The Stovebolt Site Forums The Shop Area Engine Shop What for, those dual points?

Denny, it sounds like you've got a good basic understanding of ignition principles, so let's get a bit more technical. Coil saturation is a two-edged sword. Leaving the points closed longer allows for more coil saturation, the buildup of a strong magnetic field that's able to induce a higher voltage as the field collapses across the secondary winding. That same strong field also cuts across the primary winding. A primary scope pattern will show a spike of 100 volts or more induced in the primary, which is supposed to carry 6 or 12 volts. That surge is what burns points, as it hits the winding as soon as the points break, even .001" or less. Enter the condenser- - - -which appears to be a complete circuit to the voltage surge. While the condenser is charging, it allows the surge voltage to shunt away from the points. In the fraction of a second the condenser takes to charge up and stop flowing current, the points open sufficiently to avoid arcing over.

Now comes the second phase- - - -getting rid of the stored voltage in the condenser. It's like a base runner trapped between first and second base. Once the condenser charges up fully, the voltage tries to escape by running backward through the coil, the ignition switch, the battery, and charges the grounded side of the condenser. It loses a little voltage on the way. This flywheeling action repeats for several cycles until the voltage dissipates. The descending-amplitude sine wave evident right after the "points open" signal on a scope pattern is visual evidence of the condenser discharge cycle. This must happen while the points are open, or any leftover condenser voltage discharges right across the points when they close again. The condenser must not only absorb the points-open surge; it must discharge while the points are open, or they will burn on the points-close cycle. If metal transfers one direction on a set of points, the capacitance of the condenser is too high. Metal transfer the other way indicates too low a capacitance.

I hope this increases your understanding of the condenser's role in the conventional ignition system. It's not necessary on a solid-state system, because the diodes and transistors take its place.
Jerry