When ours in 300E failed out in the boondocks I called 800.for.merc

The procedure was to keep trying, i.e. put a lot of muscle into trying to turn the key. It worked. Clearly you don't want to have to do this very many times

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Kent Christensen
Albuquerque
'07 GL320CDI, '10 CL550. '01 Porsche Boxster
Two BMW motorcycles