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Testing for a Correct Ground Connection Demonstration

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Demonstration on Testing for a Correct Ground Connection

Today we are going to show you a little bit on wiring. Basically, a wiring 101 or what to look for when you hardwire into a vehicle. Hardwire means actually getting to the wires and splicing into them and making a connection. The first thing you want to do when you hardwire a vehicle is you want to make sure you are grounded and get a test. Make sure your tester is working fine. On some cars you will see this on one of the wires. Being the same color like these black wires. You will see a lot of them ran to a screw inside the body here. Chances are that is going to be your ground. You can actually use that ground for your light tester. What we mean by a light tester is a device like this right here. Basically, this connects to the ground and what this device does is this hooks around the wire you want to test and you push it together and it precises the wire and touches the copper wire inside, creates a circuit and lights up when you have power in that wire.

Also if you do not have that ground screw in the body it is easy enough just to make your own. We just use a self tapping screw, like we did right here. To make a fresh ground, we just used this screw going to the sheet metal here. Then we go ahead and test it. Before you do any connection you want to make sure your tester is working. You do that by testing a circuit you know is live all the time. That would be basically your running lights. So in this instance we are going to go ahead and ground it to the ground screw here. Then we will go ahead and turn on the running lights. When they are on, what you can do is take your light tester and put it on the wire and pierce it like so. You can do that with every wire that is hot. The best way to do that is right behind the tail light so you know when your tail light is on that at least one of the wires has to be hot. If you go through all of these wires and one is not hot, chances are your ground is not good.

Another good ground you can use is the tail pipe because the block itself is grounded to the battery. You can clean a section of your tail pipe off, use this your clip and do that. You can do that if it is easily accessible. The older the vehicle, the more cleaning you have to do, it is up to the individual. But usually you can find a good ground and you can also make a ground on a couple pieces of steel that overlap. We can see this little wield line right here. This is actually a really good spot to put a ground in. Basically make sure it is not going to the outside or will mess up the taillight or anything. But, anywhere you see two pieces of metal overlapping, chances are you have more steel to go into, and the better chance that it will be grounded steel. With more steel the less chance you are going to strip out the screw as you tighten it down.



I cant figure out why my brake lights arnt coming on. I think its the connection on the vehicle side between the car and the plug. The running lights and turn signals all work but the brake lights on the plug and at the trailer tail lights dont come on. I just ran a ground all the way from the battery to the white wire for the plug and they still dont come on. This is a four wire flat connection. Whats going on?Thanks for your help,Robert

Patrick B.


Check the fuse/power distribution blocks first. Both in the cab and in the engine bay. It could be a fuse burned out that sends brake light signals to the 4-pole. If it is an aftermarket trailer wiring harness and has a converter box, use a circuit tester like 3808 to check before and after the box. If you have brake signal going in, typically the red wire on a converter box, but the signal is not coming out on the green and yellow wires, then the box may be bad.

Jerry N.


Just put a new 4 wire trailer light kit on my small boat trailer. Plugged kit into wire harness on truck, turned light switch on and all 4 lights came on as advertised, then turned on turn signal and all four lights were blinking, no matter if it was the right or left turn signal. Is this a bad ground wire?Thanks

Patrick B.


Bad grounds could lead to bleed over from circuit to the other. Check all your grounds and make sure they are on good, clean metal with no rust or oxidation. Check the connectors themselves, particularly on the vehicle since your trailer connector is new, and make sure it is clean. Corrosion can build up inside and cause connections to bleed over to the next. Finally double check all your wiring. The standard color code for trailer 4-Poles is as follows: White - ground, Yellow - left turn, Green - right turn, and Brown - tail/running lights. I have added a link to a our wiring FAQ page that shows the basic wiring schemes for trailers.