Starters and Ignition Systems
Ford Expedition XLT

What does an ignition coil do?

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2015-09-15 05:33:25
2015-09-15 05:33:25

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The coil is a compact, electrical transformer that boosts the battery's 12 volts to as high as 20,000 volts. The incoming 12 volts of electricity pass through a primary winding of about 200 turns of copper wire that raises the power to about 250 volts. Inside the distributor, this low-voltage circuit is continuously broken by the opening and closing of the points, each interruption causing a breakdown in the coil's electromagnetic field. Each time the field collapses, a surge of electricity passes to a secondary winding made up of more than a mile of hair-like wire twisted into 25,000 turns. At this point, the current is boosted to the high voltage needed for ignition and is then relayed to the rotor.

AnswerSome HEI distributers are known to create as much as 60,000 volts. Hard to imagine. I once had a 75 impala that put that much voltage out. And that was when HEI's were just getting started. But most of them put out about 30 to 40,000 volts.

Ignition coils are very rugged and reliable, but can fail for a variety of reasons. Heat and vibration can damage the coil's windings and insulation causing shorts or opens in the primary or secondary windings. But the number one killer of ignition coils is voltage overload caused by bad spark plugs or plug wires.


The ignition coil is basically a transformer. It converts a low voltage into a high voltage to create a spark at the spark plug.

The way it works may seem backwards, but basically, a low voltage (about 12 volt) electrical current is sent through a coil of wire (the "primary" winding), which causes an electromagnetic field to build up. When this low-voltage electrical is inturrupted (by the points in a conventional distributor, or by the engine's computer), it causes this electromagnetic field to collapse suddenly. As it collapses, it cuts across the windings of a "secondary winding" which is wound around the primary winding. Since the secondary has many more turns of wire than the primary, the magnetic field causes a much higher voltage (anywhere from 5,00 to 20,000 volts) to flow in it. This is the voltage that is sent out through the ignition leads to the spark plugs.

Why do they have it trigger the high voltage when the current is shut off rather than when it's turned on? Because, back when they first started making ignition systems for engines, they used mechanical "points" to switch the voltage on and off. When the points "made" to allow the current to flow, they had a tendency to bounce, causing the voltage to build up erratically. When they were opened, though, the current was shut off very rapidly, giving the sudden change in current flow necessary to create the higher voltage to be induced. Nowadays, with electronic ignition systems, it could be done either way, but they usually stick with the "old" way just because it's what they're used to.

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