Not to hijack this, but is there a way to bench test a tach to see if it is functional? I installed an MSD and the tach has never worked since. I tried a tach adapter last night with no luck and now I just wonder if I fried the tach during my install.

"but is there a way to bench test a tach to see if it is functional?" - project 911 -

Tachs are triggered either by a negative or positive pulse from the ignition unit.
As an example, the early Porsche tachs (not SC with a 6 pin CDI) are triggered
by a positive pulse. The SC type tach is triggered by a negative pulse.

So, the beat approach is to use a pulse generator after the tach is powered with
+12 & grd. When a generator is not available, a momentary switch connected
to either +12V or ground & the tach input can simulate a pulse generator if used
rapidly. A coarse file connected to + 12V or ground & a lead connected to the
tach input & rubbed against the file will simulate pulses and result in needle
movement.

Note: Some older tachs require a high voltage pulse from the ignition coil
and can't be tested as described above.

Thanks Loren,

I think I'll try that before pulling any more hair out.

Nope. No affiliation whatsoever...

-Wayne