses the trailer frame as a conductor. A conductor doesn’t have to be made of copper and covered with plastic insulation. A conductor can be the trailer frame and work just fine. Typically the trailer frame is called “Ground” and then people a ume mysterious and wonderful properties attributed to “Ground”. Please understand this: “Ground” is just another conductor like a piece of wire, no more and no le . The second source of confusion comes from the fact that different circuits can share conductors. Please refer to figure 3. With our trailer wiring, there are three circuits. There is the right-hand turn signal circuit, the left-hand turn signal circuit, and the running lights. The right hand and left hand turn signal circuits operate independently of each other but are wired the same way. The running lights are really one circuit, but there are two loads (one on each side).
All three of these circuit share the same conductor, between the negative side of the source and the load. In a trailer this is the trailer frame, that thing we usually call “Ground”. I know this seems confusing, but it actually helps us later on with troubleshooting. Power travels down the wire, through the load (the light bulb) and then returns through the trailer frame back to the source completing the circuit. Power from more than one circuit can travel back through the same conductor to the load, all at the same time. This would happen if you hit your brakes when the running lights were on, which would power up all three circuits at the same time. As long as there is a path from the source through the load and back to the source, it’s all going to work. The wires on the connector are color coded (see figure 2).
The most common or widely used wiring code is:
- Brown to the tail or parking lights
- Green wire to right turn signal/stoplight
- Yellow wire to the left turn signal/stoplight
- White wire to common or cha is ground
Mechanical Considerations
I’ll let you all in on a secret about electricity and electronics: 90% of all problems are mechanical ! This means any connector, crimp splice, ground lug, light socket, or bolted section of a trailer frame is where your problems usually occur. There are some simple preventative things you can do to avoid mechanical failure of your trailer lights.
Starting at the front end, let’s look at a typical trailer. We have a 4 pin connector on the end of a wiring harne that mates up with another connector on the back of the tow vehicle. The pins in this connector need to be clean and need to seat firmly with the mating connector on the tow vehicle. The "Ground" pin on this connector has to be connected to the trailer frame. See figure 4. There should be a lug on the end of that wire, and a spot cleaned off right down to shiny metal where you bolt it down. Bolting it down onto paint isn’t going to get the job done. If you’re worried about rust, paint that spot after you’ve tightened the connection down. I’ve seen dozens of trailers where that wire is broken off. How can the lights still work you ask? Well, you can get continuity through the hitch coupler and the ball. These are the guys with the blinking or intermittent trailer lights. Those parts are there to pull boats, not conduct electricity. It’s a very unreliable way to do things.
Wires break due to repeated flexing. The solution is called strain relief. See figure 4. Use cable ties to attach the wire harne to the tongue. Minimize the amount of exce wire that can flop around and you’ll reduce the chances that one of those wires will break, especially the wire that is connected to the trailer frame. The wiring harne should have some slack in it along the length of the trailer. If the wiring is pulled tight it will stretch and eventually break.
The light a emblies have to make good connection to the frame just like we did with the ground wire. Usually this is done through the mounting bolts on the light a embly. The paint and corrosion has to be cleaned off where the bolts pa through the mounting bracket so that we can have a good low resistance connection to the frame.
The light sockets deserve some special consideration, especially if they get wet. If you back your trailer lights into the lake, then you have to unplug the lights before you back in. Otherwise the hot gla bulb hits the cold water and it shatters. It also is a good idea to apply a little dielectric grease to the bulb before it’s mounted into the socket. Dielectric grease is can be found at most automotive parts stores and hardware stores. It’s used to prevent corrosion. Without it, removing an old bulb can be very difficult.
Basic problems
Different things can keep the trailer lights from working correctly. If we know what effects these different problems have it makes it easier to troubleshoot and repair the trailer lights. In no particular order:
- Bad Bulbs.
Obviously a broken filament or cracked envelope will render a bulb usele . Filaments eventually break due to vibration and shock so your going to have to replace bulbs. Keep some spares with you. If you suspect a bad bulb, the easiest test is to substitute another bulb. - Bad sockets.
The socket has a spring-loaded base designed to keep tension on the bulb. The spring will lose strength if it gets corroded ( especially if it gets wet). I’ve never had much succe trying to fix a socket. Usually you have to replace them. A bad socket will cause intermittent operation. If you suspect the socket, wiggling the bulb one way or the other might cause it to start working (temporarily) again. - Broken wires.
Anywhere that the wires can move exce ively, catch on something, or get pinched can result in a break. The most common wire to get broken is the white wire between the trailer connector and the trailer frame. - Chafed insulation.
If the wiring harne can rub against the trailer for a long enough period of time, then the insulation can wear through and short out to the trailer frame. This will cause blown fuses and erratic operation and can be very tough to find. Fixing this one usually involves pulling a new wiring harne . - Corrosion can cause all sorts of problems.
Besides the light socket, the whole trailer frame has to have good conductivity since it is used as part of the circuit. Socket to mounting bracket, mounting bracket to frame, frame section to frame section, and ground wire to frame all have to be clean and solid. Contact has to be clean shiny metal. After a solid connection is made, then paint can be used to seal it against the weather. A “Bad Ground” is actually a bad connection somewhere in the trailer frame. - Mi ing or Broken Ground Wire.
This is the connection between the connector and the trailer frame. It’s easy to break this one due to exce ive flexing or catching on something, and your lights still might work. Without the wire you might have a connection through the ball and coupler. Odds are any connection will be flaky and intermittent.
Troubleshooting
Before we can do the job of troubleshooting, we’re going to need some tools. The most versatile tool is a hand held volt/ohm meter. You can find these meters at different retail outlets, like Radio Shack or your local hardware store. Accuracy isn’t a big deal so there’s no reason to spend much money. I carry a simple digital multimeter I bought at a cheapo tool outlet for $4.00! This meter is used to determine if voltage is present, or if there is a short circuit somewhere. You can use your meter in the voltmeter mode to measure the voltage present at the connector on the back of your tow vehicle. Running lights should be a steady reading somewhere around 12 to 14 volts. A turn signal voltage reading is going to jump around because it’s turning on and off (blinking) but the meter will still show you if there is anything going on. In the resistance mode, you can use the meter to check for short circuits or to test continuity. For example, if you measure resistance between ground (trailer frame) and any of the other three pins on the connector, you should see a very low reading. That’s because a cold light bulb filament looks like a dead short, but this is telling you that there is continuity from the connector all the way through the load and to the return. If you pull out the bulb, the meter should show an open circuit (very high resistance). If it doesn’t, then you have a short circuit.
* See figure 5 for typical voltages and resistance measurement values.
The other handy device to have is a simple jig made of a four-pin trailer light connector and some light bulbs. See figure 6. Plugging this jig into the connector on the back of your towing vehicle will let you know if the problem is in your trailer, or in the towing vehicle. These things are available from several sources, like Northern Tool.
** See the list of Internet addre es at the end of this article.
Troubleshooting is the proce of elimination. Something isn’t working back there and we need to narrow down the po ibilities. The whole idea is to eliminate the parts of the trailer wiring that aren’t the problem so we can zero in on the places that could be the problem. If we can split the problem in half, and then split it in half again and then again, we can narrow in on the bad spot pretty quickly.
The first step is always to see if the problem is with the trailer wiring or if it’s with the tow vehicle. This is easily done with the test jig. Plug the test jig into the trailer light connector on the back of the vehicle and then turn on the headlights, try the turn signals, and step on the brakes to see that the correct lights light up on the test jig. If that all looks good then you know the problem is in the trailer. You can do the same tests with your voltmeter. We just split the problem in half with one simple test: Is it the towing vehicle or the trailer? If we can eliminate one of them as the culprit, then there is a lot le hardware to have to deal with!
If the problem is in the trailer, step back for a minute and think about what we learned earlier. There are three separate circuits in our trailer: running lights, a left hand turn signal, and a right hand turn signal. They all share the same return, the trailer frame or “ground”. In addition, there are two bulbs in parallel for the running lights. Here are some po ible problems and troubleshooting suggestions.
- Nothing works at all, no lights whatsoever.
Verify that things are OK at the towing vehicle with either the test jig or your voltmeter. Is the trailer wiring blowing fuses in your tow vehicle? That would suggest that there’s a short somewhere in the trailer wiring. There is one thing common to all three circuits: the ground. Since all of the circuits use this same return conductor, start looking at the trailer frame (ground) side of things. Is that ground wire from the connector attached to the frame? It’s easy to change bulbs, try that next. - No running lights.
We know that the lights on each side (right and left) are part of the same circuit. They share the same wire from the connector and then “Y” off to the left and the right. Both bulbs use the trailer frame for a return. If the turn signals work OK, then we can a ume that the ground is OK so the problem must be with the brown wire between the connector and the “Y”. Verify that things are OK at the towing vehicle with either the test jig or your voltmeter. Is there a blown fuse? Use your meter in resistance mode to verify continuity between the connector (brown wire) and ground with the bulbs in the sockets. You could try changing bulbs, even if it would be a coincidence to have both bulbs fail at the same time. If neither bulb lights up then chances are the problem is with the brown wire between the connector and the bulbs. If one of them doesn’t light up then it’s probably the bulb or the socket. - One turn signal/brake light doesn’t work.
We know that the turn signal circuits are independent of each other and separate from the running lights, other than that they share the same return wire (Ground). If we know the tow vehicle is working OK, and we know that the other turn signal and the running lights work OK, then the problem has to be between the trailer connector and the bulb. Take your meter and measure continuity between the trailer connector (the green wire for the right hand turn signal or the yellow wire for the left hand turn signal) and ground. It should show a very low resistance. If it doesn’t, then the circuit is open somewhere. Start on one end or the other and look it over. Is the bulb OK? Is the socket clean? Are the connector pins on the connector clean and shiny? - The lights work intermittently.
If both sides act flakey, then you have a bad ground connection. This usually means that the white wire from the trailer connector and the frame has broken off. You might get a ground return path through the trailer ball but that’s a poor way to do the job and usually acts up. Check that wire and fix it if nece ary. - Bad Ground.
When power flows through a corroded connection, strange things can happen if a low amount of current is flowing through the corroded connection it might work just fine. However when higher current is flowing, the connection may heat up and stop conducting! An example of this situation is when the running lights are on (low current) and then you step on the brakes (high current). The higher current heats up the connection and then it quits conducting electricity. Everything goes completely dead even though it seems to work with just the running lights on. This kind of weird behavior can sometimes be fixed by tightening all mechanical connections. This means the screw that holds the ground wire lug to the trailer frame, the nuts on the back of the trailer light a emblies, and any other bolts that hold the frame together.
I hope you find this information useful. If you have any questions I can be reached through In-Depthangling via PM or e-mail.
Steve Root
Rootski
* Figure #5 Typical Voltage and Resistance Measurements
- With bulbs in the sockets, measuring resistance between trailer connector pins and the frame of the trailer.
- Brown wire to Frame – Low or zero ohms
- Green Wire to Frame – Low or zero ohms
- Yellow wire to Frame – Low or zero ohms
- White wire to Frame – Low or zero ohms
- With no bulbs in the sockets, measuring resistance between trailer connector pins and the frame of the trailer.
- Brown wire to Frame – High or infinite ohms
- Green Wire to Frame – High or infinite ohms
- Yellow wire to Frame – High or infinite ohms
- White wire to Frame – Low or zero ohms
- On the tow vehicle, measuring voltages at the connector. Only headlights are on.
- Brown wire to Frame – 12 volts
- Green Wire to Frame – Zero volts
- Yellow wire to Frame – Zero volts
- White wire to Frame – Zero volts
- On the tow vehicle, measuring voltages at the connector. Only brake lights on.
- Brown wire to Frame – Zero volts
- Green Wire to Frame – 12 volts
- Yellow wire to Frame – 12 volts
- White wire to Frame – Zero volts
** A Google search for “trailer connector testers” resulted in a quite a few hits. Here’s an example of a handy little trailer connector tester. This devices will tell you if your tow vehicle connections are working.
