Alternator - How to test using a Voltmeter

Your Moss tech, Hans, is back once again to talk about how to test your Alternator. You will learn how to check for correct voltage at the alternator and diagnose if a diode has gone bad internally.

[@joshcole314]

This is a fantastic video! I have been in the automotive industry about 20 years, and this is expert level automotive content. Thank you!

[@timmyh13]

Thank you very much.... I can’t begin to tell you how much your clear, concise, easy to understand presentation helped clear up the confusing misinformation I’ve been finding!!!

[@scoldedcat]

I've watched 8 vid's on how to test an alternator. This is the best explanation of the lot.

[@ronaldschild157]

Hans has an excellent presentation style. I want to liken it to the beloved "Mr. Roger's voice" - and most levels of automotive fix-it types (from professionals to wrenching hobbyists) can find value in this video. The summary is excellent as well: Different era technologies call for different testing methods (my paraphrasing).

[@Jamespennington71]

This guys voice and demeanor is like..the Bob Ross of how to videos!

[@adventures223]

Dude you spark better than anyone I have ever heard explain anything period your special kick ass job

[@johnobiro5202]

I love the way this guy explains things.

[@Phillipkleinhans]

A deaf person will become an electrical engineer after seeing this!

[@beaverdam3252]

Everyone needs to know about this channel I do all my work on my vehicles myself an I'm still always learning new things I found these videos an love it great instructions

[@j-man6001]

THANKS FOR THE VIDEO! I was ready to stick an alternator into my old car's circuit! VERY GLAD I WATCHED THIS FIRST

[@downloadableskills]

The attention to Audio quality, clarity of the script, and the excellent narrator makes my podcaster heart happy!

[@henryhuynh8615]

the best instructor i've seen so far

[@adamzaloga9679]

agreed.

[@JodBronson]

Sorry Nope!!! He forgot 1 important thing called " Excite / Self Exiting ". Example: If you leave your Light on all night in your Car. The Battery will be drained.... But when you turn on the Car. Since the Alternator doesn't have enough Voltage to " Excite " the charges. You would think that the Alternator is NO GOOD, since it's NOT charging the Battery. During your measurements like aka.. Voltage Meter, it will NOT show up since the Alternator hasn't been excited. It takes a few minutes to " Excite " the Alternator and the Engine needs to be 1500-2000RPM in order for things to get back to normal again. NOTE: During this moment, a lot of People forget that " Excite " period, they ending up pulling all their hair out ://.

[@buffalobenno6256]

tHE CAR WOULD NOT START IF THE BATTERY IS DRAINED.You have to use jumping-cables .Then the magnetizeing-diods will work and it will lift voltage.Scania electrician in Sweden.Now retired!

[@rimmersbryggeri]

Id suggest a clamp meter rather than a meter like this also. Much easier to use in a consistent manner. Also swedish.

[@elizabethrowlands9419]

well done moss motors , the best teaching on utube !

[@vernjoshuatimothy7879]

i won't lie you really good at explaining the best i seen on youtube.

[@georgekolotouros230]

joshua Timothy This guy is awesome. I actually learned something.

[@JodBronson]

Nope sorry!!!

[@johnobiro5202]

I love the way this Guy explains things. Very educational. Keep it up mate.

[@poraradi1919]

GREAT VIDEO . NOW ALL I NEED IS A LESSON ON HOW TO USE A VOLTMETER CORRECTLY.

[@Mike_Hughes]

Very clear and concise. Thank you so much. By way of feedback though (and I mean this as a 'compliment'), You must be related to our Royal Family, here in the UK, to have mastered all that 'slow motion Hand Waving...

[@capefear3297]

An old dog, just learned a couple of new tricks. Thanks.

[@sterlynali6679]

You explanation was the best I ever came across .

[@stevemiddleton7034]

Clear, concise, and to the point. Thank you.

[@arbiter1056]

although there is no car in this video you explained it very well.

[@saaji55]

Nice and deep voice, you should be recording documentaries, books, and/or plays. The diodes tip" should have a separate post though. Thank you for your efforts.

[@nobbystyles4807]

easily the clearest video on this. nice one thanks.

[@reallywontsufferfools1620]

WOW ..brilliant , moss tech hans this is a very well made video congrats.

[@TruthSeekerAll]

So very clearly explained. Thanks for the video.

[@kurtwm2010]

you can bench test an alternator just fine, even with a bad diode, which would not show up in your limited testing. Problem is a bad diode can generate a current draw from the battery instead of charging it.

[@adamzaloga9679]

ive had college teachers that couldnt explain a scarf this eloquently.

[@bakervinci163]

How do you bench test the alternator ?

[@bmw-e30]

Excellent video to diagnose a faulty alternator or connection.

[@SirCavemaninthewest]

That looks just like the alternator on my 94 SL1 and 95 SL2 Saturn. I have an 120amp aftermarket installed for my SL1 and the wiring I did for it makes a sharp difference from the 95 SL2. You can shine the High beams or tap the brake pedal and the dash lights won't loose brightness.

[@MGB-learning]

Outstanding video and presentation.

[@w5cdt]

Good video. It’s a amazing how many mechanics just change parts instead of doing this test.

[@kirkwest6654]

We call those folks glorified parts changers...most just check dc volts..no ac current or volt ripple tests

[@salsarriba5695]

this videos are pure gold. so inspiring, im heading to the garage right now!

[@malcolmyoung7866]

Had my 1986 Jag Series III Alternator tested at a Lucas workshop. The technician said that there wasn't any issues with the unit despite the voltmeter on the dash working/not working fluctuating volts, unit tested by me in the manner described, same symptoms etc. Might be a cable(although have tested them too but now thinking a diode issue. Pity, the car is fantastic to drive but has never charged my battery(s) properly.....

[@Phillipkleinhans]

This guy can diagnose Dr. Phil

[@greenpedal370]

A high current alternator will not do any damage. The charging system will only deliver what the battery requires!

[@ziauddin7948]

excellent mechanism of current generation & diods function which convert AC current to DC #

[@adrianvalbuena8158]

Once upon a time there was a lonely alternator living inside his lonely castle.....

[@petruse8893]

The problem is an alternator sits usually in a tight space ,deep down under AC compressor( sometimes) and to test it one has to remove it first.Mind boggling amount of work on an old car where every screw is a miniature battlefield

[@saffron1977]

You are a great teacher, thank you very much .

[@MinhPham-pm8vk]

My car’s battery drops from 14.2 to 12.2 after either on the road running or idle park at around 25 minute mark, and both brake & battery signs on at the same time. Just bought a new battery. Is it a bad alternator or something else? Why it drops at or around 25 minute mark? Thanks for education video.

[@AubreyWalker-y2x]

Very good teacher

[@darrendixon4039]

To test diodes, commonly known as a bridge rectifier, a simple diode test will do .500 - .800 milivolts values is reasonable.

[@DanielJohnHowTo]

This is really good and helpful.

[@pclaxton78]

very clear and easy to understand...

[@OregonDARRYL]

I wanted to know if I can check it on the bench? What resistance should I get at various points? I got zero ohms from every terminal to every other terminal and ground. I assume my alternator is dead? Correct?

[@dilkal1756]

Sure, thanks, you explained best with what you had! Great job!

[@dahnejaygomez]

That was sooo good. Thanks, and now I know.

[@BL-rb7jm]

I have an older vehicle and the battery would keep draining if I didn't drive it around and I got twice a new battery. And every time it drained? I'd have to jump it while this last time. I jumped it about three times and made it just into the driveway and it died on me so I assume that must be the alternator. Now the car won't start because the battery is dead? So do you think it could be the alternator I assume it is

[@alexkleine2877]

Hans is absolutely fantastic,,, thanl you!!!!

[@davidmetcalf1093]

You got it right, nice presentation.

[@Stillsz75]

this guy should narrate books......

[@markrainford1219]

Anything leaking out of a classic British car is perfectly normal.

[@Scrubworks]

It's not an oil leak, it's Active Chassis Corrosion Inhibition.

[@imgonnagogetthepapersgetth8347]

How about anything leaking out of a British chick?

[@olegs79]

Neat video. I did not know the A/C trick to test for failed diodes. Is there any way to test just the regulator?

[@antaresatlantis6785]

IF ITS ABOVE 15 VOLTS THE REGULATOR IS BAD

[@asliasif1]

Another brilliant video sir

[@grassroot011]

Only thing is, a higher power out putting alternator can't damage systems as the components that are fed cannot draw any more current than they would with the battery in the system, and we know the batteries can deliver over seven or eight hundred amps of current. Ohm's law will tell you that. But a defective regulator can do damage allowing it to output too much Voltage. Which is electromotive force that pushes electrons. Current.

[@lukesibley2896]

I have a Sierra cosworth with a strange alternator fault. Warning light works. Battery is good and all wiring is good. “New wiring and alternator” start the engine and the warning light stays on. Rev the engine and the light will go off and the alternator will start charging at 13.8v and stays there. Until you release the throttle back to idle and it will stop charging 12v and the warning light will come back on. Any idea ? I’m thinking faulty regulator?

[@jtmonroe7856]

Saved me time and money, very informative video.

[@jofro2040]

You refer alot to British classic cars .When talking ac leaking from an alternater what are the acceptable amounts for a newer or should I say OBD2 cars?

[@johnzarollin2749]

not a stitch of video information on how to bench test an alternator. Instead we get two empty hands, blank voltmeter screen, no wires, no test leads. Hoping to see a simple B+ and ground connected voltmeter showing a minor voltage output from spinning the alt. I just bought a new alternator w/ lifetime warranty. 30 seconds after it failed to produce voltage on a vehicle with circuitry loads I was on the phone arranging to take it back to the auto parts and wondering what would the penalty be for tossing it through their window.

[@Eurocano]

Best test video i have seen simple & fast test thanks !

[@shanesmaineshop]

Great tips

[@Jbsalazar72]

does this check the diodeso

[@sandyjust98]

how does this work in a dual alternator system? I've always read to disconnect one alternator from the electrical system but upon watching the video, that's bad.

[@MikeLungu-vh9vf]

Nice lesson

[@Carlostype]

Very interesting info, thanks for sharing👍

[@bannisterjames2845]

Need serious help. I bought a brand new diesel truck 12V alternator and put it into the truck. The alternator still did not charge. I took the alternator for testing and it was found that it was not charging. My question is that the alternator was new, could it have been damaged by us as we put it in? Its fuse is still fine and there seems to be no burnt parts visible. By the way, it has an inbuilt regulator.

[@allenmckinney9533]

bannister james there are times a defective part will get through to store level. I installed a starter on my truck last year and it gave me trouble for two days before burning up, I went back and exchanged it for a new one vs a rebuilt part. $20 more but worth it.

[@sweetbrandigirl]

He's driving me crazy moving those hands

[@wencilao6199]

Well explained, thanks for sharing😄

[@radionutio81ij79]

Problem is a voltage check is OK but an amp meter check will be required as well I have seen many times, defective alternators that out put voltage but no amperage at all so don't just go by a voltage test.

[@matthewsturdee6297]

that would mean it an open circuit somewhere so test accordingly

[@donnaw.2977]

I have a 2006 Acura TL. I hot s new Battery and a new Starter. It rode good for two weeks. Now it won't start and they said the Alternator was fine. But something draining the Battery? What is it now the Alternator???

[@restore64]

Your videos are always a great help. Thanks for dedicating so much time to make these. I run a lot of electronics in my vehicle and my Honda only maintains the battery voltage it doesn't really charge it. I also need a HO alternator and was curious if a self exciting one that constantly ran at 14.8v would damage anything?

[@eventfulnonsense]

I want to know how much is the amperage output of a particular alternator. And what is the correct battery applications.

[@gensyed]

A functioning 12v 100 amp alternator always gives out 13.6 to 14.6v or thereabouts of voltage. Amperage is given to any device depending on the draw of the device or battery itself- limited to the maximum of 100 amps in this case.

[@keithshreve6736]

Excellent video and instructions! Thank you !

[@brewster46]

puzzle: my alternator light on my TR3 glows red on startup for about 10-15 seconds until i blip the throttle multiple times. this ONLY happened on first start of the day, never after. then one day it continued to glow red slightly with ammeter pegged at zero or a little less. i thought a loose fanbelt but not. what gives with this??

[@raftonpounder6696]

Brilliant. Thank you very much.

[@Johnsmith-yv8tx]

SUPER!!!! you nailed it! >>>>> GO Moss Motors GO

[@tammymarglon-desmet984]

How do I do that of it won't start pls?

[@robinhooper7702]

A bit disappointed here. Your thumb nail shows an an alternator sitting on a bench/not in the vehicle. That is what I have here, but your speak is always referencing that the alternator is installed. What I was looking for was to "bench test". Do you have a vid for this? And yes, thanks you taught me allot, never the less.

[@gschlos08]

excellent explanation!!!

[@restore64]

You forgot to explain how to test the diode. If you select "diode" on your fluke with the way you have the probes hooked up it'll show reverse voltage. If it shows OL the diode is good. If it shows reverse voltage then your diode(s) are defective and are allowing back flow and your alternator will drain your battery when the vehicle is off. If you switch your probes around it'll show forward voltage and should be between 500mV to 800mV "from what I've read". When I do the diode test on my alternator it shows 400mV forward voltage and 1.3V reverse voltage. So I can see my diode(s) are bad. My battery seems to drain really fast but my amp draw is only 30mV when the vehicle is off. Does a defective diode with back flow of 1.3V show up under amp draw? I feel like mine doesn't. But from watching your video I've learned something new. I didn't know there was more than one diode. So you're saying if one diode is bad your alternator will still show a correct charging voltage (~14V) but it won't actually be charging? Is there a way to test your alternators output to see? I assume you'd check for amperage? And to test for VAC leakage you're saying .5V or less is acceptable but the normal range is (.01-.03)?

[@wdowa94]

Step 1. Connect wires of disabled Voltmeter Step 2. Shake your hands Step 3. All done- You have to install alternator into your car to test it

[@et76039]

Excess current through the wires only happens when there are loads to create that problem. It is much better to have an alternator with excess capacity than one with insufficient.

[@SuperArchi2010]

Very Very good Very good job Very good video Very good idea 5stars

[@the1spyderryder]

I beg to differ with you in that current is drawn from its source, so if you replace an alternator who's load is drawing 30 amps from it, it doesn't matter how much larger alternator you put on it, if the output voltage remains the same, then the current drain of the load WILL REMAIN THE SAME! The engine power required to operate the alternator may change slightly, but not significantly! The only reason to go to a larger alternator is if you need more current, for example if you were adding different headlights, that draws more current, of if you were adding a high power sound system.... You also botched the explanation of the "bad diode" a defective diode has 2 failure modes, first is open, the second is leaky to shorted.... in the "open" failure mode as you said you lose the current output of that winding... where as the leaking diode allows ripple or AC to into the system and drains the battery when the reverse or opposite polarity voltage is present, it will also draw current from youre battery when the engine isn't running depending how much the diode is leaking. Effectively, an automotive alternator is 3 parts, first it's a three phase AC GENERATOR, second, it is a rectifier pack to turn the AC into DC, third it has a voltage regulator to regulate the VOLTAGE OUTPUT of the alternator .... If you are a mechanic you need to understand the difference between voltage, and current, and their impact on what its connected to.

[@darrendixon4039]

All these tests presuming the car will start, but because the alternator isn't charging the batt it will drain it and won't start. Remove the alternator a simple multimeter, ohms continuity test will determine the fault. You don't need to over elaborate

[@cbizzy2009]

A bad alternator can still have good voltage you need to check the diodes in the alternator to see if its good

[@50CC346XP]

a good alternator at idle 700-800rpm should putout a min of 14.2v if its charging over 15v at any speed the voltage regulator is bad & will boil & damage your battery ! & your not correct about a higher amp alternator overloading the wires and putting a heavier drain on the engine a 150amp alternator will only put out 150 amp if you load it to draw that much amp if you only load it to 50amps that all its going to put out not 150 and want load the engine anymore then a 75 amp alternator loaded a the same 50amps what you should have said is if you put a higher amp alternator say a 150amp where your car only came with a 75amp alternator and you TRY to Draw 150amps say with adding big power stereo amps then yes its going to load the engine more and start to over heat the wiring if you did not upgrade the wire gauge to handle the 150amps draw .

[@malikortiz2331]

218Bee mod43win One of only a handful of people that understand ampdraw. Amperage is used, so the 50 amp to 175 amp statement is correct. You never want to match need to output. If you measure current draw at 87 amps, 125 to 175 is adequate and safe , providing you have the correct wiring gauge.

[@chrissanchez5262]

Great Video

[@MeatsackMiracle]

This will not work on an external regulated system! Alternators put out a.c. voltage to the regulator. You can not read an alternator with an external regulator at the battery terminal on alternator.

[@JodBronson]

Sorry Nope !!! He forgot 1 important thing called " Excite / Self Exiting ". Example: If you leave your Light on all night in your Car. The Battery will be drained.... But when you turn on the Car. Since the Alternator doesn't have enough Voltage to " Excite " the charges. You would think that the Alternator is NO GOOD, since it's NOT charging the Battery. During your measurements like aka.. Voltage Meter, it will NOT show up since the Alternator hasn't been excited. It takes a few minutes to " Excite " the Alternator and the Engine needs to be 1500-2000RPM in order for things to get back to normal again. NOTE: During this moment, a lot of People forget that " Excite " period, they ending up pulling all their hair out ://.

[@skyphoto4114]

Just connect ur multimeter and talk infront ur alternator. That will fix the problem 😂😂😂

[@thelining4078]

There’s a secret message hidden in the hand gestures...

[@Martin_Pelayo_]

why does an alternator overcharge?

[@sara78130]

its battery faulty

[@gdog-wk5ww]

can also be the alt . the regulator goes bad , it will over charge.

[@sara78130]

regulator or battery problem

[@antaresatlantis6785]

BAD VOLTAGE REGULATOR

[@russellmcdonald6033]

You really should be a teacher.... kids don't learn this stuff anymore..... they got an "app for that" SMH great video

[@CosminRotaru]

you forgot to tell you need a "true RMS" voltmeter for the AC test. my cheap voltmeter shows 30V on AC :)

[@iRepairElectronics]

This will test will not work on alternators with external regulators.

[@laurijorgenson6683]

Love this guy!

[@Kalkaekie]

Very good !!

[@abdyfe]

he is good the man knows his shit ..very smart instructor the best