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Circuit Testing for Vehicle Wiring

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How Circuit Test for Vehicle to Trailer Wiring


Today we are going to show you a little bit of wiring. Basically, like a wiring 101 and what to look for when you are going to hardwire into a vehicle. And what hardwire means is actually getting to the wires and splicing into them making a connection. We will go ahead and do our test and ground it. Instead of piercing the wires, if you have a junction that is easy to get to go ahead and pull it apart. And instead of piercing the wire, go ahead and test on the inside here. Just check each individual contact here until you hit something. Then make sure you actually have your running lights on and then go ahead and turn them back off. It also proves to us that our ground is good and it is working with our electrical system here.

If you know which tab is your running light, you can go ahead and look on the back side of it and have a hole on the backside of it that matches the front. In our case it looks it would be the purple wire here. So the purple wire here, we know is a running light so we can go ahead and make our connection to that wire right now. You always want to do your wiring one circuit at a time. Take your running light wire, which on a 4 pole would be brown, make your connection and make your ground connection too at this point. And then, test on the other side of the 4 pole at the end where you have your running lights working. So when you test this, you want to go ahead and put your ground to the ground pole on the 4 pole. Then go ahead and use the probe to go ahead and test the running light circuit here. Do that one at a time. Also basically everything applies to the other circuits too. Your left turn, your right turn, and brake, if you have a 5 wiring system, and your running lights.

Once you have found your running light circuit, then you go ahead and make your connection. Go ahead and start with your left turn signal. So we will go ahead and turn on our left turn signal. We will just repeat the same steps for testing for that. There you have it. Now if you notice that it is blinking real fast that is because the car thinks there is not a bulb hooked up to it, because there is not. Once you make your connection and plug it back into the tail light it will go back to normal. The same thing applies to the stop light circuit. So just have somebody hold down the break pedal and you just go ahead and go through them all until you find it. Then have them let off of it then turn it back on. The reason you do that is to verify that it is your stoplight circuit. Sometimes they have a constant 12 volts going to some other electronic in your vehicle. If you test it, turn everything off and on to verify that you have the right wire. Also the same thing applies when you go over to the passenger side and look for your right turn signal, do it the same way.


Questions and Comments about this Video

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faron brandenburg 07/30/2014

this is not about the video, I have a question when I put the bulb on the right side tail light both of the tail lights go dim. so the break lights and the turn signals just light up really dim, so if I take out the right one out, the left one works turn signal and the break light. It is on a popup camper.Thanks 5982

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Rachael H 07/31/2014

Sound like to me there is ground issue, and in your case this is noticeable when all the lights are activated. I have included a FAQ thta explains some places to check for ground problems below for you. Since you are seeing the issue primarily on the right side I would check thte mouting hardware or the grounding to tthat light first. Then check the connectors on the vehicle and the trailer. Make sure they are free of corrosion, properly grounded, and all the wires appear to be in good shape. 4896




Donald T. 02/04/2013

why do the SLs blink faster when the flat plug is conected. The SLs on my old 73 blazer blink normal speed until i plug in my connector. then they blink faster. 969

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Patrick B 02/14/2013

I am not sure what you are referring to by SL. If you mean LED signal lights or LED turn signals on the trailer, it could simply be due to the fact that they draw significantly less current than an incandescent. If that is what is happening, wiring in resistors, in line on the turn circuits, will correct this. 632




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Video Edited:
Zack K
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