I have a 84 Dodge with a 78 318 and 727. I have the orange Mopar ignition box. I connected a Sears analog tach and around 2500 the tach starts shaking and then drops to zero.
I sent that one back and got another one, but it does the same thing. I connected the 2nd one to a Ford with points and it works fine.
I also connected a digital tach and above 2500 it jumped to reading 8800!
I put a factory ignition module back on and the tach drops out at 1500.
Nobody I have talked to can figure this out.
BTW the tach was connected to the Negative side of the coil per the book.