The conventional (Kettering) spark ignition system in automotive applications uses a coil which performs the dual functions of energy storage and voltage step up. It is typically about a 100:1 turns ratio. DC power is applied, producing a current of about 5 Amps through the 8 mH primary inductance, storing about 100 mJoule of energy. When the current is interrupted the points opening, (The points are a set of contacts mechanically driven by a multilobed cam synchronized to the engine), the voltage rises (L di/dt) to around 300-400 volts, which is stepped up to around 30-40 kV (open circuit) in the secondary.

The ballast resistor isn't used during cranking (when the battery voltage is usually reduced to around 6-8 volts). A typical DC coil resistance would be around an ohm, and at 12 Volts, the DC current would be 12Amps, dissipating more than a 100 watts.

To test the ignition coil connect the ohmmeter's two test leads to the coils primary terminals (+ and -). Most coils should read between 0.4 and 2 ohms. Zero resistance would indicate a shorted coil while a high resistance reading would indicate an open coil.

In simple terms coils are rated by their primary resistance—less resistance in the primary windings of the coil allows more current to flow, which makes a stronger magnetic field. //// AND A HOTTER COIL /////

Primary resistance is measured with the probes on the positive and the negative terminals of the coil. MSD’s coils would be between .03 - .7 ohms