It is the crankshaft/reference sensor that is mounted at the rear of the engine, above the flywheel area. If it is defective, don't waste your time. replace it.

Here are two threads for you to read:

http://www.peachparts.com/shopforum/showthread.php?threadid=38974&highlight=ezl

http://www.peachparts.com/shopforum/showthread.php?threadid=38728&highlight=ezl

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99 C230K Sport
87 300SDL
81 300SD