The guys are all correct as the issue can be alot of things.
Usually the issue is the alternator output is less than the car requires.
You can remove and replace the alternator for about $200.
Or you can get a alternator regulator and fix it for less than $50 and in 5 minutes.
I am out of the regulators until Friday.
They can wear into a grooved pattern and the impact of driving can make them vibrate out of the normal groove.
Since the regulator has to increase output to jump the gap it does.
You can clean up the slider rings with abrasive paper.
You can get the brushes as a separate part requiring soldering skills or you can get a Regulator brush assembly for $50.
I can supply a regulator brush assembly, the others are always suspect.
Some instruction for changing:
It is a very touchy installation and about 5% of the installation end in the ground link bent and the alternator not functional.
You can remove correct and reinstall the regulator.
The output must be 14.25 or higher on a Volt Ohm Meter not the dash gauge.
Some hints on the regulator change.
Replacing Alternator Regulator
For those who work on their cars and have not had alternator problems, here is a simple action that should save you down-time and significant costs.
Most alternators on German cars fail due to brushes wearing down (as they are supposed to) between 150k and 200K miles. Brush replacement is very easy -- it should not take over 20 min. and the brushes are attached to a new regulator.
To replace regulator assembly -- back of alternator held in place with two straight slot headed screws. This has a round transistor mounted on the outside on Bosch alternators -- the other type is similar but uses different brushes. Pop the regulator out, after removing the screws.
Fitting the regulator back is simple, but you must install it starting at an angle, to assure that the brushes compress when the regulator is flush with the alternator and the screws are tightened.
You are now ready for 200K miles of alternator life.