Turbodiesel,

I can vouch for the fact that when the ring gear looses teeth, it protests loudly. I doubt if you have never heard a horrible, loud screeching, metallic grinding noise a bunch of times that your ring gear has just quietly given up a few teeth. If this happens it is usually a combination of the ring gear teeth being fatigued by the initial starting torque cycles (which occur on the same teeth preferentially) and the pinion gear beginning to spin too early in the solenoid stroke so it tries to engage the ring gear when it is already moving at full speed. The latter is the source of the horrible noise.

I think there is something seriously wrong with the starter installation or assembly. It is not supposed to be set to allow the starter motor contacts to be activated until the pinion gear has engaged the ring gear. Otherwise the pinion gear or the ring gear would be made as a sacrificial item, and the horrible grinding noise as the spinning pinion slammed into the ring gear would be normal. So if your starter is running and the gears are not grinding or turning the motor over, there is something amiss with either the installation or the assembly. I would remove the starter and see if it is misaligned (maybe the mounting fasteners have been lost or damaged, allowing the unit to become misaligned?) relative to the ring gear, or the assembly has been compromised somehow and the relative location of the solenoid parts has shifted so the "full" extension does not actually reach the ring gear with the pinion. For this to happen all of the sudden with no noisy indications of impending failure is a little strange.

So, I line up with those suggesting removing the starter is your best course of action. If it is somehow misaligned and you never ground any gear teeth as it went from aligned to misaligned, count yourself lucky. If you have what appears to be a bit of luck and the unit starts working again, but only by the smallest margin of tooth engagement, it could badly overload the gear teeth leading to future bigger issues. I had an even like this with our 240D shortly after we bought the car. The starter died, I removed it and had it rebuilt, reinstalled it and it worked for a while. Unfortunately it was never properly assembled and, unfortunately, the pinion never engaged the ring gear correctly. This led to the dreaded screeching noise and ultimately to a ring gear replacement as well as a Bosch rebuilt starter. All of which was a lot more trouble than just buying a Bosch rebuild to start with, even if you don't consider the problems occured in the Winter, I have no garage, and had to take the starter out to find the damaged ring gear while lying on my back on a snow covered dirt driveway. Good luck, Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)