Erattic Rev counter (Tach) and pertronix ignition. Solution..
Turbo Mini Forums

I dont see any posts on this issue but wanted to post something as I could not find a solution anywhere..

I bought a VDO tacho and hooked it up properly (to the neg side of the coil and a clean supply) but it either nailed the needle to zero or showed really high, erratic rpm values. Searching the internet I found plenty of people out there had the same trouble with electronic ignitions but aside from installing a diode in series with the signal (which only worked for a few) no solutions were out there. I thought that it was probably interference so dragged my mind back 35 years to my basic electronics training and made a simple r/c filter to get rid of the crap.

It worked! I ended up with a 10k resistor in series with the signal then a 0.1 microfarad capacitor from the signal to ground. maybe some different values would work in other situations but that may be a good starting point.

website:- http://www.terryhunt.co.uk