bing tracking image

Testing and Troubleshooting a Modulite Vehicle Wiring Harness

content loading

Products Featured in this Video

Modulite Vehicle Wiring Harness Testing and Troubleshooting

All right today we are going to go over how to check over a modulite as shown right here which converts a 5 wire to a 4 wire system. We are going to go over the basics which are really simple. Basically we are just going to test input into the modulite and test the output of the modulite. That will tell us one of two things, we have no input going into the modulite, that means there is something wrong on the car side of the connector, there is nothing wrong with the modulite. If you have power going into it, but nothing coming out then you know chances are it is the modulite or there is a problem on the trailer itself. When testing you always want to test these modulites or converter boxes without the trailer hooked up first.

So you rule out either A the trailer, B the converter box, or C the car it is hooked up to. Get yourself a good light tester, one that has an incandescent light that will withdraw enough power to give it enough load for the converter to work against. And of course this is your ground connection. All right and starting off what you want to do is take what you used for your ground wire on your wire harness you want to stick your ground clip of the tester to the same place. That will tell us if you have power going into it and will also tell us if the ground you are using is good also. We will go ahead and ground this to the same ground that we used for our wire harness here.

And so at this point we will go ahead test power. The first thing we are going to test is the black wire right here. And what that is going to do is make sure we have power going to be connected directly to the battery. So first off you want to make sure you have a good fuse going up to the battery. Check that, when that is good go ahead and make sure your have power going into the box. That way you know if you have any power interruptions or a bad connection or something like that. You are going to take your light tester and pierce the wire, and check for power. And we have power right here, so we know we have power going into it. And for the rest of the wires you probably want make sure you keep the engine on or at least the key on the on position so all the electronics are working in the vehicle. So then you will simply go through the rest of the wires and check to make sure you have power going into the box. So we will do the stop light circuit first which is going to be the red wire. Okay with the brake pedal pushed down the red wire lit up so we have power going in there, so that is good. Now the same thing, we will turn on the right-turn signal. So we have a right turn going in. So we will do the same thing for the left side. So now we know we have power going through the yellow, so that connection is good. The next thing will be our running light circuit, which is the brown wire. Okay so we have power going in there. So we have all the power going into it.

If you did not have power going into it the problem is one of two things, one it is either the physical connection to the car itself or two the ground is not good enough to begin with. So what you want to do is go ahead and find a different place to ground the wire harness at. Hopefully there is a piece of sheet metal or maybe you can find a factory wiring ground somewhere beneath the carpet somewhere in the vehicle. Or if you are not sure where to ground it one other option is to run a temporary ground from the white wire up to the negative post of the battery. That way you know you have a good ground and you can test your wires that way. That is another way you can check your input, so that is how you test your ground. So you have that car side covered, now what we are going to do is check the output to the trailer.

Again it is the same thing as before. So we left off with the running lights so we will go ahead back to the running light circuit on the brown wire on output. So that is good. Now do the same thing on the right-turn signal. What you want to do is test both sides of this because what happens inside the modulite is that it combines the brake signal coming from the red with green and yellow. So it brings it down to a four wire system. Now I have a good right-turn signal so I will do the same thing with the left-turn signal. We are good there too. Okay, you do not have to worry about the ground wire on this side because we already verified the ground on the input side. So if you have everything working as shown here that means this converter is working as it is supposed to. So the problem is probably going to be further down the road. It can be A, a dirty connection going to the trailer or B, it can be a problem within the trailer itself somewhere in the wire harness or tail light.

Kyle L.


driver side blinker light stays on passenger side is fine can you help me please

Patrick B.


The best thing you can do to start narrowing down the problem is check the wires right at the box. Use a circuit tester with a sharp probe and test the wires going into the box. If everything operates as it should going in, then check the wires coming out. If not all signals are properly going in, then check your connections to the vehicle. If you check the wires in and out, and both sides are fine, then you may have a ground issue with the trailer. Make sure the ground wire on the trailer side harness is securely connected to the trailer frame and the connection is free of corrosion. If the wire signals go in the box fine, but are not coming out as they should, then the box may need to be replaced.

Info for this part was:

Employee Jeff D
Installed by:
Jeff D
Employee Joshua S
Video Edited:
Joshua S
Employee Nicholas E
Test Fit:
Nicholas E
Employee Alan C
Video by:
Alan C

At we provide the best information available about the products we sell. We take the quality of our information seriously so that you can get the right part the first time. Let us know if anything is missing or if you have any questions.

About Us
photos and videos
Original Photos & Videos

Produced to make sure you know what you are getting and you get exactly what you need.

Installations Completed

To make sure products work and fit the way they are supposed to.

etrailer call center
Phone Calls & Emails Answered

1,125,924 phone calls and 1,350,587 emails to help find the right solution.

etrailer training
Average Hours of Product Training

We get to know our products firsthand so experts can better help you.

etrailer service
Years of Quality Customer Service

Assisting our neighbors and customers, face to face at the counter.

etrailer experts
Pages of Expert Information

Created to make sure you have all the answers to your questions, from real experts.