> Maybe a schematic of the existing VW computer output

I think you'll find that the VW tach runs pretty directly off the coil.
I also would like some advice here, too, though.  I have installed a
tach in both of my Transporters, and they don't work right.  The same
tachs installed in my Vanagon GL work fine.  My trouble is that they
read badly at low rpms, working fine at high rpms.  The other bit is
that they are *not* connected to the oil-pressure circuitry that
lives in the speedometer (as the tach on your 85 would not be -- one of
these *is* an '85 tach).  Was there any difference in the Transporters
(as opposed to Vanagon/Caravelles) that would cause this?

malcolm