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.

Questions and Comments about this Video

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Gage 03/29/2019

I have a 7 way connector that goes into a 7 way receiver on the back of a 1998 ford f-150. I have checked the 7-way on the truck and it has three places where it lights up with the tester. However when I test light the 7-way connector on the trailer, nothing lights up. theres a switch box where the wire leads in the trailer and it says onoffautoIm no sure if I need to replace a fw of the fuses in the engine fuse box or need to replace the 7-way connector from the trailer wires, or the wires itself. I dont know how to get power to this 1996 Security truck camper because there is nothing online about it and I cant find the data sheet anywhere inside of the trailer. I need the lights to work in order to drive the truck with the truck camper on it. Thanks 51182

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Rachael H 03/29/2019

I would start with the truck and get the 7-way there functional first. I would check the ground light, wire first and then test by function as activated to see what is and what is not working. The functions on the 7-way are detailed in the pin configuration on the attached help article. It would not hurt to check fuses but I would isolate which ones are and aren't working. A fixing a bad ground can fix several issues. Once you get the truck figured out, it sounds like you will need an additional in-bed camper harness then you would need to consider something like the # A10-7010. This will T-into the existing harness and provide an additional 7-way in the bed. For the trailer you will need to apply a 12V power source to each function and test as needed. If there is a junction box I would check the functions going in and going out of the junction box, and again look for loose connections or bad grounds. They can be the cause of many trailer wiring issues. You will want to check each light on the trailer too, because some trailer lights ground through the mounting hardware, and that can be the cause of your short. Another quick check would be to plug the trailer into a different vehicle that has a 7-way and not your F-150, and see if there is any activity on the trailer either. 46734

George 12/18/2012

I re-wired a utility trailer. The taillights work fine but the turn signals and brake lights do not come on. I check all connections and the ground and everything appears fine. Any ideas? 670

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Patrick B 12/19/2012

Use a circuit tester to first check the vehicle trailer connector. If everything works there, then you know the problem is on the trailer. If everything does not work on the vehicle, start checking fuses and relays as a blown fuse or relay will cause function loss on a trailer connector. Check all power blocks and fuse panels both inside the cab and in the engine bay. If it is an aftermarket trailer wire set up, check the connectors and if it has a converter box, make sure all signals go into the box, and all come out. If the trailer connector is operating properly on the vehicle, then the problem is probably on the trailer. Trace each wire, one by one, looking for breaks or frays. Make sure each ground has good contact with clean metal. Paint and rust can insulate and prevent a complete circuit. 380

Leisha 11/10/2012

89 Chevrolet k1500 had a t in the wiring for a trailer plug 4 prong. My tail brake and back blinkers all quit after plugging on a trailer and only one side working with it plugged in .unplug the trailer and nothing for rear lights at all and fast front blinkers. I replaced all my fuses under thedazh on the panel. I replaced my flasher and all 6 tail light bulbs. None of these items looked blown before replacement of them. Nothing but tail lights now. I removed the t for the trailer plugand hooked my tail light plug back direct to the wireharness and everything works normal and right. So I buy a new t for plugging in the trailer and plugged it in. And now everything works except my tail lights. I plug in the trailer and now my truck and the trailers lights work even the tail lights like they should. I unplugged the trailer now I have no tail brake blinkers or anything in the back of the truck and my blinkers are going super fast in the front. I cant get it and I have to have my truck lights and trailer plug all working right as my job has me pulling a trailer in heavy traffic with a huge sign. So NP fox for this no job for me help.. 399

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Patrick B 11/12/2012

I would check the fuse block inside the engine bay as well. Check for blown fuses and bad or burned relays. It also sounds like your trailer may have a bad ground or a shorted circuit. Check the grounds on the trailer and on the vehicle. Make sure they are attached to clean, solid metal. No rust or corrosion. Then check the wiring on the trailer. Look for frays or spots where the rubber is wearing. If the trailer has a shorted circuit it could be causing the fuses, relays or bulbs to blow on the truck. Check the T-connection underneath the truck. Make sure it is clean and not corroded. Corroded connectors can cause signals to bleed over and malfunction. 164

Blake S 10/09/2012

Hello:I have a 2007 Chrysler Pacifca with a 118303. I am getting flashers, signals, and brake lights working but no running lights or tail lights. Connectors are clean, but cant figure it out. 234

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Patrick B 10/10/2012

The first and easiest thing to check is the 4-Way itself. Using a circuit tester, check the running/tail light contact on the 4-pole coming off the harness. Clip the ground of the tester to the ground on the connector and put the probe end onto the contact coming from the brown wire. If the tester lights up with the running lights on, then the problem is probably on the trailer. If it checks out though, it's time to check the box. Again use the circuit tester to check the brown wire before the box. If the signal is good, then we know signal is going to the box. If there is no signal, then there's something amiss with the connector behind the tail light. If the signal is good before the box, check the brown wire coming out of the box down to the 4 pole. If there's signal in, but no signal out, then the box is bad. Here's a link to a fairly standard circuit tester. 79

Mike C. 10/05/2012

I just installed part number ETBC7 on my 1999 Toyota Tacoma. When I plug in my trailers 7 pin plug I get all appropriate lights on my trailer with the exception of my left turn signal. My left turn signal only functions when the brake pedal is also depressed and the brake lights activated as well. This is true for a left turn signal initiated by the turn signal stalk on the steering column or by turning on the trucks hazard lights. I know it is not a problem with the trailer as its lights function properly when plugged into my other truck. 203

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Patrick B 10/08/2012

The first thing to do would be to check all the connections where you tapped the ETBC7 into the factory wiring on your Tacoma. Make sure they're clean and the wires aren't frayed. We have had issues with doing these on Toyota trucks and SUV's. Sometimes grounding at the frame isn't sufficient. A bad ground can create all sorts of oddball electrical symptoms. To check if it's a bad ground, grab a long piece of wire. Have someone hold one end onto the negative battery terminal, and touch the other end to the point where you grounded the ETBC7. If this clears the problem, that means the ground is not sufficient and the best solution is to extend the ground wire to the negative terminal on the battery. If none of these work, write back and let me know if anything happened differently while trying the ground jumper. 71

Ron Bogue 09/24/2012

I have a 2002 Toyota Siena with the factory towing package. The running lights work the turn signals work but the brake lights do not work on the vehicle four prong plug. What could be the problem? 78

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Patrick B 09/25/2012

Diagnosing trailer wiring troubles on an OEM harness can be difficult as so much of it is hidden away and tucked into the factory electrical harness. Sometimes it could be a relay in the power distribution block has gone bad, or you could have a wire frayed somewhere along the vehicle. If you have separate turn signal bulbs, typically amber, check to make sure they are not burned out. You can always get underneath the vehicle and begin tracing from the 4 pole back to see if you can pinpoint any damage or loose connections. Clean connectors with an electrical terminal cleaner and reconnect them with a bit of dielectric grease, part # 11755. It will help keep dirt and moisture off the contacts. If a problem source can't be pinpointed, sometimes it's best to add an aftermarket system. If the tail lights on the Sienna work properly, the aftermarket harness just T's into the existing wiring. 30

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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