HOW DOES AN IGNITION COIL WORK?

How does an ignition coil work?

It really boils down to magnetism. If you move a coil of wire through a magnetic field, it will produce electricity. The more turns in a coil, the more voltage (pressure) the electricity will have.

This also works in reverse - if you run current through a coil of wire, it will make a magnetic field. So it's possible to make electricity from magnetism, and to make magnetism from electricity.

Note that I said MOVE a coil. If it just sits there, no electricity is generated. We have to have some way of making something move.

Inside the ignition coil there are really two coils of wire. One has a couple of hundred turns (windings) in it and is connected to the 12 volt electrical system. The second one has thousands of turns and is connected to the spark plugs. These two coils are wound one right on top of the other so that they are very close. This way we can use most of the magnetism and not lose very much.

When you first connect the 12 volts to the coil, it takes a tiny bit of time for the magnetic field to build up. Then it just sits there. No useable current is produced in the second coil. But if you UNHOOK the coil from the battery, the magnetic field in that coil will appear to move as it dies off - this is called 'collapsing'. The part of the coil that is connected to the spark plugs isn't smart, it sees the magnetic field collapsing and 'thinks' that you are moving the field. So it generates electricity, several thousand volts of it, and you get a nice spark at the spark plugs.

Hey, pretty neat. How does the engine use this?

Well, in the old old days before they knew a lot about electricity they put a device similar to a buzzer connected to the coil. As it buzzed it would connect/disconnect the coil and make a steady stream of sparks. A big rotary switch was connected to the camshaft to distribute the current to the right spark plug at the right time (you got it - a distributor!). The 'buzzer' was called a tremblor, and you will sometimes see this in old text or translations.

This approach was fine for low speed engines (like the Model T) but it didn't control WHEN the spark jumped very well. As engine speeds started to increase a better system was needed.

Someone got the idea (actually an engineer named Kettering) to put a switch on the camshaft to connect/disconnect the coil at the right times. The switch would connect the coil most of the time, and at the right time it would disconnect it, making a single spark. Since it should make the spark at the same time that the distributor was sending current to the spark plug, it was decided to put the switch inside the distributor. This would make it easy to set the timing. The switch was a very simple one, with 2 big 'points' of contact, so it was called the points.

When you disconnect the coil, the collapsing magnetic field makes a 'kickback' voltage in the 12 volt coil too (remember, they are wound together). This kickback caused the points to burn up in a short period of time. The solution was to put a condensor across the points that was the right size to absorb this kickback and protect the points.

As time passed someone realized that on a 2 cylinder engine you didn't really need all of that equipment once you saw that firing the spark plug at top dead center everytime didn't hurt anything. You could get rid of the distributor and just have the points if you hooked up both spark plugs to fire at the same time. Since one of the 2 cylinders isn't at a position to fire the spark is wasted, so this is called a 'wasted spark' system.

Eventually the points were replaced by an electronic module. It performs exactly the same function except that it can be designed for a heavier load than the points, allowing the use of a coil that is 'hotter', making a bigger spark.

So, what goes wrong with this system?

Luckily, not a lot. Since it's simple there are only a few places where it can fail.

1. The system must get power when the ignition is on. This can be tested at the plus side of the coil.

2. The points or electronic module must connect and break power to the coil at the right time. You can put a test light across the plus and minus 12 volt terminals of the coil to test this - spin the engine and see if the light blinks on and off.

3. The windings in the coil must not be shorted out or broken open - you can put a meter across both the windings, but the easiest way to test it is to pull one spark plug and lay it on the head, with all spark plug wires connected. Unhook the ground (-) wire from the coil, but leave the + 12 volt wire connected to the other side. Turn the ignition on, and touch a grounded wire to the ground terminal on the coil. When you unhook the wire you should see a good spark in the plug. If not you have a bad coil. If you see a spark jump across the safety tab you have a bad spark plug wire or faulty spark plug wire connection.

Electricity is basically lazy. It likes to take the easiest path to ground. Normally the high voltage in the coil goes to the spark plugs, but if another easier path exists the electricity will go that way instead. Water on a connection can provide a shortcut. An exposed wire can do this too. And if the spark plugs are unhooked from the circuit the spark has no easy place to go. The safety tabs are provided to attempt to give it an easy path, but if it can find an easier way inside the coil (like jumping to the low voltage coil wires or to the frame of the bike) it will do this in a heartbeat and can burn a permanent shortcut which will ruin the coil, and it can even go back through the ignition module. So always have the spark plugs connected to protect your system.

The rest of the failures can be isolated fairly easily (swapping out the ignition module, checking for loose physical stuff like the rotor, etc.)

This Page was last updated: Monday, April 21, 2008 at 1:07:34 AM
This page was originally posted: 7/23/04; 11:55:43 AM.
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