UPDATE:

Replaced RPM sensor on bellhousing. Easy job, took half an hour including hunting down my tools and cleanup. Sensor is held on by a single allen head, pops right out. I did it from underneath - pulled front of car up on 2x6 boards - I'm skinny and was able to get right under there.

It's fixed! Tach works now. Thanks to all who replied, esp. Charmalu.

__________________
Charlie

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'66 VW 1300 96K miles
'97 E300D 239K miles
'85 300D 203K miles (sold Sep 2012)