janko,

I cannot answer your question directly, but in my experience (pretty limited compared to a qualified MB mechanic) the tumbler section is the part that sees the most wear. It is pretty normal for the lock, when engaged, to put some load on the tumbler section so it is hard to turn. You have to turn the wheel slightly to unload the tumbler so the key turns easier. If that does not help, it may be the steering lock itself, unless the tumber is really worn. I have had the entire mechanism fail so that I could get the steering wheel unlocked but the starter would not engage. I replaced the assembly as a whole, figuring I did not want to get involved with the job again the next week to fix the next piece. In the end the the only part that really seemed worn out was the tumbler. Hope this helps, Jim

Edit: The car I replaced the mechanism on was my 190E, not a
Diesel, but I think the basic hardware is pretty similar.

__________________
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)