Tach not working...

[speedo]

71 targa restoration. My tach bounces when ignition is keyed, but no response to running engine. I replaced the tach with a different tach and still no results. I took the tach out and jumpered the purple wire from the dizzie to the tach and gave it both power and ground from the engine compartment...nothing. Yes I guess there is a chance that both of my tachs are bad...but not likely. I have yet to check the continuity of the purple tach wire in the harness...but if it was not getting a signal when I jumpered it, not sure what continuity or not would tell me.
Anybody have suggestions? Would a 912 tach work on a 911 dizzie signal? I think both of these tachs are 911 tachs...but I have not confirmed their parts numbers.

Thanks


Speedo

[Noah930]

Alternator or regulator?