Tach Help

Go to the MSD site and start by reading the destructions to the box you are using. Tachs with CD/ MSD do NOT work the same nor are they wired the same as with breaker points/ Mopar electronic, / HEI or other "switched coil NEG" type systems

MSD / CDI works more like a photoflash, electronic flash. An inverter charges up a capacitor and the trigger dumps the cap across the coil. The coil NEG is grounded with CD, not switched