[Peter Serio]

Be forewarned that if you have a factory tach in your dash & it is working now off of your points distributor; and you change that to something modern, such as pertronix or GM-HEI your tach will no longer function. The original tach for all 1970 GM cars had a one wire plug-in on the back. That circuit board is designed for a points signal (a pure square wave). If you change your points out for something new you will have to replace the circuit board inside of your tachometer so that it has an accommodation for a 2 wire plug-in on the back. The HEI compatible board is more complex in that it has a transistor in the circuit. Therefore it requires a separate power supply (12 volt "+") and another wire that looks for the RPM signal from the coil.

The power wire is a 12 volt + signal (key on) from the fuse block, run thru a 1 or 1 & 1/2 amp in-line glass fuse. The other wire is the same one you have now, it's the wire from the "-" side of the coil to the back of the tach. That wire carries the RPM waveform information from the ignition system to the tach.