Was reviewing some of my old High Performance Pontiac
magazines, Oct. 1998, when I read one of the Tech Q&A submissions about the points type factory tachometers that did not read correctly when converting to an HEI distributor. The answer was provided by Paul Spotts.
The answer also provided the answer as to why do aftermarket tachs work with either type ignition, but stock tachs don't - aftermarket tachs have an extra wire, the red "hot" wire.
The factory "points style" tach uses the negative side of the coil to send a signal to the circuit board inside the tach and converts the pulses from the coil into electrical current (amps) that power the tach needle movement.
The HEI uses a circuit board that senses the number of pulses in the ignition BUT
, uses the electrical current from the 12-volt
lead on the Tach plug found on the distributor cap to power the tach needle movement.
The fix to get the older factory points style tachs to work with an HEI is to convert the older tachs with a circuit board that will work on the HEI systems.
The author recommended, and has used, the services of the R&C Tach Shop - who seems to specialize in Pontiac's, but does do other brands. R&C Tach Shop also sells a reproduction factory in-dash tach and hood mounted tach if you don't want to convert your factory tach. Here is the website which can be viewed. https://www.rctach.com/index.html
You can click on the "Services" at the top of the page.
I have never used R&C services, nor know anyone who has, but Paul Spotts is a well known Pontiac engine builder - http://www.spottsperformance.com/
- and I don't think he nor the magazine would use his services if they were not good. :thumbsup: