Ignition coils produce the spark necessary to begin the combustion process. They multiply 12 volts battery voltage to 25k to 40k volts through induction. The ignition coil is a member of both the primary and the secondary ignition circuits.
It’s constructed of two coils of wire wrapped around a soft iron core. The primary winding contains fewer (a couple hundred) windings of thicker insolated wire wrapped around the secondary windings. The secondary coil contains thousands of fine windings located inside the primary windings and wrapped around the iron core.
When turned on, the primary circuit creates a magnetic field as current passes through its windings. This magnetic field passes through the secondary winding and soft iron core. This induces voltage in the secondary windings. When the primary circuit is suddenly opened, voltage passes through and is multiplied to thousands of volts by the secondary’s many fine windings. This high voltage is then passed on to the spark plug through the secondary circuit. The more windings a coil has, the more voltage it will produce.
The primary circuit is switched on and off by the ignition control module. The module uses signals from the crankshaft and camshaft position sensors to locate cylinder #1 during start up. The PCM often contains the ignition module. Once the engine starts it uses these along with other sensors such as the ECT and the TPS to determine dwell (on time) and spark intensity.