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Trailer Wiring Harnesses Troubleshooting

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How to Troubleshoot a Trailer Wiring Harness

Today we are going to cover kind of a different subject. Basically, we are going to talk about getting your wiring installed and what is happening afterward. Okay, for instance, you went to the trouble to get your wiring installed, you plug it into your trailer, and nothing happens. There could be a lot of different things going on with it. First off, you need to figure out for sure whether it is on the towing vehicle side - the car or truck - or if it is something on the trailer. We will show you how to test the different types of wiring systems we have shown here. And that way you can tell for sure if it is your wiring harness or if it is your trailer.

First off,we have a regular T-one connector that goes straight into the vehicle itself and makes its connections and then goes out to the trailer - a real simple setup. Really these are pretty much bulletproof. The only thing that could really happen is, if you do not have the ground in the right spot, then these possibly will not work right. Other than that, these are pretty bulletproof. And the next one we have over here, we have a T-one connector with a converter installed in the middle of the harness. Basically, this is about the weak spot on this type of wiring. There are five wires coming in and it converts down to a four-wire system and back out. A lot of times, in here this could break, for whatever reason. It could be from the factory. It could be when you plug it into your trailer you have a short in it and you do not know about it. It will cook these almost in an instant. It depends on how severe the short is.

Also we have another wire harness with the hard wire. Again, it basically just has the ends here that you hard wire into the vehicle. Pretty similar to your T-one. It has the same converter and everything. All the testing procedures apply to both of them. In fact, we will use this one to show you how to test everything on the wire harnesses. First off, we are going to show you the simplest T-one connector. Again, this is the kind that just plugs into your vehicle and you do not even have to worry about ground wire, having to screw it to sheet metal or frame. This one, what you want to do is to test it. Basically there are two ways you can do it. First off, with it plugged in, you want to test behind the connector right here. You want to test these wires with a simple wire tester. You can use something like this right here. This is a real fancy version but you can get a simple one off a website or at a local auto parts store. Basically it consists of an alligator clip and a probe right here. You simply just pierce the wires. When you do this, you want to ground this to the ground on the four-pole right here. And the reason why you want to test it here first, as aside from back here, is to make sure you have got power coming out of the connection here and going to the wires. Because if you have power here but you find out you do not have anything here, that lets you know that somewhere in the wire is a break. And you could actually work your way down piercing the wire to find out where the break is and then make a repair that way. Basically, though, at that point, it probably should be taken out and replaced. But if you are stuck on the road somewhere or in a hurry, you can patch it up that way.

If you actually figure that out and you have got power going here and you want to test at the end of the four-pole, go ahead you can just use a light tester. And that lets you know that you have power coming here. And you do the same thing for the other two circuits. This controls the right turn, this is the left turn, and this is the running light. If we have everything working here, then basically your T-one connector is perfect, you do not have to worry about it, it is going to be something on your trailer. Now the other type of connector is a little bit different. You might have a T connector without a converter box in here. However, sometimes there will be a lead coming off of it that has a white wire and you do ground this to the body or to the frame of the vehicle. Again, the same testing principles apply. You also may want to test at this point where you grounded at. So we are going to pretend we have this grounded right here on the body. Go ahead and connect up your wire tester to what you are using for ground and then do your testing in the four-pole again. If you do not get anything there, it could be one of two things. It could be one, your ground is just no good, and you will not get anything because the ground has to be working on the trailer, too. In that case, to double-check the ground, what you want to do is take your alligator clip and ground it somewhere else on the frame, and then go ahead and check your four-pole again. If you get power, again, at the four-pole with the ground somewhere else, then you know that tells you this ground here is not working for whatever reason. It could be going to a body panel that has been epoxied together, or a whole body component could be sitting on top of the body mount, which is insulated by rubber and you will not get ground that way to the frame, either. Two ways to work around that would be to go ahead and find a ground wire and just simply connect to it. But a better point would be to go ahead and find a better place on the frame and just reattach it. A worst-case scenario to test it to make sure it is working is actually to run a jumper from this wire here up to the ground in your battery. That way you know for sure that you have got power coming through your wire harness. And you know you have good ground from the battery, so you know the ground is not good at the back of your vehicle. Other than that, these things really cannot be hurt. So that is pretty much it for this style of connector.

All right, next we are going to talk about testing on a wire harness, or a T-one connector, that has the converter installed like we have one here. I will show you how to test it. It is pretty similar to the first one I showed you, just a couple more points that you need to double-check while you are testing it. We will actually use this model here, the hard wire version, to test it with but the same things apply to both. First off, the converter box will not work if you do not have the proper input from it. So what you want to do is go ahead and test on this side of the box first, the input. You use your wire tester. Again, at the four-pole end go ahead and ground it. And then you want to go ahead and just pierce each wire. Like, on here, let us say we have the running light circuit on. Just go ahead and pierce the wire and make sure you have got power going through the box. And then you want to do that for the other remaining circuits, too. Like, for instance, the brake light. Separate the brake wire and pierce it and tap the brakes, and okay you have input going there. And then you would do the same thing again to the other two. Once you have all four of these going into the converter box, you know that part is fine. The next point is to go ahead test on the other side of the converter box coming out the four-pole. That way you know you have the proper current signal coming in and back out. Okay, our running light is on, and then if we hit our brakes we should have power coming out the yellow and the green. That tells us our converter is looking fine, so we have signal on both sides. Now if we only have one brake on just one of the turn signal leads, that means that there is something inside the converter that has died, we cannot fix it. It just has to be replaced. So you want to double-check and make sure, before you plug the trailer in, that this is working properly. And when you do plug the trailer in, test it or keep a good eye on it, make the test brief because it may flash not as bright as before if there is a short in it. And if it is not as bright, say for example one side is brighter than the other, that means there is a short on one side and that you want to disconnect as fast as possible and test your trailer independently to find out where the short is in the problem. So basically, for a recap, again, you want to check your connector, regardless of whether it is a converter or not. You want to check the input going in. Again, with the wires going in if you have got a signal coming in here, great. And you want to repeat it on the other side, coming out and it is usual to do it at the end of the four-pole. If you have got power coming in and power coming out, then you know the four-pole is fine and you have to move on to your trailer. Pretty much that is it in testing it.

Info for this part was:

Installed by:
Jeff D
Video by:
Kathleen M
Test Fit:
Randy B
Video Edited:
Chris R

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