So I bought a reproduction tach for my 70 road runner. Not the Tic Toc Tach, but just the standard tach.
I hooked it up and it was was working fine...after replacing the master cylinder a few weeks later the tach was bouncing all over the place.

After checking the line going to the coil...I discovered that I pinched the wire between the master cylinder and the firewall (no idea how I managed that!). After taking care of that and replacing the wire...the tach is still bouncing all over the place.

Can shorting out the lead wire actually fry the tach...? Feel like I just flushed $200 buck down the toilet!