Give your K40 relay a quick smack or wiggle with the ignition on and see if you hear anything, it WILL prevent the car from starting and is incredibly common failure on W210's of any engine.

Passenger side engine bay (US) in the big relay box near the windshield, its the one with a bunch of 10amp red fuses onit.

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Allen Kroliczek
Oak Grove Autosport | Oak Grove Autosport
01 G500, 82 300TD, quite a few more.....