I have had good luck with this situation by simply ordering a new tumbler (comes with new key) from Mercedes cut to the original code and trying the new key in the old tumbler. Half the time the old tumbler will turn with the new key. REPLACE the tumbler immediately after you get it to turn.

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Cars:
1993 400E
1993 300TE Wagon
1985 300D Turbodiesel w/ 688,000 Miles On Original Eng/Trans
1979 300SD
1977 Euro 6.9
1967 230S IMA Universal Wagon
100 Or So Parts Cars

You Cannot Change your Future Without Disturbing Your Present

It Looks A Whole Lot More Like It Did Today Than It Did Yesterday?