Ignition Coil

What they do
When the current is shut off (by the ignition module or the opening of the contact points in older ignition systems), the magnetic field causes a surge of high voltage (as much as 40,000 volts) in the coil's secondary windings. The high voltage passes to the distributor, then on to the spark plugs where it jumps the plug gap and fires the plugs.

Coil problems
Include shorts or opens in the internal wiring, and cracks around the high voltage terminal.