Coils convert low battery voltage to very high voltage to fire the spark plug. A coil contains two sets of internal windings - a primary and secondary, both wrapped around an iron core. The primary winding (input) is momentarily charged by the ignition module, but interestingly enough, high voltage in the secondary winding (output) is NOT generated until current is suddenly halted in the primary. The collapse of the primary magnetic field then causes a high voltage surge in the secondary, and thus a spark to jump in the spark plug.
For general, installation and operating information, see the "Ignition Coil Primer" PDF near the bottom.
Read our "Coil Primer" below: