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

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...
the trainer 47 what measuring voltage on the ground side means
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...
the trainer 47 what measuring voltage on the ground side means
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...
the trainer 47 what measuring voltage on the ground side means
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 con

side

r

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

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