The tach is driven by a sensor that mounts on the transmission bellhousing. You can trace the wire from near the bottom of the bellhousing up to the instrument cluster.

SteveM.

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'93 190E/D 2.5 Turbodiesel 5-speed (daily driver)
'87 190D 2.5 Turbo rustbucket - parts car
'84 Dodge Rampage diesel - Land Speed Record Holder
'13 Ram 2500 Diesel
'05 Toyota 4Runner