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!

I don't know. When that happens in my Camaro, it's time to change the spark plugs. After that it works fine again.

I'm not talking slightly off readings...it's bouncing from 1000rpm up to 4000-5000rpm..

I know the wiring is good as I can hook up another after market tach that I have it works/reads fine. The problem points to the tach...but I just can't believe shorting out the lead wire would fry it like that...

If you are disconnecting the wiring right at the tach & hooking up the new tach up right at that point & it's fine then that unfortunately pretty much confirms that it is downstream from the connections which only leaves the tach. Not sure why the green sense wire getting pinched fried it but it sounds like it has to come out. tho I would hook it up again outside the dash just to confirm before I spent the time/money to send it in

Thanks for the replies all. - Brian