Last year I had problems with the alternator for '86 300 E. Following instructions from some tech savvy and friendly people I was able to fix the problem just by replacing my voltage regulator.
Now exactly one year after the initial problem I am back to square one. On taking out the voltage regulator I see that the brushes are completely worn out. In fact they are is so worn out that they're almost close to touching the base of the voltage regulator. My question is- am I faced with a more serious problem within the whole unit? Can anybody tell me if something inside the alternator needs to be replaced or can I be back on the road just by replacing the voltage regulator?
I am attaching a copy of the VR that i took out. It may help you see how worn out the brushes are .
Any help will be greatly appreciated.

Attached Thumbnails

 

In my experience, the new VRs come with brushes that are 1/2 inch long. I know that may not sound like a sceintific statistic to use, but it does give one an idea how badly worn down yours are.

With a "cold" engine at idle, what reading do you get with a voltage ohm meter connected to the battery?

Mine reads 14.25-14.30, but I have a 100amp alt.(Bosch AL166X vs. the std. AL66X which is 80 amp).

You can get a new set of German brushes or should I say a new VR from this or any other MB WEB parts site for not a whole lot of $$$. You may have a worn commutator ring that could have accelerated the wear of your not-so-old brushes. See what stevebfl had to say about that in the following thread:

http://www.peachparts.com/shopforum/showthread.php?t=12213&page=1&pp=15&highlight=commutator+ring

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

I would imagine you can just replace the regulator but chances are you may need to replace the alternator. The contact area that the brushes press against is wearing away. These two copper contact rings establish the electrical connection to the windings, chances are the contact rings are deeply grooved and here you are with worn brushes. You may need to double check your battery, you main cable connections et. You need to ensure there is no extra load being placed on the charging system which may be causing the brushes to wear out prematurely.

If you have a high output stereo system fitted to the vehicle my only other thought may be to reduce the total output or install a higher output alternator.

The other possibility is that somehow the alternator is working extra hard due to some other problem...Years ago I had a car with 5 amplifiers (yes 5) and I regularly went thru alternators because they had to be working all the time because the 5 amps drew so much current the alternator was having to charge all the time. If the alternator is turned on all the time (charging) the brushes will wear out much sooner. If there is some other problem with the car such as bad grounds and or poor positive connections the alternator could be switched on more than it should be. If the car has bad grounds the alternator has to work overtime to overcome this poor complete circuit. It is not uncommon to see older cars go thru alternators more so than when the car is new. These were the kindsof things I saw in 25 years of experience in my car audio shop.

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83 300CDTurbo 307,000
2005 Honda Element 266,000
56 Nash Ambassador Country Club Special 34,000(under restoration presently)

http://www.peachparts.com/shopforum/showthread.php?t=125099