Trailer Wiring 101

[Jeremy airjer W]

The trailer lights are giving you trouble and you’re not sure where to begin. The first thing you need to determine is what is or is not working. There are two things that will make your lights work incorrectly. One obviously is the lack off power which will result in everything working except for one light. The other is grounding issues which will usually cause strange things to happen.

Power Issues

This first part will deal with power issues. Our objective is to narrow down the possibilities until we have isolated the problem. The information below already assumes that you have checked or replaced the bulbs just incase they where burnt out.

The first thing I want to do is find out if it is the truck or the trailer. The simplest way to check the truck is with a test light.

Since most of us use a four flat style connector I will be basing everything on that. Whether it’s an aftermarket four way connector or a 7 pin to four adapter the male terminal on the four way connector on the vehicle is the ground. Simply hook the test light to the ground then turn on the trucks lights and four way flashers and probe the other three terminals. If the other three terminals light/flash the test light than you can be reasonable confident that the vehicle side of the wiring is fine.

What if one of the terminals does not light up the test light? If it’s an aftermarket harness and everything else on the truck is working than the harness itself may be at fault since the aftermarket harness uses the same power that is provided to the lights on the vehicle. If it’s a factory wiring setup than the fuses are first on my list of things to check. Typical factory trailer wiring will have there own fuses for each function of the trailer wiring.

What if all the terminals don’t illuminate the test light? My first thought is bad ground. We have all had some pretty shady looking connectors and if there green they aren’t going to work.

If there clean then I would try finding another way of grounding the test light and rechecking the three terminals (when finding another ground source it doesn’t hurt to make sure it is a good ground by checking a wire that you know has power to it to make sure the test light lights up). If the test light lights up then we know we can concentrate on the ground if the test light still doesn’t light up then we need to dig a little deeper into the aftermarket harness or there may be a problem with the 7 to 4 adapter. If you suspect the adapter you can remove it and check the running light, left turn and right turn pins on the seven way connector. If they work at the 7 pin, than the adapter is likely your problem.

So the truck checks out fine. Now what? The first thing I would do is look at all the exposed wiring on the trailer. Some things you should look for are broken wires, pinched wires, corroded connections, and bare wires. Connection made with either wire nuts or scotch locks should also be looked at carefully as many times they can be the cause of the failure since they do not “seal” out the elements which allows the wires to corrode.

Also pay close attention to the lead buttons on the bottom of the bulbs. I have run into many of these where the buttons have worn out and causing them to loose contact with the socket. In this case the buttons have worn enough that they are touching, the symptom was that the brake lights would be on as soon as the customer turned on the tail lights.

More often than not trailer lighting problems can be resolved by repairing one of the problems mentioned above.

If the wiring looks to be in good shape, than the next step is to check for power at the socket. Again the simplest way is to use a test light. Socket for the tail/brake light will have two contacts on the bottom. One will supply power when the brake/turn signal is applied and the other when the lights are turned on.

(Photo courtesy of Shackbash)

If there is no power than its as simple as tracing that circuit back until you find the source of the power loss.

Ground Issues

So your trailer lights are acting weird one light blinks opposite the other when it’s not supposed to. When you step on the brakes all the lights go out. The lights are really dim. The lights flicker when traveling down the road. These are all possible ground issues and are not that hard to figure out.

The first thing I do when I suspect a bad ground is hook up a test light to a known good ground on the vehicle (preferable the negative battery terminal using a long jumper wire). Next, with the trailer plugged in to the vehicle and with the symptom present, touch the test light to the trailer (preferable a clean metal surface). If the test light at any point lights up then there is a main ground issue with the trailer. Usually between the trailer plug-in and where the white wire is attached to the trailer. Make sure that the area where the white wire is attached is clean and corrosion free, as well as the wire and/or connector. A star washer does a good job between the wiring terminal and the trailer frame to make a good ground contact, or in between the washer and the nut on the back of the taillight.

(Photo courtesy of Shackbash)

There still can be isolated ground issues that will not light up the test light in the scenario above. This could be anything from a corroded socket, poor contact between the lens assembly ground post and the trailer, or the socket ground wire and the trailer. All of these can be checked buy using a jumper wire to a good ground and then grounding the suspected component to see if the problem corrects itself.

(This lens assembly uses the mounting bolts to ground itself to the trailer - Photo courtesy of Shackbash)

Most trailer lighting issues can be solved with simple repairs. Remember to start with the simple things first.

-Identify any and all issues that may be present.

-Determine if the source of the problem is the vehicle or the trailer.

-If it’s the vehicle check fuses and connections

-If it’s the trailer check the wiring, bulbs, sockets, and connections

-Make sure there is a good power supply and a good ground. These are the two most common problems.

Special thanks to Shackbash, Marine_man, and 4wanderingeyes for there help!

[Jeremy airjer W]

Just to clarify.

The. Brake lights are the only thing not working? And they don't work with the running lights on or off?

If this is the case than its still an issue on the trailer side. Fuse 36 and 42 are for the turn signals and the stop lights. If the fuses where blown you would not have brake or turn signals on both the trailer and the vehicle. Since you have turn signals on the trailer and you have brake lights on the vehicle the fuses have to be good along with the wiring on the vehicle. I have a feeling this is a ground issue with the trailer and you aren't seeing the signs that this is the case.

Normally when there is a ground issue funky things will happen. Have you tried the turn signals with the running lights on to see if they still work?

[Whoaru99]

Not familiar with either the specific trailer or the specific vehicle, but this isn't a case where one of the 3 wire > 2 wire converters are necessary is it?

[Craigums]

Starting to think its a ground issue. I'm going to throw a meter on it tonight.

[Jeremy airjer W]

Not familiar with either the specific trailer or the specific vehicle, but this isn't a case where one of the 3 wire > 2 wire converters are necessary is it?

No, brake and turn are on the same bulb on the truck. 90% sure it has the factory trailer wiring.

[JIvers]

I just re-wired my whole boat trailer; I ran the new trailer wiring through soft PVC tubing to try and protect it from wear and tear.

I don't trust myself enough to solder, or otherwise attach, the trailer lights to extension cords. I picture melted wires, blown fuses, and all sorts of other problems if I tried that.

Just an update: so far, the new wiring is working perfectly. I don't think I mentioned it, but I bought the boat trailer brand new in 2004 or 2005, and the original wiring worked right for maybe a year.

Point being, if I can rip out the old wiring after messing with it for years and put in new wiring cheaply and easily and have it work right, it's pretty easy to do.

[bowhunternw]

What is the proper way to do a y splice on the wiring. I understand how to do 2 wires but how do you seal 2 into 1?

[Lip_Ripper Guy]

I take the heat shrink butt connectors, wrap the two wires tight, shrink the connector, and then seal it up better with liquid electrical tape. They make special connectors for it, but they are pretty expensive.

[Jeremy airjer W]

What is the proper way to do a y splice on the wiring. I understand how to do 2 wires but how do you seal 2 into 1?

If you get the heat shrink tubing with the glue/sealer in it when you shrink the connection the sealer will start to ooze and seal the two wires together.

If you cant find that type of shrink wrap than coat the connection liberally with liquid black tape and then use a piece of regular shrink wrap over it. The liquid tape will ooze and seal up the wires.

[somann]

Hello all, I have an issue with my trailer lights and I am reasonably sure it is the truck, not the trailer, but it has me stumped. I have a 2002 chev trailblazer with factory installed 7 pin connection. I have a cargo trailer with the 7 pin plug which plugs in directly to the factory installed receptacle. What happens is the left turn/stop light on the trailer is energized at all times. When the vehicle is on, and lights are turned on, all other lights work, but the signals act up. I believe when I turn left signal on, the left signal on the trailer works, but when right signal is turned on, both blink like the hazards are on. This is only showing up on the trailer, truck lights function correct. My trouble shooting has found that I have constant 12 volts in the truck 7 pin connector on the left rear/stop pin. The truck and all lights can be off, and I still have power on that pin. Comparing to the right turn/stop pin, I have no power unless the brake is depressed, or the right signal is on. I checked the connector ground wire where it is terminated on the frame. I removed it and cleaned the contact points down to bare clean metal and re-terminated the ground wire with no change. All fuses where checked and all fine, I did verify in the fuse box that there is a fuse for the circuit in question, and when I checked voltage from one leg of the fuse to the frame, I do also have live power all all times. Again checking the right side fuse, I have no power unless brake is pressed....working as it should. So. I am trying to figure out why constant power on the left?? I followed the wires from the rear, forward, but they are all wrapped inside protective sheaths that join up with other branches of wires as they head forward, so see anything unless you really tear some things apart. Am I overlooking something simple? Or is the wiring suspect? I am not aware of a relay that serves only the circuit in question, otherwise I would suspect that relay being stuck closed, passing current at all times. I don't believe that is the case. Where is this circuit getting 12 volts when it should not have it??

[Macgyver55]

I recently ran into a similar situation on a Silverado. After a lot of circuit testing I found the problem to be the turn signal flasher unit. It really didn't seem plausible if you think of how an old style flasher works, but after opening up the flasher and seeing how it worked I was able to actually repair the stuck relay in the flasher.

[somann]

In your case was the rear blinker on the vehicle stuck on as well as the trailer? I wonder if a separate flasher handles the trailer, because if not, I'd expect to have the rear left blinker illuminated at all times on the vehicle also. I only am seeing the issue on the trailer, the vehicle lights all function as normal with or without the trailer.

[Jeremy airjer W]

I'd have my money on the turn signal/flasher module. Everything originates from there. There are seperate circuits for the vehicle and the trailer so your symptom does make sense. If you have power on the left all the time than you can go to the left trailer turn fuse and also see if there is power there. If there is pull the fuse and check check for power on the pins in the fuse block. When you figure out which pin has power unplug the flasher module. If power goes away replace the flasher module. If power persists that you will need to start digging into the harness or fuse block.

[hunter292]

I have had problem after problem with my trailer lights this year and it seems ever time I fix the problem another one happens. The trailer is a 8'x10' 2 place snowmobile trailer and I'm just going to strip all the wire and replace the wire with new. Does anyone have an idea on how much wire I'll need? It has marker/clearance lights front and back corners and tail lights. Just a 4 flat plug is what I'll use. Thanks!!

[amateurfishing]

You did not specify if you are keeping your current lights or not, if so, just by the 4 plug connector & wiring at the big M store, think they have it in 15 & 25 ft lengths, will need extra any way (brown& white) for your ground & brake lights. never hurts to have extra laying around for emergencies...

....or if you are looking for a whole set it should be all wired and ready to go

[hunter292]

That is about how much I thought I'd need. I think I'll just get probably 35ft. Thanks!

[jeremyd]

Hi everyone, I need some advice on a trailer wiring issue. I replaced the lights and wires on my small boat trailer and the turn signals and brake lights will not work, but the tail lights do work. I also tried to install the marker lights on the side, and used the snap connector that came with the package to splice the wires, but they do not work either. My vehicle has a 7 pin round connector and the lights came with a 4 wire flat that has its own seperate ground wire. I use a 4 flat extension since there was not enough length to reach all the way to my truck. The extension has only 4 wires, with the ground together with the taillights. Could this cause the problem since the one has 5 wires and the extension has 4 wires? I have ruled out ground issues, checked all fuses and cleaned all the adapter and vehicle connections. I have an 4 flat to 7 pin adapter that has three led lights on it I guess to show that everything has power but only one lights up. I should state that the old lights worked and the lights i installed have never worked properly. Basically my question is, does this sound like my extension is the problem or is it very possible I wired it incorrectly. Any help is very much appreciated.

[Jeremy airjer W]

Welcome to HSO Jeremy!

If possible can you post a picture of your connections?

[jeremyd]

Thanks for the welcome, glad I found this site. Here is some pictures of my connections

[jeremyd]

[Dave]

Use a 12volt battery with wires attached and manually light up your trailer lights without any extensions. If they don't light, fix that first.

[Jeremy airjer W]

I was thinking on the right track. You have two brown wires. The brown wires are the tail light/side marker wires. They supply two so that it is easier to run down both sides of the trailer. These wires should have 12 volts on them. One will go down the right side of the trailer one will go down the left side of the trailer. Both of these wires are power supply wires not grounds. The tai lights and side markers are fed off these two wires.

The white wire is the only ground wire on a trailer harness.

The yellow wire is power for the left turn/brake

The green wire is power for the right turn brake.

If you used one of the brown wires for a ground than you likely blew fuses on the truck side. The LEDs not lighting up May support this theory. It is possible your seven to four pin adapter is jacked also. You can check the seven way on the vehicle with a test light. There are diagrams on the first page of this thread to help you out.

[jeremyd]

Thanks for the replies, I have checked every fuse i can find related to the trailer and they are all good, and I have also tried a brand new adapter and extension and that didn't solve the problem. I will reference the info you told me about on the first page. One more question, would the tail lights work if I wired it incorrectly? Also, im not sure I understand what you mean by connecting to a 12 volt battery to test lights. If possible could you elaborate on that. I really apprecIate the help.

[Jeremy airjer W]

Use any 12 volt battery. Car battery, boat battery, flasher battery, etc. use a jumper wire to connect the white wire of the trailer harness to the ground of the battery. Use another jumper wire to connect power to one of the other three wires. Be careful when doing this if there is a short the circuit will not be protected by a fuse unless you add one to the power jumper wire.

At this point a test light will be your best friend. You can at least confirm wether of not the vehicle is supplying power to the trailer.

Year, Make, Model, and Engine Size of your vehicle?

[jeremyd]

03 Chevy trailblazer 4.2 6 cyl Thanks again

[Jeremy airjer W]

The two fuses you are looking for are the TRL R and TRL L. They will be in the underwood fuseblock. With the turn signal switch in the right position you should have power on both sides of the TRL R fuse. If there is no power at the TRL R fuse than I would suspect a bad turn signal/hazard module.