The Trainer #47: What measuring voltage on the ground side means

if you work on cars for a living you've probably taken this measurement at least a thousand times positive meter leading meter on the positive post the battery negative meter lead on the negative post and you're not going to be too surprised when you get a measurement like the one here but with now I take the positive meter lead and put it on a body ground like here and I get a reading like this one wait a minute negative meter lead is on the negative battery post positive meter lead is on the ground how am I reading voltage if you're asking yourself that question you need to stick around and watch this edition of the trainer hey welcome back to anyone who's familiar with voltage drop testing that last reading I showed you is a big red flag saying that there's something wrong with the ground side on this vehicle to understand that though there's some basics that you have to understand first let's start with building a basic circuit every electrical circuit has a source of electro-motive potential in our case it's the vehicle battery every electrical circuit also has to have some sort of load a component that we want to do something with like an ignition coil electric window motor or a simple light bulb to make the light bulb light we have to have a path from the battery to the load and back again but in a car the ground side of the circuit uses either the chassis or the engine block as a conductor to simplify the wiring so the complete path is still there just not in a single strand of wire now the circuit wouldn't be of much use to us if we couldn't turn it on or off when we wanted so we need a control device a switch if you will that opens and closes the path this can be placed on either side of the circuit last every circuit is protected on the positive side by a circuit protection device a fusible link a fuse or a circuit breaker and that is always on the positive side to protect the wiring let's explore that for a moment why is the circuit protection device always have to be on the positive side of the circuit when we all know that current flow is the same on either side of the circuit well here's why because on the positive side of the circuit current has not had a chance to flow through the load yet and the load is the biggest single source of resistance that should be in the circuit that's what regulates the amount of current flowing through the circuit if we have a problem with a short to ground before we get to the load it's no different to peak by putting a piece of wire from one side of the battery post to the other and as technicians we all know what happens when you do that we probably accidentally Arctic Ross the wire time or two in our careers the strand of wire is going to meltdown very quickly so the circuit protection device is there to make sure that doesn't happen to any of the circuits that it protects on the positive side of that circuit write this down somewhere increasing resistance decreases current flow and decreasing resistance increases current flow and remember this the only real source of resistance in a circuit is the load any other source is unwanted and will affect how the circuit works that means that anything anything that can go wrong with an electrical circuit can be traced to a change in resistance think about it for a moment if I break a wire that creates a huge change in resistance doesn't it enough so that current will not flow and if I change the resistance of the circuit whereas going straight to ground a short to ground isn't that a change in resistance as well here's another Electrical 101 basic for you remember all voltage will be used to overcome the resistance in the circuit that means if the load is supposed to be the only real source of resistance and yes I'm going to fudge there's a little bit resistance in wire and the connectors and all that that's a very minimal amount and really not worth discussion at this point it's not going to affect the primary source of resistance and the overall current of the current flow of the circuit so if that load is supposed to be the primary reason for resistance in the circuit I should measure near battery voltage on the positive side right at the load and measure almost nothing on the other side again the voltage is being used in its entirety to overcome the resistance of the circuit let's try that out okay here we have a simple circuit a load a source and a path that connects the two the bulb is obviously lit we have a complete path current is obviously flowing so in a complete circuit the voltage in the battery should be consumed by the load which means I should measure battery voltage on the positive side which we do and next to nothing on the ground side but what happens if there's an extra source of resistance well let's find out okay now in our second example I've added a source of unwanted resistance in this case it's another light bulb it's the same resistance as the first light bulb but do I really care no this is strictly an example and it's a good way for you to learn this in your own shop building something similar to what you see here the added resistance has now increased overall resistance and what does that do to current flow of course drops it that's why this load is burning dimmer than it did before that's one indication of course there's a problem but you're not going to see them every electrical component are you what you are going to be able to is to measure the voltage present in the circuit at different spots now remember what I said all the voltage is going to be used to overcome the resistance in the circuit if there's more than one it's going to be split among them and split in proportion I don't really care if you can do the math or not that's not what I'm trying to get across here what I'm trying to get across here is that if I'm expecting to see the voltage drop on the ground side drop to nothing or next to nothing and I see this almost half that's a big red flag that there's a problem between here and here how do I find it well I keep moving down the circuit until my meter reading returns to what I would normally expect almost zero that tells me that I just passed the source of resistance and now I can work my way backward to where that reading happens bad again and just like a pitcher or baseman throwing the ball back and forth to catch a runner that's how you're going to isolate the cause in this case the the extra light bulb in the circuit and that my friends is the heart of the voltage drop testing technique if we know that voltage is supposed to drop once it crosses the load the source of resistance that it's supposed to be feeding and drop to nothing if we see anything other than that that's a big red flag that there's another source of resistance on the ground side of that circuit we have to find now let's go back and think about the measurements that I took at the very beginning of this video when I put my meter leaves between the negative post and the chassis ground where was I measuring the same as you see here between the ground side of the load and the ground side of the battery now consider what's feeding on that chassis ground there's a lot of circuits aren't there was a troubleshooter I'm going to be looking what's between where I'm at and where the negative battery post is in this case it's going to be the cabling running from the body back to the battery that's what I'm going to focus my aim on if there were something on an individual circuit well look for the things that are unique to that circuit to help you isolate where to test but this should give you a great start like I said build this in your own shop play with it if you're not comfortable with voltage drop you need to keep working at it until you are because I'm telling you guys and gals once you get this testing technique down pat and you're comfortable with it you understand what the meter is trying to tell you you're going to be solving electrical problems like you never saw before in fact you'll become the go-to person in your shop that's all the time I got for this edition the trainer thanks for watching I'll see you next month and we hope that you found this video that you just watched helpful and informative that's why we do them but let us know what you think by adding your comments here below the video player and if you did like it how about a thumbs up and share it with your friends and if you did like it of course you're not going to want to miss the next one so while you're here hit the subscribe button that way you'll be among the first to know when new content is added thanks for watching motor rage on YouTube you

Channel: Motor Age

Views: 290,079

Rating: 4.941967 out of 5

Keywords: #automechanika, chicago, motor age, the trainer, automotive repair video, car repair, auto how to, how to fix my car, auto tech, auto technician, auto service professional, engine performance, auto electrical how to, automotive drivability, obd 2, obd II, onboard diagnostics, auto computer, voltage drop, ohm testing, voltage testing

Id: qdaOSiScEu8

Length: 9min 38sec (578 seconds)

Published: Thu Oct 29 2015