If your engine is the OM617, I believer the tach (magnetic) pickup is on the harmonic balancer. It's resistance is 80 ohms per post #45 of this thread when I was working on it.

http://www.peachparts.com/shopforum/showthread.php?t=189424&highlight=tach+amp+repair&page=2

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked