The EGR computer has the tach signals running through it for some reason (nefarious or practical, depending on who you ask). I've not actually explored if you can bypass the tach signal why keeping the EGR functional...

But anyway, if you look behind the panel in the passenger side footwell next to where the door hinges are you'll find a bunch of stuff that usually will fix your tach problems by fixing it. There's the fuse that can blow and you'll have no tach and there's the EGR computer with the connector that had water or at least some corrosion in it more often than not.

Here's a thread about bypassing it (very easy):

Quick Fix: 1985 w123 Tach & A/C after EGR Failure

-Rog