This thing has a different starting device. The key has no mechanical "teeth" in it to operate a tumbler.

The identity of the key is established electronically from the socket in the dash - the key is either the right one or it is not and if it is the right key the system responds as you would want it to and the car starts. The same if it is the wrong key - the system does not allow the process to continue and as you would want, the car does not start if the wrong key is inserted in the socket.

I would suspect the business end of the key has been either scratched or has a layer of dirt on it that is interfering in the "electronic handshaking" steps. This has happened to me before and a cleaning of the socket and the key has resolved the problem for me. Without a successful handshake between the key and the socket, there is no success - the computer figures the key is not for the car and won't accept it.

Not that there can't be other problems...

Good luck,

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)