Who's using a pertronix 2.....

[saltshaker]

My Auto Meter tach isn't reading correctly...is yours? did you need a balist resistor to make it work?
Jon

[Argess]

Not a ballast resistor. Did your unit come with instructions for an adapter for marine sterndrives......two diodes and a couple resistors in parallel? If so, you can build one like that or buy a tach adapter from Summit.

A lot of tachs were designed to work with points. A point sytem will provide the tach with zero volts with the points closed and maybe anywhere from 8 to 14 (depending on wether an igntion coil ballast resistor is used) when open. With the Pertronix unit, you only get 0.9 vdc when the unit is conducting (same as points closed, but not zero volts).

With the diode adapter, you need 1.4 volts for the diodes to conduct. The 0.9 won't do it, so the tach sees "ground" through the resistor part of the adapter circuit.

Edit: Here's the diagram (you can use a single resistor of 12K, or even higher. It's not that critical. Good diodes to use are P/N 1N4007 and may still be available from Radio Shack ...in the US):

http://www.vintageperformance.com/re...s/diodefix.pdf