If your lawn mower won't start, the problem may be that the spark plug isn't firing. Sometimes, all you have to do is replace the plug, but if you've done that, and you've taken care of fuel and air circulation issues, it's a good idea to check the ignition coil for continuity. The coil is the engine component that creates the voltage to fire the plug, and if it's damaged and can't do its job, you may have to replace it. One way to determine whether or not it's working is to test its continuity with an ohmmeter or a multimeter in ohm mode.
Function of the Coil
The ignition coil is mounted next to the flywheel, which is the part of the engine that spins when you pull the starting rope. Permanent magnets mounted on the flywheel pass close to the coil as the flywheel spins, and as they do, they generate a voltage by magnetic induction. A wire connecting the coil to the spark plug conducts a current to the plug terminals, which results in the spark that ignites fuel in the combustion chamber.
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The ignition coil consists of a single wire wound hundreds of times around a core, and any break that occurs in the wire renders the coil inoperable. It's impractical to uncover the coil and examine the wire; fortunately, there's an easier way to find a break -- a continuity test. In this test, you pass a small electric current through the wire and measure resistance with an ohmmeter. If the coil is intact, the resistance is a measurable number in the order of 2,500 to 5,000 ohms. If the coil is broken, the resistance will be infinite.
Finding the Coil
To conduct the test, expose the coil housing, which is usually concealed by the engine cover. The amount of dismantling required to expose the coil housing depends on the lawn mower model, but it's usually little more than removing four or five screws. If you don't know what the coil looks like, just follow the spark plug wire from the spark plug; the coil is inside the metal box to which the spark plug wire is attached.
Setting Up the Ohmmeter
Set the ohmmeter to measure resistance in the 20 K ohm range; "K" stands for kilo, which means thousand. If you have a multimeter, turn its dial to the 20 K ohm scale, and insert its red lead in the common jack and its black lead in the jack marked with the ohm symbol.
Test the meter before checking continuity by touching the leads together and verifying that you get a reading close to 0. When the leads are not touching each other, the meter should read "1" or infinity.
Expose the lawn mower's ignition coil, and disconnect all wires attached to it except the spark plug wire (which you can't disconnect anyway). Pull the other end of the spark plug wire off the spark plug, and secure it away from the spark plug.
Insert the end of the meter's red lead inside the spark plug boot, and touch it to the metal plate in the deepest part of the boot. Scratch the lead on the plate to ensure it's in contact.
Touch the meter's black lead to the metal coil housing and watch the meter. If the meter's display indicates a value of 2.5 to 5 K ohms, then the coil is good. If, on the other hand, the meter display shows a "1" or infinity sign, or the display needle jumps to the far right of the scale, then the coil is damaged and must be replaced.