How to Test an Alternator – Plus How an Alternator Works

How to Test an Alternator – Plus How an Alternator Works

– Hey, how’s it going do it yourself-ers? Today, I am going to show you how you can
conclusively and easily diagnose issues with your alternator. But not just that, I will also take an alternator
apart and go over all the different components that make up the alternator and how they work
together as well. And as if that wasn’t enough, we’ll go over
some wiring diagrams and how you can read them and make sense of your charging system
too. Alright, so if you suspect you have issues
with your alternator and charging system in general, the first thing you need to rule
out is a bad battery. The first thing you want to do is to do a
close visual inspection of your battery terminals and your battery terminal connectors. Make sure there is no rust and corrosion between
the terminal and the connectors, ’cause otherwise that’s gonna keep current from flowing through
your charging system. And this is what I’m talking about. So if you’re having issues with your car starting
or having issues with your battery in general, and you see this on your battery terminal
connectors, this is the first thing that you need to address. You will need to remove this connector, clean
the battery terminal and replace this connector if it’s as bad as this or clean it as well. Now even if you’re not having issues with
your charging system, but have battery terminals that look like that, you should either clean
them or replace them. Just so that you don’t develop any issues
in the future. But for the sake of this video, we’ll pretend
that’s not there and go on to the next step. Which is to load test our battery using a
load tester. Now if you don’t have on of these, don’t worry. I’ll also show you how to do this using a
multi-meter, but this is simply a way that’s done at different mechanic shops out there. And the way these work is to first connect
the black side to the negative side of your battery and the red side to the positive side
of your battery. Then you want to take your voltage reading
and you should have about 12 1/2 to 13 volts. That’s because contrary to popular belief
a fully charged car battery is supposed to have 12.6 volts and not 12 volts. Because as you can see here, modern car batteries
are made up of six 2.1 battery cells connected in series together. Alright next you wanna press this button and
hold it for 10 seconds putting your car battery under load. And after 10 seconds, you wanna see your reading. And as you can see our voltage drop is down
to about 10.7 volts which indicates that we have a good battery that’s capable of putting
out enough amps. Now if the needle had dropped below 10 volts,
it would be clearly in the weak area which would mean the battery needs to be recharged
and if it had dropped below nine volts, it would pretty much mean you have a bad battery
and you should replace it. And if you find that your battery is weak
and needs to be recharged, don’t think that you can simply drive your car and have your
alternator recharge your battery. A lot of times it doesn’t work that way. What you’ll need to do is to trickle charge
your battery using a battery charger like this. Now if you don’t have a battery charger, you
can obviously remove your battery, take it to your local auto parts store and they’ll
recharge your battery for you. But I find that these battery chargers pay
for themselves even if they get you once out of a jam, it’s already paid for. And in fact if you’re interested in one of
these and how much they cost along with any other tools or products I’m gonna be using
in this video, I put links to them in this special box down below, so don’t be afraid
to click on them and check them out. Now let me show how you can test your battery
using a simple multimeter as well. First thing you wanna do is to obviously turn
on your multimeter and then you put your dial at 20 on the DC voltage scale. And then you wanna put your red test lead
on the positive side and the black test lead on the negative side and take your voltage
reading. As you can see, we’ve got 12.6 volts which
is about perfect. Now if you get a reading that’s more than
12.6 volts, have no fear, that’s simply surface charge on your battery terminals. However, if you want to have an accurate testing
procedure, you’ll need to get rid of that surface charge. And you can easily do that by turning on your
headlights until you read 12.6 volts at your battery. Next I’m gonna use couple of these wires with
these alligator clips at the end of them to attach my test leads to my car’s battery like
this. And next we wanna get in our car and start
the engine and right when we crank the engine in the first second or so we wanna make sure
and write down how low our battery voltage gets. And we’re basically load testing our battery
using our starter because at no time is the battery under more stress than the time your
starter try to get your engine going from a dead stop. And once again we don’t wanna see our voltage
dip below 10.5 volts. So as you just saw the lowest our battery
voltage got while we were cranking the engine was 10.9 volts, which means we have a good
battery on this car. So now we can move on to testing our alternator. With our test leads still attached to the
negative and the positive terminals of our battery, we’re going to start the car and
the voltage reading you wanna see at the battery with the engine running is 13.5 to about 15
volts. Alright so as you can see we’re getting about
14 1/2 volts which is just about right. Now what we’re gonna do next is to raise our
RPM to about 2,500 and make sure that our voltage here doesn’t keep rising and stays
steady. And by doing that we make sure our voltage
regulator, which is this guy, is doing its job. Which is to maintain the voltage and currents
that’s being produced at your alternator within a specific range to your battery. Now I’ll explain in more detail how everything
you see here works together later on in this video. Alright, so let’s do the 2,500 RPM test. So yeah and you wanna keep it at 2,500 for
about 30 seconds to a minute to make sure that voltage stays steady. Alright, so if you get a voltage reading that’s
above 15 volts, than the most likely culprit is going to be your voltage regulator. Now most alternators these days come with
internal voltage regulators, so you simply might want to replace your alternator, but
if you feel like it you can try to source one of these, open up your alternator and
replace the regulator only. But if you have a voltage reading at your
battery that’s below 13 1/2 volts, here’s what you wanna do next. First thing you wanna do is to turn the engine
off and then check the condition of your drive belt that’s turning your alternator pulley. We wanna make sure that it’s not excessively
loose, it’s not badly damaged or torn. Also do a close visual inspection of the pulley
itself, make sure it doesn’t have any obvious signs of damage and that also it’s not making
any bearing or any other types of noises while the engine is running as well. Next you wanna do a close visual inspection
of this connector that goes to your alternator, but more importantly this cable that’s attached
to this positive post that’s on your alternator. See this positive post is what’s attached
to this stator inside your alternator and that’s where the current is produced and that’s
how the power from your alternator gets transferred through this cable back to the positive post
of your battery and that’s how your battery gets recharged. So the next check is with the engine running,
we’re gonna do a voltage test on the post itself to see how much voltage we have at
the post. ‘Cause our alternator could be producing enough
current at the post, but for whatever reason, like corrosion or damage to this cable, some
of that could get lost by the time it gets to the positive post of our battery. Alright so let’s first start our engine. Now we’ll keep our black test leads still
attached to the negative side of our battery. And then with the negative post lead we gonna
touch the post on our alternator. And as you can see, we got 14.4, 14 1/2 volts
here, which is what we have at the battery as well. Alright, so if the voltage you get at the
positive post of your alternator is 14 1/2 volts, but let’s say you only have 12 1/2
or less than 13 volts at the positive post of your battery, that means that your alternator’s
working properly, but that voltage is being lost somewhere from that post to the positive
post of your battery. That’s the case, here’s how you can find out
where that voltage is getting lost. So once again you grab your multimeter, keeping
the dial on the same setting, we’re going to do a voltage drop test, which is a test
to see how much voltage is lost between every two points from that post to the battery. Now to have an accurate voltage drop test,
you’ll need current flow through the circuit. And as the car is sitting right now, it doesn’t
have a whole lot of current flow, so what we’re going to do next is to turn on all the
accessories inside our car to put some demand on the alternator and have some current flow. So let’s turn on our headlights, put our fan
to the max position, turn on our AC. Turn on our radio. And I think that should be good enough. And we’re gonna start our voltage drop test
at the point that’s going to be the most likely culprit, which is this corroded terminal right
here. So we get one test lead and put it on the
post itself, grab the other test lead and put it on the terminal. And whatever number we see on our multimeter
that’s going to be the amount of voltage we’re losing when voltage goes from this terminal
to the positive post of the battery. As you can see, we got about .01, .02, depending
on where I’m putting this black test lead, which is really nothing. Alright, so the next place you should check
is from where it goes from the stud to this cable. So again one test lead on the stud and the
other one on the cable itself. And we gotta record our voltage loss and again
there isn’t any on this car. So nine out of 10 times your voltage loss
is going to be in one of those two places, especially this one. But if it’s not there, then you simply just
keep going down this line. So on this car, this line from there goes
to the junction box first, then it goes through this 120 amp main fuse, then back out the
junction box and to the car battery. And if you’re wondering on this car, the total
voltage loss from the positive post of the alternator to the positive post of the battery
is .07, .08 volts, which is well within spec of .2 volts. Alright so that was the voltage drop test
from the positive side of the alternator to the battery, but you also need to do a voltage
drop test from the negative side of the alternator to the battery as well. Because in order for current to flow, the
alternator needs to be grounded. Now your alternator is grounded through your
engine which is grounded through your chassis by cable, by grounding cable and your chassis
is obviously grounded back to the negative side of your battery since that’s also connected
to the chassis as well. So in order to test the ground circuit, you
put one test lead on the negative side of your battery and grab the other test lead
and put it on the case of your alternator. And if you get a reading, you wanna make sure
it’s not above 0.2 volts. And as you can see here, we have no voltage
loss whatsoever. And if you get a reading that’s above 0.2
volts, you wanna check your alternator to your engine. So you get one test lead on the alternator
casing, grab the other one, put it on your engine and see how much voltage you lose there. And then you can simply work your way back
to it. You go from the engine to the chassis. And by the way on the chassis side, you can
put your test lead on any of these bolts or screws that are bolted into your chassis. And then you do a voltage drop test from your
chassis to your negative battery terminal as well. You’ll find the voltage drop believe me. Nine out of 10 times, it’ll be at your battery
terminals. Make that 95 out of 100 times. Alright so that’s how you do a voltage drop
test to see where you’re losing voltage when you go from a working alternator to the post
of your car’s battery. But now let’s move back a little bit and go
to where we check voltage at the positive post of our alternator while the engine was
running. And let’s say that we only have battery voltage
or just below battery voltage at this post. If that’s the case, you could have a bad alternator,
but before you condemn the alternator you need to check and make sure that the actually
voltage is being supplied to the voltage regulator inside your alternator through the connector
that’s on there. See these two wires here? One of them carries battery voltage to the
voltage regulator inside your alternator and your alternator needs that battery voltage
in order to be able to produce current which in turn will recharge your battery. Alright so here’s how your alternator works. So a lot of times you have two wires going
to two wire connector on your alternator. These go straight to your voltage regulator. So one of them carries battery voltage to
one of these brushes that you see here and the other one has ground since your voltage
regulator is grounded through your alternator which is grounded through your engine. So we have both power and ground on these
spring loaded brushes. And those two brushes make contact with these
two slip rings that are on your rotor. See on this alternator, this voltage regulator
goes on like this and right there you can see it make contact with the slip rings. So power and ground comes through these two
slip rings and it goes through this winding around your rotor. You might see it better at this angle. Each one of these slip rings is connected
to one end of this winding that’s going through this rotor. And as you have both power and ground through
this field winding, you create an electromagnetic field. Now your electromagnetic field goes inside
of your stator. Which is basically three sets of windings
that go through here as you can see. And as your drive bolt turns the rotor and
the field winding with the electromagnetic field around it, that field induces current
through your stator. Now since the current that’s being produced
at your stator it’s AC and the car or your battery uses DC voltage, that needs to be
converted to DC voltage. And that’s where these diodes come in. You have two diodes for each winding that’s
going through your stator and since you have three windings, you have six diodes in total
which turn that AC current into DC. And this piece with the diodes in it it’s
called the rectifier. And that from there, the DC voltage or current
goes from this positive post of your alternator back to the positive post of your battery
and recharges your battery. Now since your engine spins at different RPMs,
it’s going to vary greatly the amount of current that’s going to get produced at your stator. But that’s where your voltage regulator comes
in. It varies the voltage that’s going through
your rotor based on how fast or how much current is being produced at your stator and makes
sure that current or voltage stays within a certain range. So in so many words, if you don’t have battery
voltage going to your alternator, you’re not gonna have an electromagnetic field and you’re
not going to be able to produce any current. Now lucky for you checking for that is pretty
easy. What you wanna do is to get in your car, put
your key in the ignition and turn it to the on position. Then grab yourself a test light. Attach one end to the ground side of your
battery and then with the other end, probe the two pins on your connector and you should
have power in at least one of them. If you don’t have power there, the next thing
you need to check is the fuse for your alternator at your junction box. Alright so now let’s make sense of it on a
wiring diagram as well. Now this wiring diagram is for an F150 since
I don’t have the wiring diagram for the Subaru that we got here today, but they work very
similar to each other. So here’s the power side that comes in, goes
through the fuse, through your junction box. And it gets fed to your regulator here. It also gets split off from this side and
goes to your brushes. These are supposed to be your brushes on this
diagram which go to your rotor. This here is your stator with three windings
inside there. As they spin, they produce current, which
goes through these diodes and goes back around or out of this B plus or the positive post
of your alternator back to your battery. Now similar to this Subaru, there’s also a
second wire in that connector. The second wire is for the battery warning
lamp that’s inside your cluster. Talking about that guy right there. So when you turn your key to the run or start
position, you allow current to flow through this fuse to this lamp inside your dash. Now when the engine is not spinning, your
voltage regulator grounds this wire and illuminates the battery warning lamp inside your dash. But once the engine starts spinning and starts
producing current at your alternator, the regulator opens this up and your battery warning
lamp or light goes out. Jose if you don’t stop horsing around while
I’m shooting, I’m gonna return you. I’m gonna take you back right where I found
you man. Right on the side of the road. Now if you’re interested in knowing more about
alternators, how they work or maybe how you can open them up and rebuild them for example,
there is a YouTuber which I put a link to channel right below on the screen somewhere,
which has a YouTube channel based solely on alternators. And it’s a very informative channel, I definitely
recommend you check him out. So yeah, thanks for watching. I’ll see you guys next time.

About the Author: Michael Flood


  1. I feel like I can now charge people to fix their alternator problems ๐Ÿ˜. Thanks for all the info, very detailed. I'm going through charging problems and cannot afford to pay to have it checked, so again, many thanks. This will help me know if my alternator has really crapped out, as the test they did at AutoZone only checked the voltage at the battery and they didn't bother to actually check the alternator itself.

  2. Very thorough and understandable explanation, right down to the details such as where to set your multimeter dial, how the alternator and voltage regulator work, etc.ย  I already knew a lot of this stuff, but your video is an excellent refresher.ย  Plus, I gained quite a bit of knew knowledge from the details you presented.ย  Having the disassembled alternator was an excellent teaching tool.ย  And, the camera work was well done. Keep up the good work – I look forward to viewing more or your instructional videos!ย  Thank you!

  3. You did a great job!!! Congratulations Man and You must know that i m waiting for the next super cool videos about how to repairs cars!

  4. If the voltage drops a bit low while starting. The battery could still be good, the starter could be dragging, causing it to pull more current.

  5. I dont have the knowledge of testing. But when the battery lite comes on the dashboard..i think its a sign alternator is going bad. It means the car is completely on full battery mode only. Hope this input is right. Happy 2019 to RnW community !

  6. I have a 2000 toyota camry ce I have 2 alternators available to me one is 80 amps and the other is 90 amps can i use either one? the car came with 80 amps will the 90 amp hurt my car?

  7. nice explanation sir, you did really make me learn something about alternator, how it work and how to diagnose if it has gone wrong already,, thankz, thumbs up..!

  8. I have a alternator off a 88 jeep and the regulator has 4 pins "P L I S"… which pin should i use for the keyed field wire and sense wire? and why is there 4?

  9. Searched for a diagnosing video. After 10 videos, you were the one to finally make me understand everything I need to know.

  10. Thank you – Your video has gave/ return my life peace, happiness and satisfaction! Thanks to you the 5v fuse of my RAV4 97 was causing the vibration. I will replace the regulator, and provide feedback, however, the vibration on idle was caused by the blown fuse to the Alternator. You deserve more than a thank you, I wish I can compensate you, instead I included you in my thread of forum

  11. What does it mean when it say B1+ and B2+, because mine is connected with b1+ and itโ€™s not charging my car .its a new alternator and new battery and i donโ€™t know why itโ€™s not charging….please help

  12. This guy is a good mechanic. Employers need people with brains like this. I'm in the mining industry and the majority of the electricians we get are not worth employing.

  13. Wonderfully thorough video and answered my question about the two terminals on the back of the alternator. I knew one was + battery voltage to the stator and the other was to the gauge.

  14. Great video as usual. In my experience it is the voltage regulator that needs to be replaced most of the time, especially on Mercedes.

  15. Great stuff! Is it possible to get 3 AC outputs from a car alternator and connect directly to 3 transformers? Could you please show how to do it? Thanks & Regards.

  16. If you don't have the exciting clamp for your alternator how do you find out which one is positive or negative?

  17. awesome video! One point. You actually SAID it…lead acid batteries have a maximum voltage per cell of 2.2 volts. Multiply by 6 and ….bada bing bada boom….13.2 volts…not 12.6.

  18. Brother thank you. I absolutely hate everything electrical and all the problems! ๐Ÿ˜  Especially intermiting one's ๐Ÿ˜ค!! You just explained and showed it perfectly, simple and made it nonintimidating. You are a mechanics Mechanics๐Ÿ”ง๐Ÿ”ฉ๐Ÿš—. Thank you ๐Ÿ‘

  19. My battery is at 12.05 doing nothing. My battery is at 11.50 to 12.00 while car is running. My alternator reads the same as battery. Shouldn't it be higher while running?

  20. .ljtfddssssss jiolwoleooskweoeoels,wl3pepe8eoeoeoekeieieieieiekelrorkrkrkkrekeekddkkdekekekekekm zksksslsoweleo3ke

  21. hey my friend maybe you can help me, i did replace the alternator regulator on my e 350 mercedes, when i start the engine it generate 15v maybe a bit more,, but after a few second it drops to 13v, is it because the battery is full charge, is it suppose to stay at 15volt all the way. have an idea ?

  22. I like the informative demonstration here! Ive worked with alts (inside them) for quite a while and this was great.

  23. If your alternator is putting out 14+ volts there is no theoretical reason driving around won't recharge a low battery.

  24. Excellent explanation. I wonder how you test for parasitic current that is discharging your battery due to bad diodes or any other means due to problems in the alternator. I have tested all my fuses using a not disturbance approach
    The other question can you have a parasitic current due to bad reply I wonder if it is possible to pass a rely test as you pulled out but can you have a condition that creates a parasitic current as you turn the engine off and it goes to sleep even though it passes the rely test
    2002 ford wind-star disconnecting the negative post over night so I donโ€™t drain the battery

  25. Just one thing wouldn't you need cold cranking amps instead of voltage we had batteries in the military that had the correct voltage but no cold cranking amps

  26. Well Done! -in particular compared to some of the other videos on how alternators work. Theory was clear and useful hands on video shots of what the components look like. Clear troubleshooting how-tos and why. Thank you!

  27. Awesome video. Very informative. Extremely helpful. Great explanation of working your way throughout the entire charging cycle to make clear what makes it work. Before I could only say motor spins alternator which charges the battery. I know there are a lot of wiring diagrams out there. At least now, I actually can make sense of one too. Super neat knowing what I've learned right now. I'm in the process of installing a car stereo system with multiple amplifiers and batteries. I've seen other car audio youtubers show all the "where's" to make connections but leave out the "why's" or "how" it works. I just want to learn more now. Thank you so much brother.

  28. Rachets and wrenches: ๐Ÿ‘+๐Ÿ‘.
    Muchisisimas gracias…
    Amigo nesesitaba un poco de ayuda para saber algo de como llegan los cables del alternador a la bateria, porque no le entendia!!!
    Y con este video aprendi mucho,
    tanto que yo mismo le arregle varios problemas que tenia,
    tanto el alternador, como el cableado, y hasta un relay descubri que no servia.
    No soy mecanico y ni idea, LOL…
    Y deveras, muchas gracias por
    subir el video. Me ahorro mi carro,
    malamente lo iba a tirar, porque si
    que gaste algo de dinero en el
    sin resultados a mi favor.
    Y la gran satisfaccion de arreglarlo yo mismo, vale mas que el carro. Gracias ๐Ÿ˜ƒ…๐Ÿ‘ Gracias…..

  29. you should have a multimeter with a min/max function. The more features on the DMM the better. It makes your job easier.

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