300 sd tach sending unit?

[jones]

Tach, clock, odomoter do not work on my 83 300 sd. Went to junkyard yesterday and picked up two instrument clusters. Checked both on car. Now have working odometer and clock, but no tach response on any unit. Coincidence? Three bad tach units? Only one wire connection for tach-so must have electronic
sending unit of some sort.

Where is tach sending unit on 300 sd? Are these things particularly quirky, or am I just unlucky enough to have gotten 3 bad clusters?

No real need for tach, so I can live with this. But it would be nice to see that needle moving in a purposeful way.

Help anyone?