Voltage check on car electrical components

[Hondaboi1729]

Hi

I watched this video of a guy testing a fusebox with a multimeter:
if you skip to 6:10 he uses a multimeter on a particular fuse and
when putting the black lead on good ground of car and red lead on either side of the fuse he gets the same reading on both sides. He even comments that
there should be the same reading! yet I don't see how:

the fuse must have SOME resistance so when connecting to the end which is furthest from the negative terminal battery won't the reading be slightly less? electron has to travel through the wires to nearest end of the fuse. the eelectron has to lose potential overcoming the resistance in the fuse when it passes through the first end of fuse and reaches the other end. (the one further away) so at that end won't the reading for potential difference/voltage be less?

 

[jerromyjon]

The resistance across a fuse is very little. Typically if you set the multimeter to ohms it will show the same across the fuse as it does with the leads touched directly together.

 

[Hondaboi1729]

Ok thanks - But theoretically because of this very little resistance the true voltage (not multimeter reading ) at the other end will be less right?
and suppose you had an actual component with non negligable resistance- you can still do a voltage test at either end because both terminals must show a nonzero reading if receiving power from battery
but even the multimeter will show a smaller voltage reading at one end right?

 

[jerromyjon]

The multimeter just isn't sensitive enough to display the voltage drop which would probably be in the .001-.0001 volt range. It is only a very short smaller gauge conductor. It think it takes meters of typical conductors to see one tenth of 12 volts drop.

 

[Hondaboi1729]

this is irrespective of the component or are you talking about fuses in your above reply?

 

[jerromyjon]

It is only a very short smaller gauge conductor.

The fuses, you can see the length of the circuit... if you think about the black lead grounding through the chassis to the battery cable then the negative post of the battery and the positive running through the wires to feed the fuse you typically have .1-.2 volt drop from battery voltage, the fuse resistance is minuscule compared to that.

 

[Hondaboi1729]

ok- but for other components with two terminals that have non negligable resistance there will be a much more noticable difference in voltage by putting the red multimeter lead first on one terminal and then the other right because of the much larger resistance?

 

[jerromyjon]

Sure. An ignition coil, solenoid, headlights would have considerable voltage drop.

 

[Hondaboi1729]

what's the protocol for showing appreciation for a particular post? pressing like? just avoids saying thank you profusely

 

[Hondaboi1729]

In that same video the dude said in order to do an ohm's test on a fuse on the other hand- the fuse must be removed .

1) why need to remove for resistance test?
2) do I need to disconnect the power before I remove the fuse?

 

[CWatters]

In that same video the dude said in order to do an ohm's test on a fuse on the other hand- the fuse must be removed .

1) why need to remove for resistance test?

To measure the resistance a meter puts a small voltage across the fuse and measures the current. It calculates the resistance using R=V/I. If the fuse is still in the car the meter cannot tell if the current is flowing in the fuse or through the wiring of the car.

2) do I need to disconnect the power before I remove the fuse?

Yes. As a general rule you should always disconnect the power for safety reasons if nothing else. A car may only use 12V but a car battery can put out a lot of current if you accidentally short circuit it while removing a fuse.

 

[Averagesupernova]

Sure. An ignition coil, solenoid, headlights would have considerable voltage drop.

Not with just a voltmeter. Do some circuit analysis with a several megohm load (being the voltmeter) in series with say a several hundred ohm load (such as the coil of an automotive relay). You will find there is virtually no drop through the relay coil.

 

[Hondaboi1729]

Not with just a voltmeter. Do some circuit analysis with a several megohm load (being the voltmeter) in series with say a several hundred ohm load (such as the coil of an automotive relay). You will find there is virtually no drop through the relay coil.

Ermm so no matter what component the difference between a voltage test on either terminal of the component will not be much different? is that what you are saying?

 

[Hondaboi1729]

also for a voltage test why not just measure the voltage between the two terminals themselves on the multimeter instead of bothering to connect one multimeter lead to ground and the other lead to one of the component's (e.g a fuse) teminals.

 

[Averagesupernova]

also for a voltage test why not just measure the voltage between the two terminals themselves on the multimeter instead of bothering to connect one multimeter lead to ground and the other lead to one of the component's (e.g a fuse) teminals.

Go ahead and try it. Something tells me that no matter what we tell you, you will become more confused before you become less confused.

 

[jerromyjon]

Ermm so no matter what component the difference between a voltage test on either terminal of the component will not be much different? is that what you are saying?

Check the voltage drop on a running starter motor which cranks the engine... anywhere in the system available voltage would decrease.

 

[jerromyjon]

also for a voltage test why not just measure the voltage between the two terminals themselves on the multimeter instead of bothering to connect one multimeter lead to ground and the other lead to one of the component's (e.g a fuse) teminals.

And where would that get you? For voltage you would detect none. For Ohms check you touch the leads together and the meter I typically use shows .6-.7 ohms just from the leads and then you subtract that from the resistance of something you are checking to see what the resistance is and if that checks with the normal.

 

[Hondaboi1729]

because in school all I learned was putting the voltmeter in parallel with a particular component? it was only when dealing with real life stuff like cars and motorbikes was this connecting to ground business came in.
Obviously if you see a voltage reading when you put a voltmeter in parallel with a component and it gives a non zero reading it is receiving power from the battery.
Once again you would detect no voltage because the resistance in the fuse is so little? And we would be circling back to the same discussion for other components too with other high resistance components too where I was directed to see to for myself? I just don't see what the issue is in coming to a conclusion in theory based on physical fact instead without having to try it?
If something has high resistance like a starter motor why is it not clear cut to conclude you'll get a reasonable voltage reading with voltmeter in parallel to it on both terminals. and when doing voltage drop test twice with one lead of voltmeter/multimeter connected to ground and the other on each terminal there will be a difference... I'm happy to try it- but what is wrong with the theory first is all I'm interested in...

 

[Averagesupernova]

@Hondaboi1729 . If you take the time to learn some circuit analysis you will understand it. Yes, voltage is always measured in parallel with a component or set of components. If you draw some simple schematics out you will realize that is what you are doing when one voltmeter probe is connected to ground.
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A starter motor does NOT have a high resistance. Quite the opposite. The jest of it without going to a lot of 'book lurn'n' is this: A high resistance component (automotive relay coil) in series with a lower resistance component (incandescent headlight) with this group hooked to the battery will have the total voltage divided up between the two components. The higher resistance component will have the most voltage across it. Sometimes we want NO voltage across one of the components such as a fuse until too much current flows and blows the fuse. Then we would want ALL of the voltage across the open fuse in which case no current flows.
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In the end, the best way to wrap your head around it is to learn basic circuit analysis.