I don't know about the 300SD, but I have a 1984 and 1985 300D. Both use the tach pickup on the transmission bell-housing. See if yours has that. It is on the L side near the top. It routes thru the EGR Box (behind R kick-panel) on both, I recall. If the OVP relay above fails (common), the EGR Box doesn't pass the tach signal thru. I removed both and jumper'ed across the EGR connector, using a post you can search for.

Both engines still have the pickup on the crankshaft damper, which runs to a connector on the L inner fender. There are no electronics there and it is termed "diagnostic port". Apparently dealers could connect an instrument to read rpm. Earlier cars had an amplifier which plugged into that connector. I would take out that clutter but the cable is hard-wired to a harness, so would have to snip wires to remove it.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans