How do you check ignition coil operation?

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An ignition coil is simply a high voltage transformer. It takes low voltage (12 volts)and turns it into high voltage (aprox 30,000-60-000 volts). The low voltage input is called the primary side, and the high voltage output is called the secondary side. Most cars have a constant 12 volts positive going to one of the primary inputs, and the ignition system provides a switched ground to the other input on the primary, that pulses on and off as the engine is turning. This on/off ground pulse provides signal for a spark that comes out of the secondary side. You can use a 12 volt test light on the primary side. First, turn the ignition key to the "on" position. Check for 12 volts of power to the input wires at the coil. These will be the small diameter wires, not the spark plug wire. You should see you test light light up on both wires. The reason is that you are seeing 12 volts input on one wire, and you are seeing that same 12 volts coming through the coil windings, and out the other side. Now, have someone crank the car over. You should now see, with the engine being cranked, 12 volts steady power on one wire, and a repeating on/off ground signal ("flicker") on the other input wire. If you are missing either the power, or the switched ground, then you need to see what is wrong in the ignition system. The coil is likely ok. But if you have power to one wire, and a switched on/off ground signal to the other wire, with the engine being cranked, and the coil is not emitting a spark from the output side, then you have a bad coil.
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