knarf,

My experience is that the tumbler section which is what the key bumps and ridges interact with to allow the key to turn, gets worn out. In addition, you have to move the steering wheel slightly one way or the other to get the load off the tumbler section from the wheel or the key cannot be turned either. This feature becomes more and more apparent as the key or tumbler wears.

Turning the key to postion "1" is an essential step in the tumbler, or any other part of the ignition switch, replacement. If you cannot get the key to turn at all the job is much more complicated. If you are interested do a search on this and you will see some pretty detailed instructions on how to replace the tumbler with and without the keyworking.

Hope this helps, Jim

__________________
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)