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This Old Trailer: Troubleshooting the Ground Wire

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Troubleshooting the Ground Wire Episode of This Old Trailer

The first thing you want to do when you first start working on electrical problems is you want to divide and conquer. You have two parts here, the truck and the trailer. You want to verify which one actually has the problem. Chances are it is going to be on the trailer, but then again you still want to check the truck too. So use a separate light tester, hook a ground not to the frame but to the 4-pull ground itself because this is what is going to carry the ground to the trailer. :52

So we will hook up our ground lead to the tester to the ground, we have our running lights on so we are good to go. We will do the same test with the left turn and right turn. Once we verify our truck is working fine then we can start working on the trailer. The hitch should only be used for pulling purposes, not for anything electrical. And that is the proper way how the trailer should be hooked up to the tow vehicle. 1:19

Just to give you a quick idea how electricity flows in the trailer what happens is the electricity flows through the brown, yellow, and the green wire goes through the lights and has to flow back in a circle like I said, so it all has to come back to ground. On trailers it goes to the frame, and from the trailer it has to go through the ground on the truck. And if there is any interruption it is going to cause the trailer lights not to work at all or do some weird stuff. We have our ground wire right here, as you can see it is loose in the air so it is not going to do you any good. We have wires disconnected so lets go out back and show you what is going to happen when you have no ground wire attached to the trailer.1:54

You can see here we actually have the lights on on the trailer, yet we still have the ground disconnected up front. Basically what is happening is starting to get ground any way it can. In this instance it is getting it through the ball on the truck. Now you get a brand new hitch with everything painted and nice and new, chances are this is not going to work this way. But on an older vehicle, with the parts worn in and used a lot where you still have metal to metal contact it probably will work. However, it is not very good on an electrical system, because when you go down the road you are going to get a constant on off all the time. You have probably seen guys going down the road and see their lights flicker every time they hit a bump. We will shake the trailer and get the same effect. And now we have the turn signal on. Again, the ground is not hooked up front, but it is getting ground from the ball. We will go ahead and shake the trailer again. See how it goes off? That is not good. So to fix that we will go ahead and reattach our ground. 2:51

Go ahead and run the ground screw through the trailer. Now that we have it reattached go ahead and test it back at the other end of the trailer. All right we have the running light circuit back on, go ahead and check the trailer, simulate it bouncing down the road.



Great video, very helpful. Our marker/tail lights work fine, but we cant seem to get the brake and turn signals working at all. Voltage from the car checks out fine. Green and yellow wires seem fine, and we buzzed them out to each bulb socket. The Brown wire connections also seem fine, but we are noticing that we have close to a short between Brown and Ground. Is that normal? If we remove all the bulbs and disconnect the cable to the car, the near short goes away. Thanks!

Etrailer Expert

Rachael H.


Grounding issues are the most common causes of trailer wiring issues. I would check and make sure that all the ground connections are secure.

Tom M.


Suddenly trailer and boat lights do not work with Toyota FJ. Have a Tacoma and lights work. Found no blown fuses in the FJ.

Etrailer Expert

Rachael H.


If the trailer lights work with a different vehicle then the issue is isolated to the vehicle and checking the fuses is a great start, but typically if no lights are working on the trailer at all then it is usually a grounding issue. I would make sure that the ground wire is properly secured on the vehicle trailer wiring harness. I have also included a couple of help articles that explain some other trouble shooting items to check.

Taylor M.


@RachaelH why would electricity be moving through my ground and powering different lights.

David B.


If you have a hot ground then there is a short or you don't actually have the ground wire. Read the linked answer page and let me know if it helps ok.



Thanks for the helpful video. Our trailer lights all stopped working a few days ago. We tested the car and all is in working shape with power coming through, which pointed to a trailer issue. We re-wired the trailer, strengthened the ground on the front of the frame, and cleaned up a couple of corroded grounds on the 2 side running lights. When we re-wired it, we needed to splice the brown wire for the running lights to cover both sides. When we hooked it back up, the spliced brown wire was touching the trailer frame and all lights came back up, but when we switch on the turn signal both left and right simultaneously blinked. When the spliced brown wire isnt touching the frame, none of the lights work again. Sounds like a grounding issue still, but we cant figure it out. Any ideas?

Patrick B.


Sounds like some wire colors were mixed up at one point. If the left and right sides blink simultaneously, they are on the same circuit and shouldn't be. It may be best to re-wire the whole trailer so you know exactly how it's done. Here's a link to an FAQ article that should help quite a bit with trailer wiring.

Jeff E.


I had a ground issue with a tilt trailer. I had to move the ground from the hitch area to the front of the actual trailer. The ground stopped working after I applied WD40 to the pivot point of the trailer.

Patrick B.


If the ground stopped working when you coated the pivot point with WD-40, that means the electrical system was grounding through the pivot point. If the lights are mounted to and grounded to the body of the tilt bucket, then they would lose ground if conductivity in the pivot point were altered. It may be necessary to ground the lights to the frame of the trailer, below the pivot point.

Christine C.


This video would have been helpful if it had included some diagrams and definitions of key terms. For those of us who are novices, the video skipped the basics. For example: What are the parts of the hitch/ball? How does the tester work? What is a 4-pull ground? The run-on sentence describing brown, yellow, and green wires is grammatically confusing. What do the brown and yellow wires do if the green goes to the lights? What are running lights? What is a down tongue? What is stud grounding? What is a ground screw? My problem is that rust forms all around the ground screw even though it is stainless steel. In the video, the hole was already drilled and there was no description of how the screw was removed. Recently, I ground down a section of the frame, wire-brushed the eye connector, and clamped the ground wire directly to the clean metal frame. Still no lights. The wiring harness is fairly new and worked fine in the past until the rust formed. I cant go anywhere legally until I get this problem solved.

Patrick B.


Electrical gremlins are frustrating, that's for sure. And no one video can cover it all. The first thing we want to do is isolate the problem between the trailer and the vehicle. First, check the 4 pole on the vehicle. If it works fine, then you know the problem is the trailer. I've added a link to that video. To check through the trailer, it may take running down the wires looking for knicks and breaks. We have an FAQ page that covers a ton of info on trailer wiring including what wire is what function. You can always contact us by phone and we can walk through testing procedures with you.

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Employee Cole B
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