I see this on EVERY car I get - even ones that have been "dealer maintained!" It just amazes me that this does not get inspected and changed. When it fails it will leave you stranded with a dead battery. I like to call this "run insurance". The only time in my life when I have been stranded on the freeway with a dead car was when the voltage regulator went out in my 1986 230E. The brushes wore down to a point where they were making enough contact to not have the dash warning light come on, but not good enough to keep the battery charged. When the voltage got low enough in the battery the ignition system shut down and the car quit dead going 70 mph.
Problem & Solution
Common among these chassis:
A badly worn regulator can also give you fits when trying to troubleshoot a charging or dead battery problem. One or both of the brushes can wear down to the point where they are only making slight contact on the armature of the alternator. That means sometimes it will charge - other times it will cut out. You may see a flickering charge light on your dash if this is happening, but sometimes the light won't come on unless you have total failure.
First indication of a problem is your wipers start to slow down or the lights start to dim. And the engine won't start the next time you try. As these old Mercedes approach 20+ years the brushes are wearing out and the alternators are failing. Many owners are having their expensive alternators replaced without knowing that it might just be the inexpensive voltage regulator mounted on the back on the alternator. I am now taking a preventative approach with my customers and just replacing all the old voltage regulators. This small regulator is easy to install by jacking up your Benz and going underneath to the back of the alternator. I especially like the quality German Monark brand regulator because it has extra long brushes. If you have not replaced this unit in the last 5 years I would highly recommend you do....... One of the best run insurance measures you can take! Replacements are available for 70's, 80's and early 90's models.
Removal is a simple matter of removing two screws. Look what I found today when I removed the regulator on my newly acquired 300SDL. Notice how short the brushes are. It would have not taken long for this alternator to stop charging altogether.
If you look closely you will see the deep scoring in the commutator rings. It is deeper than I like to see but I am going to install a new voltage regulator and see if I can get another 25,000 miles out of the alternator.