Just for info, MSD is a completely different mule of a different color. There is NO DC (battery) current through the coil with a CD/ MSD. The MSD generates a high voltage pulse sort of like a photoflash, and the trigger device (points or distributor) causes the box to discharge across the coil. This, in part, is why you cannot hook a tach to an MSD driven coil. Most CD setups/ MSD, also, the coil NEG is grounded. What some of us call "switching" type systems, like Mopar ECU, Pertronix, GM HEI and others, work more like points.........they swtich the coil NEG on/ off with battery supplied to the coil +