How Does a Coil Work on a Mower?

Magnetic Field to Electrical Pulse

• The armature in a coil is a metal U-shaped bar. The two ends of the U-shape, called legs, point directly at the engine’s flywheel with a small gap in between. As the flywheel turns, it creates a magnetic field between the armature and the flywheel. The armature then turns that magnetic field into an electrical pulse.

Primary and Secondary Coils

• The primary coil is thick gauge wire of either copper or aluminum, wrapped around both legs of the armature. The magnetic-to-electric conversion starts in the primary coil. The electric flow in the primary coil is low and flows to the secondary coil, where the electric charge is amplified. The secondary coil is a thin copper or aluminum wrap directly over the primary coil. For every one metal wrap in the primary coil, the secondary coil has 60 to 100 wraps.

Coil to Plug Spark

• The electronic ignition is used in lawn mower engines to send the spark from this coil through the plug wire, where it ignites the gas and air mixture in the combustion chamber, via the spark plug. As the flywheel turns, creating electricity in the coil, an internal thyristor switches the electrical current load through to the plug at the specific time the gasoline mixture reaches the chamber, enabling it to detonate.

Magnets and Flywheels

• The permanent magnets work as the flywheel spins, creating a magnetic field between the coil and the flywheel. On twin-cylinder engine mowers, there can be multiple magnets in a series, alternating north and south poles, which are used to create enough electricity to fire multiple coils. Generally, the magnets are riveted on or molded into the flywheel.

Coil Wear and Replacement

• All of these parts are internal and not visible when the mower engine is in use. Mower coils seldom require replacement. However, occasionally the aluminum or copper wires in the coil can deteriorate due to age, engine overheating or climate conditions. When the coils are incapable of carrying a current, the spark plug will not be able to spark and the mower engine will not run. Homeowners with a moderate amount of experience with small engine repair can often disassemble the engine to replace the coil on their lawnmower, while others may prefer to have a mower technician replace the coil.