I did a little more digging.
The MB part number 0015455932 is on the tach amplifier for both my 78 300SD and my 83 300SD.
But this part was only used until the end of the 83 model year.
Your 85 300SD doesn't have a tach amplifier in the diagnostic port.
The tach signal is routed through an early EGR computer (MB part number 0035455132). See attached post number 1 and thumbnail number 3 by
Jeremy5848.
http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/226815-w123-parts-sale.html
The EGR computer receives its power from the OVP relay. See attached post number 3 also by
Jeremy5848.
http://www.peachparts.com/shopforum/diesel-discussion/280208-tach-signal-wire-fuse.html
As
Jeremy5848 suggests, for a failed or erratic tach in a 85 300SD, the likely culprit is either the OVP fuse, or the OVP relay itself.