I hope you got it as I have seen tach's cause problems but usually what I see is a tach shorts to ground which would drop your volts and keep the car from running. You have to remember that when the points are open you will see 12 volts so the voltage you read at the coil with the eng running is an average of the points open and closed. If you turn the ign key on with the eng off and the points closed and then read the voltages in the ign system you should get 12 volts at the ign side of the ballast. About 6 or 7 volts on the coil side of the ballast and the coil + terminal. It should 0 volts on the coil negative terminal and at the points. Now tap the eng over until the points are open and it will read 12 volts at both sides of the ballast and 12 volts at the coil pos and neg terminals and 12 volts at the points pos side. Thats because it has no ground with the points open so when the eng is running and you read the volts at the coil terminals its reading and average at the pos side of the 6 or 7 with the points closed to the 13 to 14 with the points open and eng running. When you get 13 on the coil neg side it tells me either its loosing its ground or maybe the tach is shorting voltage to the coil neg side and putting 12 volts to the coil neg terminal. Lets hope that was it and you have it fixed. Ron