How To Test an Alternator Part 2. Rectifier, Diode Pack, Parasitic Battery Drain

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With the AC voltage test, cheaper multimeters may give a spurious reading around 20-30V AC. This figure is totally inaccurate and should be ignored.
This is due to how low end multimeters calculate AC voltage so for an accurate AC reading, you'll need a multimeter stamped "True RMS". As an Amazon Associate I earn from qualifying purchases. Using my Amazon links does not cost you any extra.

[@jeanbrodeur9667]

I not only found it useful, it solved my parasitic drain problem.

[@Invzame]

This is the best video I have seen on this subject. Your explanation was simple and concise.

[@johnlocke_1]

Very informative. Thank you very much for taking the time to record this. Cheers, mate.

[@FabiWann]

Now I know how to test my alternator in the weekend. Thanks!

[@donaldsansoucy3815]

Really excellent job of explaining the alternator tests and what to do. Thanks

[@cayrick]

Excellent video. When I did the ac voltage check you state the max is .5v. Mine showed .7. Ironically I sent my Honda to the dealer and they offered to send the alt to a repair shop and have it back in the car in 3 hrs. 6 hrs later they told me to go home and return tomorrow because the shop was haviing parts issues with the rectifier. Got the car back and it seemed to hesitate then crank (1 yrl old batt). Starter tested fine. After sitting for many days the batt. went dead. Only when I watched your video did I find out about the ac test. Then you talked about the battery leak issue. It all made sense. Ordered a new rectifier.

[@JenniferTalbot]

this is the simplest diode test rather than the 4 individual diodes, all was great.

[@dcs4555]

well put. your explanation was very understandable. now that there are a zillion exceptions listed earlier, i wish i could help but the details are probably gonna solve alot. jmho ie: a 1.6a drain (meter + on the alternator output)is a bad alt. a drain w/ alt disconn = something else still draining. only reason i'm here is because my wifes 2014 buick is draining, and it was @ 1987 when i last did this kind of work lol. bikes - not sure what switches are involved. new cars - open doors but close their latches, unhook trunk & dome lights, and don't leave the dam glove box open like i did when getting started. i use the dim filament test light method (in series to neg batt post). good luck a new battery is expensive.

[@BJMoorhouse]

Thanks for this; nice and clear, I'll be doing this test on my Veloster in the morning.

[@Spodokamono]

What a great video this is, Very helpful, thanks for a brilliant tuition, have a charging problem so this will hopefully confirm if the Alternator diode pack is working correctly.

[@williamwilliam7100]

Very good and direct what u need to no.

[@malcolmyoung7866]

Thanks for this.. having watched a couple of videos on this subject this one is just that bit better… with an excellent verbal suscriptor if the why’s and wherefores..

[@PauloM463]

Helpful video. My car battery has a parasitic drain of 1.33A due to the alternator. I ddidn't know if it was a regulator issue or the rectifier. This video gave me a clue. Thank you

[@sergiuxy21]

I just watched your battery drain video ,very good made! My first problem was with my Webasto that didn't work and after a diagnosis i saw there was an high voltage error where my webasto is blocking it self. After that i start digging,i had an audio drain that i fixed with a relay. Maybe i have more parasitic drain from Confort Access(its a common issue at Bmw). I drive everyday,and every morning my battery voltage is 11.60... So maybe just a bad battery or parasitic over the night and the power module its pumping up to 15 volts to charge the AGM battery faster. No problem starting the car!

[@bogdanchiolan]

You may be right...I'll check that pulley exactly as you said and I'll let you know if that's the problem. Thank you very much for helping me!!!You're the man!!!

[@kingdom50002010]

thanks a lot, you clarify many things to me

[@bmw-e30]

Very good explanatory video. Apart from changing brushes, home maintenance on an alternator is a no no. Impossible to get the screws off a used Bosch for a start.

[@slowvag8v]

Great series

[@billhanson827]

Excellent information. Thank you!

[@colin5064]

Fantastic video very well explained and demonstrated first clss

[@jamiespruce4547]

Excellent video. Well explained. Thank you for posting 👍🏻

[@sirobb]

Glad it was helpful!

[@paulderrick6095]

Thank you for your informative vid, I especially liked that you showed a cheap multimeter as most of the punters that watch these vids only have a cheap one (like me) Now I know its millivolts as it doesnt say on my meter. I found out I have knackered diodes with these tests whereas before I thought it was the regulator.

[@sirobb]

What results did you get on the 3 tests? Also see the video description for the caveat relating to cheap meters. Thanks for the comment.

[@ijohnny.]

Excellent presentation, thanks much.

[@seanharris3217]

Great video, should help me with my truck

[@roughas100]

Mate, wished I looked at this video while my faulty alternator was still in my 4wd, but was still very helpful. I thought I had a voltage reg problem as the alternator was under charging the battery, max DC about 13v output. After pulling apart the alternator I measured all 4 diodes , 2 were OK , using the diode check on my multimeter tested about 520 each , the other 2 were open circuit in both directions. When testing at the large + terminal I had a reading of about 950 . Also the diode terminals looked a bit burnt too, ended up replacing the alternator as I damaged the voltage reg & brush holder , the screws were rusted in. For some reason I thought alternators were all 3 pole / phase, like most electricity grids, but my Toyota seemed to be 4 pole/phase.

[@sirobb]

You got there in the end, that's the main thing.👍 Thanks for the comment.

[@watchmemow3817]

Thanks for this, very helpful in outlining how to test for a faulty diode.

[@brianlongman9559]

First off, great video and very informative. I've had a parasitic drain for a while in which the battery(new) would die overnight. My car was a 2008 Saturn Astra XR (essentially an Astra H) with 75000 miles. The alternator B+ post is not accessible from the top and certainly not while engine running so my tests were slightly different. At any rate, the parasitic draw was 380ma after all electronics went to sleep. After fully charging battery, I ensured I had 12.6v. Next, I did a voltage check at battery posts with engine running: 14.15v. Ok, next I checked AC voltage: 0.39vac. Hmmm... couldn't do diode test without yanking alternator (which is a PITA on this car). So next, I went and did a draw test whilst yanking every fuse and relay one at a time... no change, still showing 380ma draw. So I tear the thing apart and the moment the alternator broke free from the block, the draw dropped to

[@sirobb]

That sounds very strange and I can't offer any suggestions. Let me know how it gets resolved though.

[@brianlongman9559]

@@sirobb I purchased a new remanufactured alternator and reinstalled. There was no spark when reconnecting the neutral like there was before. Draw test was

[@sirobb]

Thanks for the update. Very unusual symptoms but I'm glad you got a positive result.

[@dunckeroo1987]

About 380 MA ? It's probably that little diode trio pack that runs the internal regulator and field rotor coil. Reverse voltage though alternator, using test light to limit current, should be around 1.2 volts -- if it is around 0.6 volt; then one of the main diodes is shorted and will cause alternator to run hot. If one of the main diodes is shorted then the internal regulator circuit might be drawing current from positive though shorted a main diode.

[@jeremyhanson9569]

I had a similar experience on my 03 Dodge Ram 3500 Cummins. I bought it off my dad who had been dealing with a parasitic drain for years. I started by replacing the two batteries which had failed due to the constant drain. Then I did the standard test of pulling fuses with no luck. Then I stumbled across a video where they said the alternator was the problem. Had it tested at auto parts store an they said it was good, but when I disconnected the alternator the drain went from 1 amp to 15ma. WEIRD!

[@colin5064]

Excellent informative video

[@jacksmith3189]

Good information Thanks

[@Teddy_Bass]

Thanks for this post

[@mchabi18]

Great job ! Thank you.

[@Tyler_deJong]

Excellent video, thank you!

[@sirobb]

You're welcome and thanks for the comment.

[@Evs_Projects]

great info

[@hp11208]

Good video

[@ghulabkhan762]

Good job

[@jeffs7267]

Thank you!

[@davidc4162]

Thank you for this video, very helpful! Based on the second test here my rectifier is bad, I am seeing 30V AC! I didn't directly test on the alternator as it is inaccessible but instead measured from jump points in the engine bay which are relatively close to the alternator (battery is in the trunk). I hope this is still a fair test, using the jump points? E46 BMW if you're familiar with the car. Thanks again!!!

[@sirobb]

David C You're welcome David and thanks for the comment. Please read the video description. You need a better multimeter.

[@davidc4162]

My apologies the description was hidden on my phone's app. I'm showing 14.10V at idle so I think the alternator is OK, but intermittent problems are inherently difficult to diagnose! I'm going to have the battery tested in case it has a sporadic internal short or some other fault. Thank you again!!!

[@DumbSkippy]

Thank you for your tutorial. I think I have a bad diode.

[@bluestar465]

This may be my parasitic drain problem! I will do test procedure to check.

[@RandallFlaggNY]

Thanks. I learned something here.

[@sirobb]

Excellent.👍

[@frundlemud]

Very helpful ta.

[@philkfoto]

Very interesting, glad I watched this. My question is if I did the same test without removing the main lead is the result accurate or inaccurate?

[@edwardharvey5839]

Brilliant!

[@user-dj2sn1nu8q]

Superb. Best video on this topic by far. Can you comment on diode test millivolt readings outside the normal range of 500-800? Does parasitic drain increase / decrease proportionately? I suspect my alternator has a parasitic drain issue even though my alternator charges normally at 14.4V. Specifically, I have a consistent diode test reading (using 3 different meters) of 1.067 millivolts when I put the red meter lead on the alternator case and the black meter lead on the "B+" post - outside of the car on the bench. When leads are reversed, I show no reading as expected. Your thoughts are greatly appreciated. I know others have the same / similar questions. Thanks in advance!!!!

[@sergiuxy21]

Hi man,very nice video!! I have a Bmw and I'm getting 15.10 volts at battery with engine on. And also i have some battery drain cuz my battery is getting empty if i don't drive the car for more than a week. It can be voltage regulator and for the drain the diodes??

[@wbudd]

Hi SiRobb, 91 Honda CRX Si 1.6L here. Battery is 8 months old but doesn't hold a charge. I got a jump and was able to drive to car parts store. Battery tested ok, but tech who works there said alternator diodes tested as bad. The only way I can get car to start is if someone gives me a jump. When I turn car off and turn on ignition, slight click then nothing (no crank, no start). All fuses tested ok. Couldn't do DMM series parasitic draw test because car won't start. Just now I removed all electrical connections from alternator and did isolated diode test. With red lead on alternator case and blk lead on post I get a bit over 500V. However reverse test (red lead on post, blk on case) I continually get 1.7V. Thanks for any insight. My DMM is true RMS. Sorry for long post. Your vids are great.

[@sirobb]

William Budd Hi. What are the voltages (both AC and DC) measured at the alternator and measured at the battery at idle and 2000 revs with full electrical load?

[@wbudd]

Simon, thanks for your quick response. I'm on a bit of a steep learning curve assessing/diagnosing these parasitic drain issues and didn't provide enough good data. But I'm hampered because my car won't start unless jumped. I have a fully charged Clore JNC300XL jump n carry (900 peak amps; 225 cranking amps) but it isn't providing sufficient voltage to turn my engine over even tho its cranking amps are in line w/my battery. As soon as I can find a good samaritan to give me a jump I will collect the data you mentioned. Regarding my previous msg, prior to getting battery checked at auto parts store, I did voltage check no key in ignition. It was 3.3V. I didn't have presence of mind to check voltages later when some one jumped me so I could make trip to the parts store.

[@sirobb]

William Budd Ok, well at just 3.3V, your battery is almost certainly beyond help so will need replacing regardless, hopefully under warranty? The boost packs are really designed to give a helping hand to a below par battery not to do all the work of starting the car. They rarely have enough amperage for that. As soon as the battery is replaced, get straight on with the various voltage tests. I also have a separate parasitic drain video in my list.

[@Jubi316]

I tested the diode like in the Video and got nothing going one way then switched the probes and tried a got 455 is that good enough ? You said 500 to 800 ?

[@sirobb]

It could well be ok but there are 3 tests in the video so what was the DC charging voltage and AC volts?

[@michaelrand8291]

Hello Rob, On the Rectifier diode test, on the forward test I am getting a reading of 1.02v and on the reverse test zero. Would you consider the the rectifier is faulty?

[@sirobb]

Sounds ok. There are 3 tests in the video though to confirm.

[@hamidizadi]

When i perform your alternator diode test with the red lead on housing and black lead on alternator B post the voltage reading starts off at 525mV and then quickly drops to about 485mv and slowly drops 1mV every 20-30s. Is that normal. Cause my value is not steady like what you demonstrated. I have a denso alternator and using a klein clamp meter. Also one more question. When you performed a voltage drop test on the negative side of your starter motor with your ignition disabled, in your other video. You stated that the acceptable voltage drop is no more than 0.2V. I'm getting result of about 0.35V. Some say up to 0.5V is acceptable and others say no more than 0.2V. The starter seems to be working just fine by the test results are a bit skewed. Any help would be greatly appreciated.

[@valeriegriner5644]

My battery test and starter test(cranking)...were normal. My charging system test results came back: DIODE SHORT. I have a 2007 Jeep Grand Cherokee. Are DIODES something you can buy and replace? I'm a 62 y/o single female and I don't know too much about engines(except for lawn mower engines!) I had that "parasitic drain" problem that left me stranded over the weekend. Since my car was jumped off, it has cranked okay for two days. My question is....are DIODES something you can buy and replace...instead of replacing the alternator? Thanks so much for the video and any info. you can offer. I just spent $521 for a new a/c compressor, so I'm hoping this won't be an expensive fix!

[@paulknowles9727]

Hi buddy, thanks for the vid. I've a 2005 Fiat Ducato 2.0JTD. I've done your tests and not getting a reading in reverse, but i only see a battery drain when Alt connected??? :O My mind is about to go on strike. 3 days I've been dealing with this. Battery is brand new, and charges fine at around 14v. takes about 2 days to run the battery to low cranking volts. Reading from 350 to 700 milliamp drain when alt connected. Reading 550 on Diode Test one way. I think I was confused about how you tested the AC current.? Any ideas?

[@faisalmahmood7589]

hi what if the reading is only going one way but it is low bwt 100??? also what about the wire that goes to the cluster does that have a one way diode or can current flow both ways throught that?

[@robikihm9855]

Thanks , was great :)

[@thomaskaragiannis4546]

Hi, thanks for the video. I have a question for you if you don't mind. I have a faded GEN light on when driving and when i stop the engine gen light comes bright red as it should be. At the same time with the faded light on, battery is charging, measured at 2500-3000rpm 14.4 volt and 13.3 with engine at idle. I have also measured the diode at 0850 (but forgot to measure the other way) Also measure the AC which was 003 volts only as per your instructions. Any ideas? Do you think that the voltage regulator might be faulty?

[@sirobb]

Check the system voltage in other locations throughout the car. The alternator may be working well enough to output sufficient voltage but voltage drop through poor connections is then causing the warning.

[@thomaskaragiannis4546]

@@sirobb Thanks for the reply, it was finally a cold solder in one of the three diodes. Measuring with the multimeter is not always reliable as you have to unsolder the diode you are measuring.

[@ncyankee101]

Hi SiRobb, I have a question - there are three separate coil windings in a alternator, correct? Does the diode test get all the diodes at once, or is there a dedicated diode for each coil? If the latter is the case, you would need to rotate the pulley to bring each diode into the circuit, right?

[@sirobb]

Pretty certain that no rotation of the pulley is required. Regardless, a correct DC voltage reading on a running engine as shown in the video is usually enough to confirm.

[@ncyankee101]

@@sirobb After I asked this question, it occurred to me to find a circuit diagram of an alternator circuit, and yes all diodes share common connections at both ends. It looks like the three windings operate concurrently not sequentially. The issue I have been having is, a $60 battery tester I bought a little ago was giving me crazy ripple readings. I recently got a true RMS meter and was able to check the AC voltage and it seems to be about where it is supposed to be, and the diodes pass. I guess the ripple function on the battery tester is not reliable, at least not when taken across the battery - I might try it again off the alternator.

[@seanswilson]

I watched your video yesterday and then ran your checks on a 2006 Nissan Pathfinder. Until recently the vehicle has been fine, but there is some debate as to why the battery is suddenly draining if sat for 48 hours. I'm getting conflicting information that the battery had been connected the wrong way round, though no reason as to why battery had been removed in the first place. Secondary to that is that it's physically impossible to connect it wrong due to the lengths of the terminal cables and battery posts to one side of the battery. I was also told that it had been jump started, again with the confusion that the jump leads were connected wrong. Unfortunately, I can't confirm any of that. For the first test I got 12.8V across the battery, and then 14.3V and 14.5V on the DC check. When I did the AC check I was getting 28.1V, and on the diode checks I was getting 0.532 and 1. I was for asking what the 28.1V on the AC check would mean, so I watched your video again to see if it was mentioned, and then by chance, just read the full video description, where you mention the 20-30V reading that some multimeters would show. Assuming that mine falls into that category, and the alternator is otherwise okay, is it possible that connecting the jump leads the wrong way around (even though it wouldn't have jumped the car) could have fried something else in the car? I've completely isolated the alternator anyway and will be checking it again later today and again tomorrow to see if the drain is still there on the car. BTW Finally, just to say that I thought your description of a diode was brilliant. I've known what they are and what they do, but could never think of a way of describing their actions.

[@sirobb]

Thank you for your comment. Your alternator sounds OK so far so my best suggestion is to follow my parasitic drain test video here kzbin.info/www/bejne/kHSwgnhoqcyFiac It's a very simple method of finding what your standby drain is and how to find it.

[@brucewalls548]

Great video. Just did this test as I can’t find any drain elsewhere. Reading 400 Millivolts with black lead to alternator terminal for battery positive and red lead to alternator case, and 2v when I switch them around?!? Neither of those values sound right? I am using a good multi meter with diode function.

[@floofsale]

Ya i got about 450 so im thinking this is my problem.. time to take the alternator off and get it somewhere to get rebuilt or fixed

[@pward2180]

Hi, Thank you for the informative video. I am trying to ascertain whether 2 old alternators I have are ok, but can only test them off the car, as I no longer have the cars from which they came. I have obtained a reading of around 1200mv on each, using the diode test you described, with no reading the other way round. Does this indicate a fault? I only have a cheap meter. Thanks.

[@sirobb]

+P Ward It's impossible to know the precise reading for every diode pack using every available multimeter so my first impression is that your diodes at least are ok.

[@pward2180]

Phew ! Thank you for the quick response. My first thought was that as they are both the same make (Paris Rhone) and basically the same model and that they both gave similar readings, I was hopeful that they are ok as far as the diodes are concerned. It is helpful to get your opinion. Clearly there are a number of other considerations, such as the voltage regulator and integrity of the windings, etc. that I can't test quite as easily, but at least it's a start. Thanks again.

[@JisINSANE3]

i had a 01 chrysler voyager, And I could not figure out why all my instruments would jump all over the place and the thing would go into limp home mode stuck in second gear. I noticed a whine from the alternator, Had a good 13.8v charge. Could not figure that little bit would screw anything up. Then I thought well wait 14.4 is the normal where is the lost voltage. Tested it for a/c and it was putting out 1.5volts a/c current. This was years ago. Replaced the ALT and back to normal.

[@maar5725]

بعد شُكر الله ؛ شـــُكراً لّك . Thank you Very Excellent . My Advice to all of you ; After you clean those wiring and cable connections grease them with silicon grease . Remember ; sometimes ' cleaning the posts and connections is not enough because corrosion penetrate inside the wire or cable end which require replacement ،،،،،،، or cutting wires and or cable ends and re-crimping them .

[@velthog]

As Temperly measure until fixed or replaced would removing the black lead stop this parasitic drain (it’s easier to get off and on quickly) or as it is red going to the alt would it have to be red. Finally can you not do the diode test by just disconnecting the battery lead as I cannot get to my alt as easy as yours? Thanks for the helpful vid.

[@sirobb]

Yes you can do the test with the black battery lead removed instead. This will also stop the drain but will power down the entire car.

[@p00ky76]

Great video, I have an alternator that doesn't seem to be working, battery voltage goes down after starting up the vehicle :s I've done a bunch of tests including checking AC voltage which was as high as 25v when the car was idling :o Would that be the kind of AC voltage out you would expect with a blown Rectifier? I haven't done the last test you did but shall try it in the morning. A Rectifier replacement would certainly be more cost efficient, fingers crossed :)

[@sirobb]

p00ky76 I suspect that your multimeter isn't True RMS as per the video description so you're getting an unreliable AC voltage. You'll have to rely on your DC voltage figures and the diode test.

[@christopher_barr]

Very helpful video, cheers Si! For other viewers, if the problem is your alternator, dont throw it (and your money) away! I just made a video about Alternator Troubleshooting & Repair. I had a non-functioning alternator and in the video I go through the fault-finding process and dismantle the alternator. In the end the problem was with the diode plate, a $25 component, which is easy to replace. If you want to see the video, go to my channel and you'll find it there... Happy fixing! Cheers, Chris 👍 p.s. Si, where are you from? I'm originally from Blackpool, and your accent sounds pretty close... :-)

[@RuralTowner]

Having this problem one my early 80s VW Caddy diesel. Battery voltage doesn't change (12.7-8) even with everything on. Headlights really dim. My dash light will go out once engine is revved but when it's dark I can see it glowing faintly. Tested everything else...including replaced the regulator with a good used. No change. Seeing explanations on bad diodes causing power leakage makes sense. In the bed I have an electric lift pump for the VO tank and even with it off...a few times when I would turn the key on I could hear it trying to weakly spin. Not all the time. After running for long periods it more than once acted like weak battery when trying to start...which shouldn't be the case. Alternator causing a bleeding ground out when considering possible diode failure. Just did the AC test with my Wal-Mart multi-meter on the lowest AC setting (m200) and reads well into the 1/10s of volts. Probably high from being a cheap meter so isn't sensitive enough but conclusive enough to point out to bad diodes.

[@sirobb]

What was your result from doing the specific diode test shown in the video?

[@RuralTowner]

@@sirobb Unfortunately didn't write anything down or recall specifics other than some bouncy numbers of 0.1+. Narrowed my problem partially to iffy grounds from battery to frame to body & motor. I get 13.0-1 with only the engine running (only draw being the S/O solenoid) but 12.8-9 with everything on. 1 more possible connection culprit to check first...the boot w/ 3 female spade connectors that plugs into the alt. A PITA so thinking they are dirty. Have a mind to cut it off & do a boot delete & just use new ends.

[@velthog]

Hi I have a parasitic drain and started with the alternator and have done the 3 tests, charging is OK, there is NOTHING when red on post so I assume diode is woeking.... but when red on alt body and black on post it is only reading 458 what does this mean? and the fuse test is next? Any advice on this low reading is appreciated.

[@sirobb]

The general range is 500-800 one way so 458 one way wouldn't cause me an issue as long as charging voltage is around 13.8 Volts DC or more. There's a more detailed parasitic drain test video in my Playlist called Cars.

[@velthog]

@@sirobb thanks for prompt response my worry was the diode had gone which test says it hasnt, which was £130 new or £70 fixed. Tomorrow the fuse tests THANKS Ian.

[@catgolfer1]

On the diode test, I am getting a reading of 502 and OL. Is the 502 still within the tolerance? You got 532. Great video, David 🐈

[@sirobb]

Sounds good on the diodes. There are 3 tests in the video though so check them all.

[@sergiuxy21]

Thanks,so then is most likely the voltage regulator for the 15 volt issue?

[@sirobb]

sergiuxy21 Not necessarily. In winter, higher alternator output around 15V isn't unusual particularly when cold. If it is still 15V once the car is fully warmed up, you may have a regulation problem or a battery in poor condition so check your battery voltage like in my video here kzbin.info/www/bejne/qYiseZV6ds2pars I'm using diagnostics but you can use a multimeter for the readings.

[@micthegreasemonkey2918]

In the diode test u removed a single spade connector plug that i dont hav on my alt it just has a 2 wire plug do i disconnect this?

[@sirobb]

You disconnect the battery negative first then any alternator leads so it's just the alternator bolted to the engine block.

[@rangersasc]

Cheers

[@faisalmahmood7589]

hi what if the reading is only going one way but it is low bwt 100??? also what about the wire that goes from the alternator to the cluster (small thinner (blue one)) does that have a one way diode or can current flow both ways throught that? cars a 325i e30

[@sirobb]

Faisal Mahmood The test is in 3 parts so as long as you're getting around 14V DC with the engine running and less than 0.5V AC, your diodes are most likely ok. The small wire is switched so only produces voltage with ignition on and doesn't require a diode.

[@faisalmahmood7589]

SiRobb thanks..👍 was going to change the alternator beacuse the small wire was showing current both ways but seems alternator is ok as other tests are ok... maybe the new battery was bad anyway

[@valentinbacalu]

When I put the black on the alrernator and red on the post I got the normal OL, when I put the red on the body and black on the post I got 0.000, yet the alternator is overcharging when its running at 15.2-15.4V, what do I make of this? Is it draining the battery and overcharging at the same time?

[@dcs4555]

imho the V regulator sounds bad. if that's internal then rebuild alt. if external better option

[@Cdswjp]

My friend, good to run into you again, hope you have been fine. I have a question for you please. If the first indicates a faulty rectifier due to voltage decreasing when engine is revved, would it be safe to say that results for the AC voltage test must return a value greater than 0.0 V? --- For clarity and background for why I am asking, my voltage did drop when I revved the engine however when I did the AC test I received a result of 0.0volts. It is possible I made a mistake somewhere as well because I was not expecting a 0.0V on the AC test.

[@sirobb]

A flat zero on AC voltage means you would need a higher quality meter to get an accurate reading. If the DC voltage is high enough and the alternator passes the diode test though, all should be well with the rectifier. What is the DC voltage?

[@Cdswjp]

SiRobb with red lead to B+ post and black at alternator case - 12.24 V when car is off, at idle 11.82 V, 11.80 V at 2000rpm, and 11.65 V with accessories on. Brand new battery and alternator regulator. Battery is 12.94 V when removed from vehicle entirely

[@sirobb]

Those figures are low so do the diode test.

[@Cdswjp]

SiRobb very good - cheers and a good rest of your day to you my friend

[@monirobaid5809]

Hi thanks for this video it's very helpful but I have a question I want to perform the Ac voltage test on the alternator but my multimeter has several numbers on each setting, which number of ac Volts I should set it to ?

[@sirobb]

Use the setting which allows you to accurately measure

[@monirobaid5809]

@@sirobb on your multimeter showing 0•+3 numbers so your results showing 0•140 my multimeter has 700V 200V 20V 2V On the Ac setting so when I set it on the 20v the numbers was 0.01 the last number keeps bouncing between 1 and 2 when I bring the RPM TO 2500 it will hold at 1 So I've watched the settings to 2V and still 3 digits but this time starting with ▪035 the number 35 bouncing up and down up to 50 before I rise the RPM TO 2500 then it will go up to ▪090 when I rise up the RPM, the only test I have left is the last one when you disconnect the b post from the alternator, I forgot to mention the first test which is the DCV, with the car running lights on heat on radio on 14V RPM 2500 14.8V the car running lights off heat off radio off 14.16V RPM 2500 14.24 (note) the car was setting for about two years do to battery drain overnight now I want to face this issue seriously, my car is a 2003 Mercedes benz e500 just replaced both batteries this week brand new from the Mercedes dealership, I removed the alternator and took it to the alternator rebuilder shop to check if anything wrong with it, I replaced the voltage regulator also this week Sorry, I went too long to write, But I am in a state of frustration with this issue

[@sirobb]

Your alternator sounds OK so next I would do a proper parasitic drain test.

[@monirobaid5809]

@@sirobb hi, how are you doing, I'm back just performed the last test in this video, again my voltmeter has several settings on the ohms section too, the first setting has the buzzer mark and something looks like this >+ the second setting is 200 ohms the third is 2k the fourth is 20k fifth is 200k 2M then 20M So I used the first setting, Ok now the results that I came up with As follows negative lead to the B post and positive lead to the alternator casing (484) positive lead to the b post and negative lead to the alternator casing no numbers just the number 1 on the left side of the screen, Am I good? note I've only removed the B post and i left the voltage regulator connector on, also the skinny wire i didn't pay attention to that until after the test but its not connecting to anywhere on the alternator, and again thanks very much for your time

[@sirobb]

Yes, your alternator is good as I said in my previous reply. You need to to a parasitic drain test at the battery negative terminal. There's a parasitic drain test video in my list.

[@djovidius3545]

hi there, i just checked mine and: when i put black lead of multimeter on plus from alternator it shows 402. after i switch them for 2 seconds its shows 1200 then go to 1 and stay there. after i take the leads out and touch again red on + and black on - nothing apears, just staing at 1. black to + and red to alternator body = 402 red to + and black to alternator body = it goes for 2 sec to 1200 (oscilating fast) next move i put again red to + and black to alternator body = and nothing happen. if i switch black to + and red to alternator body and then red to + and black to alternator body its go again with that 2 sec to 1200. is this ok or my alternator´s diode are gone and is draining my battery?

[@floofsale]

402 sounds like u found the problem

[@Photostudioww]

👍👍👍👍

[@restore64]

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. I've learned from watching other videos that 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?

[@sirobb]

I LOVE NINTENDO Yes, you can check Amperage with a current clamp. You'd be able to see the alternator output as well as the charging rate of the battery. You still haven't told me the DC voltage and AC voltage as per the other tests. Almost impossible to solve without information.

[@restore64]

Sorry. VDC (idle -> 14.3, 2500rpm -> 14.4) - VAC (idle -> 0.03, 2500rpm -> 0.05) Looks all normal to me.

[@sirobb]

I LOVE NINTENDO Sounds normal to me too along with the standby current drain. Perhaps something is turning back on periodically in the car? Do you have any aftermarket accessories fitted or devices left in charging?

[@restore64]

No, the vehicle is completely factory. I tested all the fuses and there's nothing going on there. The only thing that shows defective is my diode test that I learned from watching your video. "Posted results above". I went to an auto parts store to test one of their alternators to make sure I was doing the test correctly. And when I test a good alternator the reverse voltage shows OL like it should so I know my diodes are bad it just doesn't appear to show up as a draw.

[@restore64]

So it turns out 4/6 diodes in my rectifier were bad. I got my rectifier replaced, reinstalled my alternator, and all seems well. But when I use a Fluke 234 Plus to measure the ADC coming out of the alternator at 2000RPM it only goes to about 40 ADC. My service manual says the reading should be 85 ADC. Any ideas?

[@sergium6317]

I did all your tests and ok. but still have the 15.2 volts at the battery and the dash computer only reads 14.7. What can it be?

[@sirobb]

Ser Shark What car have you got and are you testing the voltage on a cold or warm engine?

[@jja8010]

What if black to B+ and red to alt body and it is reading more than the value of 500-800? Reading is about 1200+. No value on red to B+ and black to alt body. Battery is not draining. Lights are flickering. Dc volts is also around 13.5-13.9V at idle. But when at high rpm sometimes it goes up to 14.5V. Also tried the ac volts with a reading of 29. Is it a sign I have a bad diode? Car still starts fine.

[@dcs4555]

mho is the alt is fine, just diff diodes in various models. lights flickering is probably a diff beast altogether. probably loose connection. if both sides then start at their source - headlight conn or column

[@masterdebater8757]

flicker randomly all the time, when hit bumps only when driving, factory lighting kits/bulbs, car audio installed, do they just dim like when ac compressor/cooling fans run kick on? Anything more than .5-1volt AC is unacceptable means AC current bypassing rectifier and getting out put into a DC system does the light flicker speed up as the engine revs? If this is the case that is your reason AC power is in phase with the revolution of alternator this is why AC generators RPM has to be set specifically to match the needed hertz cyclic rate 50 or 60 hertz is 50 or 60 pulses per second. The DC output of modern car charge systems with non direct connected fields (ECM Controlled) generally use ambient temps to determine the needed output voltage the colder it is the higher the voltage it calls for... In cold start normal weather a car may charge at 13.4-13.8 as car warms road warms it could drop very slightly. In cold start cold weather system could charge at 14volts and up. This all depends on the vehicle and type of control that is designed in. Some newer cars have economy modes dropping charge voltages into the 12volt ranges once every parameter it looks at is satisfactory. Next time give us a bit more info for a better reply.. Year. make of vehicle, a bit more specific about the actual problems.. The info you gave is all within spec for some cars and possibly out of spec for others the only outlier here is the AC voltage.

[@lathcar]

I was unable to disconnect the voltage regulator and stator connections on my alternator (2006 Ford F250), but instead disconnected the connection to the positive battery post. The diode test showed ~970 mv. The reverse leads showed OL. All other alternator tests were good. There are no symptoms of a bad alternator except the check engine light remains on, and the Ford Technical Service Bulletin suggests a possible diode problem...as determined by an O-scope. I am wondering if the higher mv reading was due to not being able to disconnect the voltage regulator and stator connectors, or that there is a diode problem triggering this check engine light? Thanks for the video.

[@sirobb]

+lathcar when you say other tests were good, what were the AC and DC volts readings?

[@lathcar]

The DC output was 14.5v engine running, and the AC was

[@sirobb]

lathcar From those figures, I see no diode problem and strongly doubt that an oscilloscope would confirm any different. Eliminate voltage drop to battery by doing the same AC and DC voltage test on the battery *posts* and if they check out, it could be something as simple as a poor connection to the dash tell tale light. Your charging system can be A1 but if the system doesn't know that, you'll get a warning light. Diagnostics live data will help also as that gives the system voltage which can be different to the battery or alternator output voltage.

[@lathcar]

I took it in to a parts store and had them do a diagnostic. Nothing wrong w/ the alternator or battery. The check engine light remains on, had the Ford dealer reset it, but it eventually came back on. This has become quite the mystery since I have done about everything suggested by the error code - the alternator being the last thing. Thanks again for your reply.

[@genedoerr9519]

SiRobb, Thank you so much I have trying to locate a drain on my battery for a while now... when i do the dido test i get no reading Black lead on the case Red lead on the post. When I go Red on the case and Black on the post I get a reading of 850. The alt. is putting out 13.99 volts DC and the AC voltage is around .01 or so. Should I be looking elsewhere for the problem?

[@sirobb]

Your diodes and DC output are fine so look elsewhere. I have a separate detailed parasitic drain test video in my list.

[@genedoerr9519]

@@sirobb Thank you again!... so just to be clear we are talking about an 1988 VW Cabriolet... on the parasitic draw test, key out, I got a reading of 0.02 using the 10A setting... that doesn't seem like a lot to me... but the battery does go drop down to around 12.40 or so after a few days of sitting... what do you think?

[@sirobb]

20mA shouldn't cause a problem in such a short time so either the drain is spiking over an extended period due to for example a faulty door or boot open switch, a faulty glove box lid (if there's a light inside) or your battery is end of life. You may have to leave the meter in series for several hours and use the high/low function if the meter has it to record any anomaly. I have a How to test a car battery properly video showing the difference in behaviour between batteries in good and poor health.

[@genedoerr9519]

@@sirobb Yes, I have watched all your videos on the battery/Alt. subjects... also I noticed that the drain is intermittent... I have one more ground spot to check and clean, but I am starting to think it's the battery...

[@sirobb]

@@genedoerr9519 OK, good luck and I hope you find the source of the issue.

[@arm2644]

Hello. I have performed a diode test and on diode mode of the multimeter it reads 1 in one direction and 1141 in the other direction. When using the 2000 ohm scale it reads 1 in one direction and 825 ohms in the other direction. Is my diode rectifier pack bad or is it normal. My alternator is mitsubishi 120 amp alternator with 8 diodes 40amps

[@sirobb]

The diode figures show one way current flow so suggest that the diodes are ok but there are 3 tests in the video so what are the readings for DC Volts output and AC Volts output when measured from the alternator?

[@arm2644]

@@sirobb that is the problem I am trying to determine if the voltage regulator is bad by checking all other components, since the alternator is not charging at all and the on board monitor says: faulty charging system

[@sirobb]

There's also a video of mine showing regulator replacement and testing of slip rings so perhaps that might help.

[@nobs846]

When I did the reverse test my meter read like 1600 for a split second then went to one. It did that a couple times , why did it show that high of a reading then go to one. Is that normal or is my alt bad.

[@dcs4555]

for a split sec? i'd look up other vehicle types - i think zener diodes might allow that, not sure. sounds like gate V but i'm old and senile

[@bogdanchiolan]

What I'm trying to say is that the belt runs the same although I've changed all the components(including that pulley)

[@mowzol]

I'm about to swap out my rectifier..do I need high melting point solder to reattach to sator wires or will any solder do?

[@thehamburglar9mm]

just regular electrical solder. how did it go? what wattage iron did you use?

[@masterdebater8757]

@@thehamburglar9mm dont you love that how people never return SMH> we both know he went and bought an alternator right?

[@thehamburglar9mm]

@@masterdebater8757 haha most likely. Not what he told his friends though.

[@kellismith4329]

Just swap out the alternator, don’tmess with the rectalfire

[@joesmithiii6287]

JB the engineer, why didn't you mention doing an AC voltage test while car running with and without accessories turned on and at about 2k rpms? diodes should not let very much AC into charging system at all. isn't that what we are looking for with the tests?

[@joesmithiii6287]

Raymond Duke because there are enough people doing it right out there, unlike others.

[@FelixLanzalaco]

well I have a parasitic drain of 300ma which goes when I disconnect the alternator.. used the method here to conclude my diodes were bad and the info here is wrong. On diode method given here I had 1.3 volt one way and 500ma the other. It seems like wasted time and expense taking it back to the garage with the info here as they told me the ripple test found it was ok and that newer diode packs are transistors which feed current differently, so you need to find manufacture spec to know whats what with these readings. Am going to do a simpler AC check now.. I suspect the parasitic drain must be coming back through the alternator circuit, because it shares circuit with the starter circuit and must be passing something from that forward to the battery.

[@fuckaudolphin2591]

300ma is well within spec my f250 by spec up to 500ma

[@floofsale]

@@fuckaudolphin2591 ur confusing 30 milli amps to 300 milli amps... And 50 milli amps to 500 millo amps which is 0.5 amps or 0.05 amps

[@GreenOrgyKing]

Great video. I'm a little confused as to why multimeter hot -> casing, and multimeter negative -> post? My problem is 11v - 12V with engine running, and 25V AC voltage measured across regular +/ground points on the vehicle. According to this test however, the alternator passes the test. I even disassembled the diode assembly, desoldered, and tested each diode with diode check function, pass. However one of the brushes in voltage regulator may not be contacting. Any ideas?

[@masterdebater8757]

The rectifier is where AC is converted to DC, if your getting AC at the post then rectifier/alternator is bad. Low voltage could be that or another issue some newer cars feed the magnetic field current with the ECM (computer) via one of the small wires by changing the DC input to the field they can modulate the alts output. This is maybe for economy. Some vehicles alt output drops below 12.6 due to the ECM. Some people have this section of the control fail or just dont like it and feed the Feild a battery + source after the ignition switch to negated that damaged or undesired operation. Old timers would do a toggle switch on race cars to keep parasitic loses down during their pass down the track. Car audio guys will add diodes to the field wire which tells the alternator a lie (minus X volts) that the battery is lower than it actually is. Doing this they get the alternator to supply slightly higher voltages closer to where they prefer. The average case of this mod would look like 1-4 specific diodes used to raise the alternator voltage from like 13.2-13.5 up to 14-15 volts the diode used sets the change (lie) most aim at diodes giving -.5volts reading on the field. Diodes can be parallel or series or both depending on the needs, series allows a smaller diode set to handle a bit more amps because they do get hot being a resistance to burn of or hold back voltage. In the case of the car audio alt mod there isnt a bunch of amps flowing as if you was to say mod a computer power supply bundling the 12volts to power something at high resistance like a cloud making drip vape... rambling yadda yadda... Any who, you have a bad alternator rectifier most alts have limited life warranty if it has been replaced from parts store so if your lucky you get a free one. That stray AC current isnt good for DC systems and should be below .500 volts AC most would prefer it be half of that if not zero.

[@GreenOrgyKing]

@@masterdebater8757 OK I really appreciate this response for so many reasons. Perhaps you would enjoy the mystery of how this issue concluded, as thus far neither your comments, my brains or the internet have come up with an explanation.: Shortly after posting that comment above i did another test: there is a (+) terminal on most BMW's with two very heavy gauge wires connected, and on mine on (red) cable goes to the starter and alternator, while the other goes to every other function of the vehicle AND the battery leads. Obviously this is set up for ease of diagnosis. The 25V AC voltage continued to read NOT on the alternator/starter side, when I interrupted the circuit. But on the other side.

[@GreenOrgyKing]

@@masterdebater8757 So I ended up replacing the alternator, the serpentine belt tensioner, and only then were things back to normal. I still don't know if the problem was maybe due to some kind of short circuit loop between the casing and the windings in the old alternator, or somehow possibly due to incorrect tension and slippage on the pulley?

[@GreenOrgyKing]

@@masterdebater8757 But i tested everything that would be acting as a rectifier. In fact i saved the big diodes because they are incredibly robust and will try to resurrect some electric vehicles via custom rectifier circuit

[@masterdebater8757]

@@GreenOrgyKing if your breaking the Circuit to the PDC power distribution center and the AC flow stops i would venture to guess your removing the ECMs control over the Field and it may have a redundancy in place to read straight battery at that point. Would imagine doing that would kill the engine thus negate the test so im not sure what you mean to say.

[@geoffreyrawlings9736]

Hi can't get a alternator reading from battery I've placed my leads on the alternator post and to earth still not getting a full charge reading do you think the alternator needs replacement.I'm worried because when I was fiddling in engine bay alternator started to work until I switched car off .back to not working.great channel excellent content,

[@sirobb]

What exactly are the readings you're managing to get?

[@paulclose3426]

I have 14.2v (dc) at the alternator and 30v (ac) - but the diode test is perfect (626 in one direction, 1 in opposite) - any Ideas as to what's going on?

[@sirobb]

Hi. The video description will explain.

[@paulclose3426]

@@sirobb - ha found out that cheaper digital meters (non-RMS AC reading) give spurious reading in the region of +/- 30v ac... and should be ignored. The quick test was reversing the black/red probes gave a reading of 0.00v ac - thanks for your prompt reply! My alternator is ok, now to start the parasitc drain dance...;-). Sorry I didn't see your description as it was in the dropdown bit... my error.

[@sirobb]

@@paulclose3426 No problem. I have a detailed parasitic drain video in my list also.

[@bogdanchiolan]

First of all I want to thank you a lot!!!! really helpful videos, but I want to ask you something(if you don't mind of course): I have an 2002 s80 d5 and my engine does a strange noise...my timing belt doesn't run smooth at all even if I changed it (I changed also the crankshaft pulley and another think but i don't know how it's called in english :) ). Can you please help me? I'd really appreciate that. I think I can make you a video with what's happening there...but I don't know how to give it to you... Once again I want to thank you for what are you doing and wish you all the best!!!!

[@sirobb]

Hi and thank you for your comment. If you upload your video on KZbin and put the link here, I will be happy to have a look.

[@bogdanchiolan]

+SiRobb Thank you very much! I'll make the video tomorrow

[@ltownsgrimreaper]

Hey I have a draw going on in my truck so I tried the last test and it came to 327 with the red on the case and black on the post and it came to 1 when reversed what does this low reading mean since you said 500-800 is in spec

[@sirobb]

ltownsgrimreaper The 500-800 is a general industry standard. You have a definite one way reading on the diode test and remember that there are 3 tests in the video so as long as the DC volts are over 13.5 and the AC volts is below 0.5, the rectifier isn't the problem.

[@AndyFletcherX31]

Your rectifier may be using low voltage drop Schottky diodes which turn on at about 0.4V. These produce less heat than conventional diodes at a given load current and are therefore useful for high current alternators.

[@tomke92z]

Hi, this is very helpful! My alternator is not charging the battery (the voltage at the alternator is the same as at the battery). I did the diode pack test and I got 720 mV in one direction and a positive value which was constantly increasing in the other direction. This proves that my diode pack is wrong and I should replace it, right? BTW. I did not disconnect all the wires from the alternator but my battery was fully disconnected (both plus and minus) so I guess the reading was still correct?

[@yon1438]

sir u just said that if the diode going wrong or problem that are 3 sympthoms things will happen..then what is it??

[@sirobb]

All 3 are explained in the video.

[@sergium6317]

Bmw 730d Agm battery. Cold and warm testing. One thing I know that it can't go over 14.8 cuz at 15v my webasto blocks himself.

[@hammerchisel2477]

For diode testing, how about removing the Negative from the battery then remove the B+ cable (Merc C220) from the alternator? Just seems safer to perform such tests with no power in the system.

[@sirobb]

Sure, you can do it whichever way is the safest for you.

[@hammerchisel2477]

@@sirobb The alternator was tested today and seems fine so there must be a drain elsewhere. Thanks for the know how by the way!

[@sirobb]

No problem. There is a detailed parasitic drain test video in my list.

[@marianjavorka7712]

There is a significant delay in your charging. Wouldnt that be regulator faulty or rectifier pack?

[@sirobb]

This video is 3 years old and my car is still working perfectly. Which delay are you talking about?

[@marianjavorka7712]

@@sirobb Just from start up I was expecting faster response.

[@sirobb]

No, that's normal.

[@bogdanchiolan]

It's a new one, made exactly by INA...I don't have a part number but I assure you that everything is ok with that pulley

[@sirobb]

Have you tested the pulley clutch mechanism?

[@kr33p1n]

So is the 3rd test done with the car off? It doesn't sound like the car is running

[@mohabatkhanmalak1161]

Third test is done using the diode mode on the multimeter, and this is specifically to test diodes.

[@kr33p1n]

@@mohabatkhanmalak1161 but is the car off? or is the car running?

[@milkman2591]

The car is most certainly off on the third test

[@nectarcape]

I took my alternator apart, because it was draining constantly some 200 mamps. However when I measured the diodes on the rectifier they were working fine - not seeping current wrong way. Can it be the voltage regulator? When I connect the voltage regulator, the alternator has continuity both ways. Slightly moving the regulator affects the continuity. Is it possible that inside the regulator there is a short or something that makes the alternator to drain the battery?

[@sirobb]

What were the AC and DC output readings?

[@nectarcape]

@@sirobb DC output was normal I suppose. The battery was being charged. I don't know about AC...

[@blinkerfluid4712]

my diode test comes up fine but i get 13.4 vdc and 28vac at idle is that the rectifier?

[@sirobb]

Karl Griffiths You need a better multimeter to get an accurate AC voltage unfortunately as per the video description.

[@antaresatlantis6785]

SOUNDS LIKE YOUR ALTERNATOR IS NOT GROUNDED PROPERLY

[@igortanyarascovschi1225]

hello SiRobb I would really appreciate your thoughts< my battery is in the boot not in the engine bay This are my multimeter results> 14.32 V the battery in the boot (although changind quiet a bit from 14.32 max to 13. 7 in a very short period of time) 14.48 V positive and negative in the engine bay 14.29 V DC alternator positive and the casing 41.7 V AC alternator positive and casing( although couldn't of measure it if not put the max on multimeter, too unstable and changing with different specs) 0.455 V diode test on alternator 107.5 mV the two positive posts I want to find out my problem as the voltage is not stable at all, noticed with my headlights dimming and back to normal again quiet often

[@sirobb]

Most likely for me would be voltage drop in the main connections or worn brushes or brush springs in the regulator. The brush springs can stick and cause irregular contact with the copper slip rings in the alternator.

[@igortanyarascovschi1225]

many thx im going to replace my regulator with a new one@@sirobb