Car died twice - Losing all electrical power - PeachParts Mercedes-Benz Forum

#1 06-22-2002, 09:26 PM

While driving the 300 SEL yesterday, first the A/C shutoff, then the ABS light came on, then the car just shut down totally and would not start again. There be enough of power that the windows would work, radio would turn on, but the engine would not crank.

Towed it to my Tech and he said that the there was a faulty circuit in the battery so he replaced it. No big deal was within warranty.

Took the car out today and it did it all over again after driving for 25 minutes. Could it be the alternator? Is it possible that the alternator was charging properly at the shop when the car was colder so the Tech did not notice, but once warmed up, the alternator or voltage regulator stopped working. Could it be another bad battery?

Help appreciated? Have to tow it again.

Gilly · #2 06-22-2002, 10:07 PM

I would guess the alternator also, "however",
there is a part in the alternator called the voltage regulator that can fail. It's held on to the back of the alternator with 2 screws, easy to remove, inspect, and replace if neccesary. The regulator has a pair of carbon brushes which will wear out, causing this problem when eventually it dosn't contact the rings inside the alternator.
The voltage regulator is available seperately.
Nice thing is that the alternator doesn't have to be removed, the regulator can be easily removed without doing any additional work, and there is a ton of room on an SEL engine bay to do it.
Around here, I think the regulator is about a $50-60 part.
Or if you know the alternator is original and has alot of miles on it, a more expensive route is to put in a reman alternator, but I'd bet the regulator would fix it.
Gilly

#3 06-22-2002, 11:13 PM

Thanks Gilly. On the 420 there is plenty of room. On the 300, looks like I have to work from the bottom of the car as the air pump blocks most of the view to the alternator.

A little confused. I see on the back of the alternator the large wire that goes to the battery, but not 2 screws. Do you simply loosen the 2 screws and then remove the voltage regulator? Does installing the new one take much expertise to ensure that the new brushes touch or is it simple?

Maybe a dumb question, but inside the alternator is foreign to me.

Thanks a lot for your post!.

#4 06-23-2002, 12:14 AM

I was able to find a pic of the voltage regulator. Should I disconnect the battery terminal?

Is it as easy as just loosening two screws, remove old regualtor, put new regulator in place and tighten?

Gilly · #5 06-23-2002, 04:24 AM

You do need to kind of finesse it in there. You won't harm anything if you don't force it. Hard to describe how to do it. If i am describing it correctly, I think the way I do it is to put it in at a slight angle, so I can start one of the screws, then twist it so I can start the other screw. It'll feel like it's sitting flat against the back of the alternator. The important thing is to get it off so you can look at the brushes. The tip on one of them will ptobably look kind of dull, like it hasn't been contacting the ring.
You don't need to disconnect the battery even.
Gilly

Mike McDonald · #6 06-23-2002, 04:52 AM

I had the same exact problem with 560SEL, went through 2 batteries, when the alternator got hot it shorted out and just killed the battery in about 30 sec. Replaced the alternator and no problems since.

#7 06-23-2002, 08:23 AM

Thanks guys. I am going to post the pic of the regulator that I took out this morning. Compare this pic to the new one located above.

The 2 post looks shorter than the new one and even have a "U" groove in them from wear. Does this look like this could be the symptom.

Even though I just wore down my second battery, once I replace this regulator, will the alternator charge it back to normal or is it ruined?

#8 06-23-2002, 08:24 AM

here is another angle.

engatwork · #9 06-23-2002, 08:40 AM

From what I can tell it is worn out. You should be able to charge the battery back up unless it is worn out from old age. You may end up having to replace the battery.

#10 06-23-2002, 10:26 AM

Just out of curiosity, does anyone know the theory on how this regualtor works? do those 2 posts or brushes somehow regulate the amps as the alternator rings spins inside? Exactly what is going on inside? Fascinated on how that little things does so much.

joe p · #11 06-23-2002, 10:32 AM

Looking at that image you attached of the regulator you might want to fix or at least slow down the oil leaks as well. :-)

Joe

stevebfl · #12 06-23-2002, 01:04 PM

An alternator makes electricity by spinning a magnet within a coccoon of windings. There is an electrical potenial that grows inside these windings as the poles of the magnet sweeps through. First polarity in one direction and then the next. The AC electrical potenial backs up against diodes (polarity gates) and the respective voltage passes in both directions (negative and positive) into the battery.

The regulators job is to control the output current by the level that the battery is below a control voltage. It does this by controlling the DC current that is passing through the rotating field. That current called field current is controlled by pulsing the voltage very fast through the rotating field. By changing the duty cycle the output is varied.

To finsh this brief concept, there is one other important point. The power to flow through the alternator as it starts spinning comes from the battery via the ignition switch and the alternator light. This small current flow is necessary to jump start the alternator (these Bosch alternators do not work if the alternator light bulb is blown). As a small AC potential builds the phases each have a smaller diode that shunts part of the output to the same point as the beginning source from the alt. light. The light now has battery voltage on both sides and goes out. The significant current used by the rotating field is now totally provided to the regulator by this diode pack; called the trio diode in three phase alternators.

#13 06-23-2002, 09:13 PM

Thanks Steve. Physics is coming back to me. I printed your message out and will have to read it again. Very impressed with your in depth knowledge. Interesting point about the bulb. Apparently if the bulb fails, replace it ASAP as the charging system will no longer operate?

What happens when the brushes wear, do does the electric potential build up/magnetic field terminate?

UPDATE: REPLACED THE REGULATOR THIS AFTERNOON AND AFTER A JUMP START, THE HORN BLEW LOUD, THE LIGHTS WERE BRIGHT, ETC. I DROVE IT AROUND TOWN FOR 15 MINUTES TO BE SAFE. PRICE OF PART $34. THANK YOU ALL FOR YOUR HELP AND MONEY SAVINGS FOR ME! A TECH WOULD CHARGES ~ $250 + LABOR FOR A NEW ALTERNATOR = ~$350.

stevebfl · #14 06-23-2002, 09:27 PM

The field current is what passes through the brushes. Since alternator output varies by the field current good contact here is imperative.

And, yes, a bad bulb will keep the alternator from ever starting. Domestic alternators with switched ignition power aren't like this.

Gilly · #15 06-23-2002, 10:22 PM

I was off both working and partying all day, glad Steve got the tech theory out of the way for you, he's great at that.
The brushes on the old one really looked shot alright. The picture I would really need to see is the tips of the 2 brushes, where it contacts the rings. Usually, one of them will look really dull, and the other will be shinier, as usually one fails first and the other still makes contact. Have any problem getting it installed?
Your battery should be just as good as it was before, the alternator will charge it up just fine now.
Gilly

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