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I have a W140, 1998, S320 Mercedes and I've heard there is a voltage regulator inside the alternator that eventually will wear out and leave you stock in the road. That regulator has a couple of brush which wear out and has to be replaced. Is that true?, and in the case, how you find out it is time to replace it?, is there a recommended mileage you must do it?

Thanks

Francisco
 

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Please not that most US Cars on the Road with US type alternators have the voltage regulators inside of the alternator to the extent that you have to remove the alternator and take it apart to get at the voltage regulator. And, yes the Brushes on them wear out and the voltage regulators sometimes die on them. And that is just part of driving any Car.

I looked over on auttohausaz.com and a Bosch voltage regulator is inexpensive. However as you stated likely inside of the alternator (I don't know for sure because the pic of the alternator had a plastic cover over the rear of it).

The Voltage Regulator indeed has the brushes on it. In the service manual it would say the minimum length the brushes could be.

Go to your favorite parts place and look up the Voltage Regulator part number and also the number for a replacement alternator. Use those numbers and see if you can find a youtube vid on replacing the voltage regulator.

It could be once the rear plastic cover is off you can replace the voltage regulator easily.
 

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I have a W140, 1998, S320 Mercedes and I've heard there is a voltage regulator inside the alternator that eventually will wear out and leave you stock in the road. That regulator has a couple of brush which wear out and has to be replaced. Is that true?, and in the case, how you find out it is time to replace it?, is there a recommended mileage you must do it?

Thanks

Francisco
Frank : With all due respect , You are worrying about the wrong thing . I have a few Mercedes some even 40 years old. Do you know why a 3 year / 5 year / or even 10 year warranty begins on day 1 ? Its simply because all mechanical parts may " Break " on day one I have actually had cases where I bought a new regulator or a compressor for my 450 SL and they both failed in the first week . So there is no set time to change any mechanical item on our vehicles.
 

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I have to agree with gmbuylow - don't obsess over the voltage regulator. There are hundreds (thousands?) of particular parts that could go bad and if they did would leave you stranded. They usually don't go bad... The best thing is routine maintenance by a qualified mechanic. Find these things and replace them before they go bad.

I'm not certain on your model, I know the earlier ones better but you can change the voltage regulator (and brushes) without removing the alternator although you have to get to it from underneath.

Good Luck
 

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I have a W140, 1998, S320 Mercedes and I've heard there is a voltage regulator inside the alternator that eventually will wear out and leave you stock in the road. That regulator has a couple of brush which wear out and has to be replaced. Is that true?, and in the case, how you find out it is time to replace it?, is there a recommended mileage you must do it?

Thanks

Francisco
Alternator failure means no added juice to your battery. Car will not stop 'dead in the tracks' as long as battery has enough voltage for ignition. So, even after the alternator light comes on - as occurs when the serpentine belt breaks - car should continue to run for quite a while. You can help by turning off accessories; radio, climate control....... Btw: 1. that's where dieselers have an advantage; once started, engine runs on compression-firing until; fuel is cut-off. 2. per my experience, jump starts can blow an MB alt. MB Alts are 3-phase.
 

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I have to differ on being able to run your car along way if a serpentine belt breaks. Sure the Battery will last a while but the serpentine belt also turns the Water Pump and you are going to over heat.
 

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Discussion Starter #8
Thanks you all for your comments, I was worried about driving in the freeway and suddenly your motor will die because the brushes on the regulator won't make contact, but as brbhan said: the battery won't charge but you will be able to put aside to stop in a safe way, that was my primary concern, now, you said MB Alt are 3 phase? How that is possible with a 12 Volts system?
 

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They are AC because they are no different than any other modern generator, and three phase for the same reason the AC generator that powers your home right now is three phase, your alt is just smaller. And it's all cars btw, I've never seen anything other than 3-phase. Of course you could have a single phase AC alternator but the story I heard is ages ago they decided the smoother output was easier on the battery. I don't think it matters, I think it has to do with the smaller parts needed and smaller overall size of the alt, which is of course also cheaper.
I've never seen one fail from worn brushes, they've all had failed regulators, but I have seen some with a bearing that was just horrible but they did still work. This is why I always hated that they put the reg inside. It should be outside so you can swap it easily. Some are, but I think it should be mandatory.
So when your alt fails you will be feeding from the battery for power so most likely you will notice the starter cranking the car slower, like the battery is dying. At that point I suggest you check it before you drive because it may not start next time. It's possible it may die while driving but usually not. To test them I do two things; one is check the batt voltage while the eng is running, which should be up ~14.5V but maybe down to 13.5. Either way the batt can't produce that so if you get 13.5+ the alt is working, at least to a degree. If it reads 12.5 or less then it's not charging, and if so then odds are you'll see more like 10-11V because the batt has been draining. To test it further you can turn on the headlights and high beam and again check the voltage, which will now be lower if the alt is dead. Sometimes the alt is trying but the batt is bad which can sometimes give similar symptoms, so an easy way to check is disconnect the batt. If it stays running you know the alt is working, at least to some degree. If the car dies then the alt is dead. This used to be my go-to check but one car, which died when I pulled the batt wire, the radio wanted a code to work again and F me if I knew what it was so no more radio.
Yet another check is to see of the alt is magnetic. So while idling put something like a wrench against the side of it, if working you should be able to feel it or even hear it rattle against it as the mag field pulls and releases the steel tool. The greater the load the greater the field so you can turn on all the lights, AC etc to make it easier to tell.
Lastly, the AC is simply converted to DC via Diodes, just like any charger or whatever else you plug into the 120VAC wall but outputs DC. Diodes are one way check valves so you have a pair at the output so if trons are going one direction they can only go through one of the diodes, then when it changes direction trons flow through the other one. The battery is on the other side of those diodes so the one diode is always positive and the other always negative, which is DC. The power comes out in spirts, because that's how it's made in AC, but the polarity is right now. Since it comes out in spirts this is where three phase is smoother. Picture yelling, but then you have to stop to inhale so your yelling would be in spirts. Get three people timed equally apart and they'll overlap pretty good so it's hard to tell when one person is petering out because another kicking in. Same with single vs three phase because three phase is just three sets of single phase evenly spaced apart, both physically and the time frame they give power. Since all three are on check valves it makes a pretty steady stream of DC where a single phase would be like that one person yelling, meaning there are times where no power is made. If your battery died and you had a single phase alternator then the engine would not fire in that moment there was no power, but it would fire a moment later when there was and would no doubt cause a backfire. It would likely do this several times a second so that would be fun and just may blow your muffler apart... So there's another point to have three phase. You could have two-phase but three is way better. You could have four but it isn't much better than three so basically three is the goldilocks number.
A straight up DC generator, like they used way back when, is similar in size and shape, kinda, but operates differently and WILL eat brushes. You see, in a DC gen all the power goes through the brushes and they're making and breaking contact many many times per engine rpm. It would be like turning a switch on/off 100 times a second while 10 amps were going thru it, it's gunna wear out... An alternators brushes do not transfer the power the alt makes and they don't make and break contact like that so they last a really long time. DC gens can also ignite gas fumes under the hood where an alt will not.
So the Alt wins and 3-phase wins so that's what we have, plus diodes since there's no such thing as an AC battery.
 
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