I don't know how the circuit works on that particular series, but in
my 1980 300SD, they use a pickup from the engine that in turn goes
through a small conversion circuit that converts it into a signal that
then drives the tach.   That circuit is located inside the cap of what
I think they call the "diagnostic connector", which is located on the
drivers side, under the hood, mounted near the fender, forward of the
master brake cylinder, near the fuse for the glow plugs, etc.

It's there so they can hook a shop instrument to it to read the engine
speed easily for service.   These little circuits go bad frequently
and result in either intermittent, erratic tach, or no tach.    The
cap is black, about 1 1/4" in diameter and just screws off by hand.
Unfortunately, the simple little circuit is incapsulated inside, so
you have to replace it.