Note, the '85 300D does not have a little black plastic cylinder on the L fender. The circuit is in an aluminum box, similar to the climate control box, located under the dash on the Rt side, you have to remove the glove box to get to it.
I have never seen the schematic diagram, if anyone has a copy of that I would like to see it.
Are you sure the tach sender at the crank isn't bad, like the wire is torn or something? Using a spare module to swap in place of the questionable one is the first thing I would try. I use a module for that. And then swapping the dash gauge (or entire instrument cluster would be the next thing to try (more difficult than swapping the tach module).

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!