My take on your 'scope picture.

You're correct about the term 'ringing' and what you have shown there is a classic inductor/capacitor current discharge waveform. While there is no actual capacitor on the secondary windings, there is 'stray' capacitance so that's normal.

The number of up/down excursions will give you an idea of how long it takes to dissipate all of the stored energy from the coil; a few 'rings' means lower resistance load while a lot of rings means higher resistance.

The second set of waves could be from "point bounce" if the points manage to drop back closed briefly because of a worn rubbing block on the points, distortion in the distributor cam lobe, or possible both combined with a too-small points gap. It could also be caused by arcing across the points due to a weak condenser or a small flashover in the secondary either internal or exteral.


Now I'd like to go over what I remember about 'Kettering' spark Ignition. Points, coil, distributor, spark plug(s). Origin of the the vintage engine's 'tune-up' of "Points, Plugs, Condenser (another name for a capacitor) and set the timing" which is now a thing of the past thanks to electronic ignitions.

The coil is actually a complete 'COIL' assembly and consists of primary and secondary insulated wire windings around an iron core. A basic electromagnet. The primary windings are the low-voltage side, and the secondary windings are the high-voltage side.

Apply 12v across the primary windings (sometimes including a 'ballast' resistor in series) and current flows through them. Energy from the voltage and current is stored in a magnetic field in the iron 'core.'

The higher the current and the more 'turns' of wire in the primary coil the higher the magnetic field strength. The proper current and number of turns has been worked out already by the engineers.

The points stop the this primary current flow when the contacts open. Energy cannot be stored in a magnetic field, except at superconducting temperatures, and it doesn't get that cold even in Canada.

The field collapses and the stored energy, looking for a way out builds voltage rapidly; that energy wants to go somewhere and will try anything to do so!

The secondary windings have a lot more turns than the primary, so the voltage builds to very high levels, 25,000 volts or better!

A spark gap in a spark plug looks like a dandy place for this energy to go. The high voltage causes a spark to leap the gap, and the energy heat things up (like a 14:1 air/fuel mixture) quite nicely!

The collapsing field's energy also builds up voltage in the primary windings. While we want the energy to jump the spark plug's gap, we DON'T want sparks to jump the gap in the points, because sparks cause metal erosion and produce carbon deposits. So, we put a capacitor (condenser) across the contacts. The energy from the field collapse in the primary winding is absorbed by and stored in the capacitor rather than arcing across the points.

Now all we have to to is steer this voltage with a 'distributor' to select which cyclinder to 'fire.' Non-electronic multi-cylinder distributors combine the points, capacitor and high-voltage steering functions, with one 'coil' to make the high voltage spark.

The resistance of both the primary and secondary windings can be measured with an ohmeter, and it's an easy test to identify a coil as a cause of "no spark." If there is no resistance there is no continuity in the wires, so current can't flow and the coil won't make spark. The proper resistance values are published in the technical specifications for each engine's ignition system.

Hope this helps anybody, but it was a fun exercise for me!

BCingU, Jim