Car Electrical Systems Explained - How To Diagnose A Failing Alternator vs. A Failing Battery

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Is your battery warning light illuminated on your dashboard? Are you experiencing a lower than ideal car battery voltage or car battery amps? If that's the case, it may be time to look into servicing your vehicle's charging system-but how do you do that?
In this video, Nate Vincent, our Motorsports Director, takes you on a deep dive on how to diagnose a failing alternator vs a failing car battery. From testing the voltage and amperage using a multimeter, to going over the internal components of the alternator itself, everything you need to know to find the source of your problem is covered here.
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Read this guide on our blog: blog.fcpeuro.com/car-electric...
00:00 - Electrical Issues Intro
00:38 - Why Is My Battery Light On?
01:00 - Inspecting Your Drive Belt
02:15 - Using a Multimeter to Test Your Car Battery
05:36- How To Check Your Vehicle's Battery Voltage with a Scan Tool
08:40 - How a Decoupler Pulley Works
09:45 - Inspecting a Decoupler Pulley
10:45 - What is a Voltage Regulator?
12:18 - Testing the Voltage Regulator
12:30 - Is my Alternator Bearing bad?
12:52 - What is inside the Alternator? - Stator and Rotor and Rectifier Explained
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[@gwgrote5]

Nate, I know this "how to" is 2 yrs old but it is excellent and has helped me. I am the second owner of a Lexus. Lexus has all of the records of what was done to the car prior to me, which was frequent oil changes and detailing. Now that I'm approaching 100K miles, I realize that nothing was done on it but the few things that needed replacing during my ownership.....speed sensor and rear brakes. I do not fix my own cars, but I will definitely buy a multimeter tester because I'm seeing an alternator in my future. Great job! 😎

[@RedondoBeach2]

Clear, concise, easy to understand usable information. No assumptions about a DIY mechanic's knowledge. The benchmark for repair videos. Great job. Please make a video on troubleshooting cooling fans and associated electrical system components on a 2004 VW GTI VR6 2.8L.

[@lauriezerulla2448]

11o

[@milanmalogorski3882]

Nmn

[@salmothymus]

first time here.....subscribed immediately. Kudos to the young presenter...very clear and just about right amount of technical for my level of knowledge (liked that pulley comparison with bicycle crank) 👍

[@ronaldbryant7710]

This is the best site and video I have seen on this red battery light issue, thank you for up the very simple explanation, with clear and concise instructions. Thank you!!!

[@beagletube]

Great stuff. Just learned a whole bunch. Thanks!

[@MySSS03]

One of the best, best best video to understand without moving the camera where it goes everywhere. Thank you

[@murkyturkey5238]

This was perfect 👍 A++

[@Davidmc23]

You and your team did an excellent job on this video. Thank you, it's been very helpful.

[@fcpeuro]

Thank you for watching David!

[@jaymorris3468]

Thank you for showing where the multimeter dial and leads go to on the multimeter, awesome, a lot leave that bit out.

[@Rivanni1]

Thanks so much! Great video so very helpful. Love the visuals!

[@kdao1007]

Thank you for the in depth lesson. Was great, and thanks for making these amazing videos for DIY

[@fcpeuro]

Glad you like them, and found it helpful!

[@fortifiedgentleman8827]

Simple explanation, very clear 👍

[@witnessmalinga2377]

A very nice and easy to follow tutorial. looking forward to get more of such videos. Thank you so much

[@fcpeuro]

Thanks for the love!

[@johngreathead7512]

Be aware of your multimeter's cables when working around a running engine. They could become entangled in the drive belt, likely destroying the meter and possibly injuring you.

[@LastDickOnEarth]

@@borick8744 XD

[@dreadaught]

First-hand experience? OUCH.

[@OcRefrig]

Yea, I’m surprised He did not Mention a little safety around a running engine for Women especially that might want to try thier hand at auto repair. Take off any jewelry, necklaces etc while working around a running engine.

[@FelipeRodriguez-eh6ty]

@@OcRefrigdont be a 🤡, he can’t teach common sense

[@VintageLPs]

@@OcRefrigDo you know how many men wear bracelets and rings? Also, men, keep your ponytails away from the running alternator belt. There……I fixed your comment! 🤣

[@BonesDT]

Fantastic and thorough video! Thank you.

[@carolynelouis4867]

Thanks well Explained , best and most informative videos .

[@jstreett1]

Very helpful video! Thanks!

[@junebrooks665]

Great video i must say and extremely well put forward, you should be on television as a speaker well done young man

[@aob.aob83]

Thank you so much for the information

[@renostubbs8504]

For a guy man you know your stuff,glad that I came across this channel. Thank you for sharing your knowledge and expertise

[@Michael-st3wg]

How you know he's gey 🏳️‍🌈???

[@faizurrehman7699]

Great video. Learned something today...👍

[@cesarpalmero8323]

Quick and simple. Keep it that way. Many thanks.

[@moffatk7948]

Very insightful video on charging system, covers checks on battery, alternator, obd scanner, explain alternator how it works even a DIY person can understand 👌🏻

[@stephenmorrissey1254]

I had no idea alternators could adjust voltage depending on demand. Must be why these things are so expensive. Thank you for the in-depth article!

[@glasser2819]

yep this is Bosch engineers going to town on efficiency to save gasoline. If you have a fancy sensor (Temp, Amp, Volt) on your battery terminal good chance your ECU has "opportunistic charging" feature. Great while it works then hundreds ways to fail 😉

[@fcpeuro]

@@glasser2819 Things are constantly evolving ^.^

[@ginalolo4373]

I needed a few jumpstarts over the past few weeks, I was told the blower and the resistor for my Q5 need to be replaced, my battery is good, the diagnosis recommended humidity sensor too, are all these working together to cool the car? I'm more concerned about the car not wanting to start now than the AC blowing hot air. So, I'm thinking of maybe replacing the alternator first. I will very much appreciate any opinions you may have on that. Thanks much for your informative video!

[@ericmakondo965]

Appreciate the information on the video

[@AnthonyBoettcher24]

Thank you so much! Hoping to figure out my charging problem on my W124 with this knowledge

[@jolenemoyanchildkiller7621]

1999 ford taurus se. We had similar issues u explained in the video, After replacing alternator and battery..Battery light won't go out. after couple months. car died. dash lights, gauges, blinker lights, stereo. and battery wont charge. check megafuse before looking in the alternator?? your video very good.

[@hafiz198022]

Excellent quality video

[@mikimiki195]

Wow this is great video thanks 👍🏻

[@garydenny7904]

Brilliant presentation

[@mospheric]

One of the best, most informative videos I've seen. Well-done. Too bad my alternator is so buried, it's almost inaccessible.

[@phonkyfeel1]

To your point: Well said! If your alternator is buried, then this doesn’t really apply. I love how people try to make things sound so simple and quick/easy. My wife’s Q5 alternator--childbirth is probably easier than removing the alternator in this car. Or testing it. 🤣😂🤣😂.

[@NateDawgdidit]

Good stuff right there thanks million

[@muhammadrezasadeqi2919]

Just like all of your videos, this is also very informative and an awesome video, Thank you so much FCP EURO, for your great videos and contents, keep up this awesome work and please upload more videos. As I'm an automotive technician, these videos are supremely useful for me. Thank you so much FCP EURO.!

[@fcpeuro]

You are very welcome Muhammadreza Sadeqi , glad you enjoy the content we've been producing!

[@muhammadrezasadeqi2919]

@@fcpeuro Thank you so much.!

[@lifeslooker]

Great video; did you do a video when the alternator was actually changed? This is fascinating. Learning all the time. Regardless of lockdown. Thank you.

[@fcpeuro]

Hello Jason, Here is the video of the alternator being replaced on this exact Volvo C30. kzbin.info/www/bejne/g3ikaJiJhNmDp6s

[@t238]

Learnt a lot thank you ! My car was charging - but on a long journey stops charging on the way back! - everything is new - i am completely stumped - new connectors new wires , now wont charge at all! Focus mk1

[@DirtBikeSessions]

Excellent video 👍

[@robertalfred1724]

I would like to see more of these

[@timski9063]

Do a video on radiators and when they need tobe replaced. Thanks ps good vid on alternators.

[@martinandleesmith9609]

Good plain-language demo. Thanks

[@mra8035]

Hi - Great Vid! - We need you more guys like you on you tube! ......I've got a 2016 330e petrol phev (F30) and recently got this Battery warning message come up intermittantly, especially when i start the car with no charge to the electric battery and run from petrol. When i go to 'save option' to build some charge to the battery, the warning message comes up and then disappears allowing me to charge - but the error re-appears stopping the charge and then goes away and then allows me to charge again. Could this be the alternator? In addition - Ive recently had it recalled for the starter motor from BMW UK - Could this be anything to do with it?

[@kboudy10]

Awesome video and explanation

[@fcpeuro]

Thanks Kevin glad you liked the video, we hope to do more like this!

[@xamen_moi6979]

Great tips.

[@alexcornell8970]

Great video !!!

[@muhammadwaqar3406]

excellent video

[@david_5668]

Very informative. 🎉

[@dlopez61]

Really enjoyed the video! Congratulations on such a thorough and informative rundown of such a common problem. I would like to ask you if the old myth of pulling the positive off the battery, while the vehicle is on, and the vehicle turns off, does it mean the alternator is bad? Appreciate your feedback. Thanks again. Dodge caravan RT 2016.

[@judahlion2067]

Yes

[@Pakistanijugars]

The same is my question-if I remove the positive from my battery while engine is on , the engine stops after few seconds Does it mean my alternator is faulty ???

[@JunePleasantArt-qv9hi]

Yes. The red of ,car dies, alt is in need of help

[@kingcory21]

More please!

[@jeffreykarg5608]

Very nice video; Thank you. My xtool has an additional variable it reports on the alternator, % loading which ranges from 72% - 99%. When idling, it read 89%. What is the relavance of % loading.

[@donwolfjr1]

Wow... I'm so glad I watched this video because I almost didn't! Lol. I learned quite a few things here and I'm really impressed!!! Excellent video, very well made and Nate did an awesome job with it!!! Again...wow!!! Thank you!!! : ) Don W., Ohio, USA

[@fcpeuro]

Thanks for checking in, Don!

[@donwolfjr1]

You're welcome, n thanks again for the video!!! 🙂👍

[@grahamball364]

I have a Dodge nitro 2.8 I live in the UK England, my problem is that the battery light came on and I had to drive 20 miles on my return journey. The Dodge came to us not after the power supply died. I get a jumpstart or another vehicle to Home that evening with the lights which was about half a mile. , I think that you diagnosis and health towards finding the fault with my vehicle is that the alternator on the Dodge 2.8 is at fault, Charged the battery or in the process of charging to see if the car start on the battery and then I will do the test that shown video this is a great helping her as a tutoring video and I thank you for your help. Have a great day..🇬🇧🤝🇺🇸🇬🇧👌

[@brucegarven7988]

Very helpful and useful video, thank you! Would a faulty alternator fuse also be able to stop the alternator charging the battery? I have a 2006 2.5 Toyota Hilux with a new battery that runs the vehicle for a day or two if the battery is charged up but then it starts to fade, the lights fade, battery light goes on and a while later the vehicle 'dies' as though being turned off

[@pgreenx]

Could it also be a loose wire connecting the alternator and battery? Is it worth checking the voltage AT the alternator instead of the battery?

[@andrewdoherty8847]

A great video Nate. It is no wonder I like old vehicles - 2x '80's MB. 1/ I always carry a spare belt, but make sure it is the right one. I got caught. On systems with twin belts, eg 380, make sure they are the same actual, not nominal, length. 2/ I start with the battery. If it is not 12.7 volts something is wrong. I then turn on all the lights and re-read it. If the voltage has collapsed the battery is shot. 3/ If the battery is not holding charge check the standing current drain. The drain should be about 10 mAmps. Setting to Amps DC connect one meter end to the positive wire and the other to the positive post. Then, without loosing continuity remove the positive wire from the post. Two people are handy for this manoeuvre. Warning. On a modern car if you loose continuity you may have to reset all the control systems. See Facts Over Feelings92. 4/ If the standing current is fine but the battery is not holding charge look for irregular discharges. My wireless standby capacitor periodically turned into a 4 ohm resistor. Not enough to blow the fuse but enough to flatten an occasional use car. 5/ If the alternator is over 14 volts it has lost battery contact. Either a dirty battery post or the control unit is faulty.

[@twerktospec]

Or you could just live in the current world

[@andrewdoherty8847]

@@twerktospec No, no, no! New-age electrons are skinny weaklings compared to the ones 50 years ago. There are so many more of them these days and food is shorted. 🙂.

[@bigbluechopper24]

Thanks for another great video. Would be good to see a video that helps you test how much life is left in your car battery. I.e. does it need to be changed now or in the near future or is it fine etc

[@vildrotax]

An easy test to do is to measure the voltage on the battery as you are starting the car. If it goes below 10V then you know that it needs to be changed. If it is cold outside then you may want to change it at a slightly higher voltage, like 10,3V. This needs to be done on a battery that has been charged up.

[@markpalarca3111]

Thanks!

[@claudiorassouli1240]

Great video

[@ChandraSQLVideos]

Good video..👏👍

[@joseduranjr4963]

Nate: I have a 2017 VOLVO xc60 with auto start/stop. I’ve replaced the auxiliary battery twice already. Sometimes the system works and sometimes it doesn’t work. I don’t have an alternator issue or belt issue. I’m curious why my system works sometimes and at other times it doesn’t work. Wires are ok. Belt is ok. Alternator is ok. What could be the issue?

[@johnbutler6391]

Great info. I’ve today had a new alternator fitted however I doubt the mechanic actually tested the original alternator. Cost $1100 dollars. The vehicle has 47000klms. And services by time rather than klms. I’ll check out your recommendations. Regards

[@KenStOnge-vg4fi]

Great vid.

[@OcRefrig]

Good video ! You covered a Failing alternator well but I missed how to diagnose a failing battery 🔋.

[@activatedcarbon158]

With a multimeter readout, if it is less than 12.6 , chances are it is failing

[@ItsDaniPlays]

He literally explained it in the video after he did the multimeter test. You failed to watch the video.

[@confidential8738]

I have an alternator overcharging at 15v 14.9v at batt and alternator. 04 buick LeSabre. 3rd alternator so cant be the voltage regulator. what else could cause that?

[@user-rx5kg2ks2d]

Great video, so my issue is that when my car is off the voltage is perfect but when i start engine and on all the functions such as lights, stereo, wipers the voltage goes to around 15.5, is there something wrong with my alternator and do you think it could be repaired by replacing the voltage regulator, any response would be greatly appreciated, car is jaguar xf 2013

[@98XC1]

Dear FCP. Please make an addendum to your video with instruction to DIY not to disconect(or connect) the battery terminals on a running car as some of the comments below were suggesting.

[@SUB-Entertainment]

I have a cigarette lighter usb with volt It shows when car running the voltage goes to 14 when not running it shows 12.2 so point is u can just buy 5 bucks usb with colt display and plug it into your cigarette lighter it will show voltage before and during engine run. Good explanation on how to fix

[@amram7139]

Hi I have a nightmare going on with my 07 Honda CRV. OK I had my alternator replaced with a high output model sourced from a reputable alternator manufacture in Tucson, Arizona. When having the alternator installed, I recall the gentleman doing it stating that he needed a smaller belt because the pulley was smaller than the stock model. The alternator I purchased was designed specifically for the make and model gentleman installing it had me go to O’Reillys to purchase a different belt which was unavailable that only one size made for this car he said he adjusted the tensioner on the belt and said it was fine . The vehicle ran fine. Everything was good until I had a can of clearcoat that blew up in the driver side cabinet underneath the steering wheel. I had the vehicle, detailed, and what I could tell, cleaned to the best of their ability which I little find out they used a power washer to spray most of the stuff off under my steering wheel , it wouldn’t start at first and I let her drive for about an hour. The vehicle started. I went home and the following day I couldn’t get my vehicle to start and there’s a flashing green key on the dashboard at that point. I thought it could be my key fob, because on top of the spray tan blowing up, my key fob broke in the middle key came out of the fob self Which worked by holding the follow-up to the ignition for a couple times and then stopped completely. I went to arrowhead Honda dealership and purchased a new cut key to have a number and put the internals from the broken five into the new fob. got home and the vehicle and I start, I had on the come out to my vehicle in my house with a new key new internals and had a program to the vehicle which was successful but the green light continued flashing and when I start after a few different Lock Smith‘s coming to the house one of them gave me a code that Red immobilizer is not communicating with the IMOS. They told me to replace the multiplex control unit Which was sprayed on the steering wheel because it’s located on the fuse box in the driver side cabin last night I replace that multiplex control unit and it is in the vehicle and about to have Locksmith come out to program the keys and see if this works. It cost me $100 to have a key programmed and if that multiplex control unit is not it then I will have to go from there. I hope you can kind of understand my confusion and there’s different things that could be wrong and it could possibly be the alternator pulley not doing his job correctly as well. After watching this I believe that could be an option. If you could please contact me and give me a little bit of advice and what you recommend me doing. I’m at a complete and total loss. Thank you so much.

[@glasser2819]

beware of the alternators controlled by the ECU based on feedback from the battery sensor.

[@JohnPooley-te9ei]

Nice1..FCP & thank u

[@hubster4477]

I heard a simple test, with engine running remove the positive cable from battery, if the engine Keeps running its a bad battery. Is this a good test?

[@Blacklist2013]

Had battery light and red check engine light on. Faulty alternator as diagnosed by breakdown and garage. Alternator replaced but both battery light and red check engine light still there. Car voltage is over 13.8 when running but the both lights have decided to stay

[@HillBilly-1]

You forgot a very important part of the charging system. Same problem as this AFTER replacing BOTH alt and battery. Replacing the starter cured the problem. Hot wire from battery and alt both go to the starter solenoid. Bad solenoid can burn the hot wires going to alt and battery.

[@judahlion2067]

Facts something happened to my nissan changed my battery and put 3 alternators

[@nyacoustics8373]

in my c4 corvete i just put a brand new battery one week ago. I haven't driven the much in the last week. I started it today and it ran fine. About 10 minutes of just idling the car died. i went to restart it and got nothing. no interior lights or dash lights. i tried to jump it and got interior and dash lights briefly and but no start,. just clicks and clunks but the car wouldn't turn over. I noticed last week the volt warning light was on. I took my multi meter and put it on the battery while it was just sitting not running and was unable to start. The reading was only 9.7 volts. Do you think this is a battery issue. Should I try recharging it? or is this more of an alternator problem?

[@salvatore_davi]

1:57 I notice that the belt spins around the ac compressor and the pulley turns when the ac is turned on and the pulley turns intermittently when it’s off, is that normal?

[@deyvidaleksandrov5276]

Hello i have BMW 730d 2008 and when i start the car it says Alternator fault and some times indicators left or right failure but for short time like seconds ??????!

[@janicemasaira]

Thanks bro

[@fcpeuro]

Anytime, Bro.

[@davidespada01]

informative

[@smorrow0816]

My red battery light is on intermittently. I checked the volts and while the engine is running, it is around 14.7 volts. Is that cause by too much voltage? hence, the voltage regulator is not working?

[@tjsworld2338]

I just used a battery charger on my CTs vsport last night because it was reading 12.4 and I plugged right on the battery did I hurt the battery

[@alchem7065]

Please help, my car charging lamp blinks on an off while running. Battery is not charging. When a new alternator is add it becomes ok for few days an the same problem comes again, please help

[@alcraig1]

Really good video. But, did you know that you can see your alternator voltage, in real time, while driving, by installing an OBDII plug and loading the TorquePro ap?

[@alcraig1]

@@412Walk doing all what?

[@reallyhappenings5597]

Decent scanners also have live display

[@alcraig1]

@@reallyhappenings5597 True, but having various displays on my phone (which is mounted on my dash) is very helpful.

[@judet8290]

@FCP Euro where can I get one of those t shirts?

[@rodneyosei-bugyei7621]

wow thanks a lot! would check this on my Lexus R72 (oops, there's no car like that🤣). Perhaps, not yet.

[@alexybodom]

is it posible that the alternator is pushing to much amps causing the battery to buldge ??? i have that kind of problem with a motorcycle of mine BMW r1150rt with "real" alternator the voltage ist in spec 14 volts but the battery is buldging

[@EddieRiff]

My car is the Volvo S40 1.8 (non GDI) 123k miles original Bosch alternater. Test: @2000rpm I did the positive circuit drop and got a steady 0.54 under load (all the ACC's on) @2000rpm I did the negative circuit drop and got a steady 0.12 under load (all the ACC's on) And yes both battery terminals positive mostly getting hot. I do summize the regulater is on the blink. It throws a p0340 code. Yes a cam p sensor circuit code. Apparently it's the signal noise the regulater gives off that throws a random code esp when it shares the same wiring harness/chassis.

[@constantreader6678]

Good to know for newer cars. My 1985 Porsche? Not so much. No OBD II port. Old school mechanic needed!

[@MartinSage]

Simple question. I replaced my battery a week ago. With engine running I see 12V (same as engine stopped). Belt to alternator good and spinning alternator. Today my alternator light goes on. I suspect a bad alternator so I have been charging my battery daily with a 10/2amp Sears charger. Question: Can I drive in the day for less than 1hr? It's been raining so I can't replace the alternator till the weather is dry (no garage).

[@TheMrBennito]

Clear , yet somewhat high pace for a not exactly knowledgeable 67 year old. the vid , amongst other things, convinced me to get an affordable multimeter :)

[@reallyhappenings5597]

Multimeters are cheap, powerful, versatile

[@tomboyd7109]

Another common failure point is the brushes. They wear out over time (get too short to make good contact) or when they get wet. The tell-tale is that the alternator idiot light comes on or flickers at idle and goes out when you rev it up. They are cheap and usually fairly easy to replace. Sometimes they are attached to the regulator or need soldering to replace. Again fairly easy considering the price of an alternator.

[@PrivateOrdover]

Help ! I recently replaced my alternator and now my check engine lite came on ! What do I need to do ? Thanks !

[@nevillerichards7354]

Hi. I'd like to ask a question. I have a Volvo v50 t5 2005. I am getting warning lights on the dash, not of a battery but brake failure, unti skid service needed, windshield wipers just working without the stork not been touch, no indicate working. So my mechanic suggested the altinater, but seen videos that it might be cem. Any advice? Please 🙏

[@egrunauer47]

My c250 2012 starts and slowly shuts down by itself, I barely drive my car and try to warm it up every 2 weeks cause it just sits there. I took it to my mechanic and the battery was reading a bit low in the 12v before all this and he was saying it my be the alternator and that I had to drive it more but I wasn't able too drive it at all. Last week had to open the door manually cause the fob stopped working, after I turn the car on and slowly shutoff by itself the fob was working and was able to lock/unlock my car with it. I was thinking of getting a new battery and start from there but ill run these diagnose test and see what I can find.

[@kylebento9721]

This video was made for me.

[@fcpeuro]

You're welcome, Kyle.

[@penjejakarus5033]

So if not charging,the rectifier are the cause right?

[@Altruisticme]

Truck cranks but doesn't turn over. With jump start it does turn over. Next day same situation. Before crank battery volt 11.36, same volts while engine running. Alternator was repaired 3 months ago as it was overcharging like 15v Question - it it battery this time or alternator ?

[@motologist1917]

hey brother plz reply back , i am having ford fiesta diesel , whenever i start the car battery warning light comes on the dash but if i revv the car till 4k rpm it disappears , and wont come back till i turn off the car , what is the issue

[@Pinkielover]

My alternator rarely goes over 13.4 V and it never goes below 13 even when rolling down windows..Even when I got the car new.. I've taken the alternator out brought it to the shop and had them test it ,, they said the alternator is perfect.. Most of the time after the car is running it's down to 13 V most of the time...I have an obd voltmeter Attached to my aftermarket head unit radio so it's a pretty accurate voltmeter.. I do notice when it's colder bellow 70 outside My voltage will be up to 13.6 sometimes when I initially started .... I'm just annoyed because it should be closer to 14 volts. But they're telling me it's fine.. if I put loads on the system you could see the voltage dip for less than a second and the alternator kicks in and compensates so the alternator is working but the voltage is always the same never more than 13 or 13.4 V 13 V if it's idling a lot if I drive down the road it will go up to 13.2.. I don't have a German car it's a Mazda I believe mine is ECM controlled.. it has 2 control wires , and I believe it uses a pulse system from Ford to determine the proper voltage not 100% sure.. but yeah the alternator has a regulator on it.. Definitely notice a difference when it's colder out the voltage is higher..

[@melivakarauwale1365]

Hey thanks for this, just a question what happens when battery light and power steering lights comes on, i had experienced this lately and i could hear a hissing sound coming from the alternator ends. Hadn't checked the voltage of the battery yet but do you think one reason for the alternator failing also was due to the engine mounts or vigorous movements around the alternator.

[@theycallmebigfoot]

Did you ever figure it out?

[@IceESole]

What was the end result?

[@q8kingo358]

Brother may i ask my car battery is 95 AH - 450 A , if i pot 100 AH - and more than 450A is that will damage the car ( passat R36 ) ??? am grateful 💜🙏🏻

[@happycat0411]

If the battery is failing then use a manual load tester on the battery. If no load tester is available then use a multi-meter and look for a reading for around 12.2 ~ 12.3 volts with the car off. With the car running the readings will be around 14.0 ~ 14.4 volts. You can use a 12 volt cigarette socket voltage output tester as well as they are cheap and work really really well. If you are really careful you can disconnect the battery while the car is running. If the car dies when the battery is disconnected then the problem is with alternator. The 14.0 volts ~14.4 volts indicates that the alternator is working properly. You can always boost the car to get it running and then drive it around for 20 minutes. If you try to start the car again and you get nothing, the problem is the battery as the battery should be charged enough to restart the car at this point.

[@imperuk]

Im avionics engineer so cars are not my strong skill but I noticed voltage fluctuations up and down and batt warning light on when high and when down so I suspected in something regulating the voltage when too low and when too high., and eureka! 90% sure is voltage regulator issue. Lets confirm with multimeters Thks master