Slant 6 coil

FIRST thing we MUST know is what type of ignition, AKA is this an MSD, or other capacitive discharge, or a "switched" system like Mopar breakerless, HEI, or breaker points

Desired voltage does not often happen because of wiring voltage drop WHICH ALSO causes over charging

Power to the "key" side of the ballast "should" be exactly same as battery, and when charging/ running, should be nominal 14V

COIL + voltage VARIES because of the ballast resistor, and will vary "running" about 9-12 with the key side of ballast at about 14

Key on/ not running, coil+ will vary about 6-10V with key side of ballast at 12--12.6V

Coil - is NOT measurable with most meters because it is a pulse. The neg side of the coil actually produces some fairly high voltage--over 100V pulses, and is enough to give you a very good tingle.

I have no idea why a tach would not work IF it is not a cap discharge type ignition. On a CDI, the coil - is actually GROUND