The Magneto Coil – Unraveling the mystery of yet another electronic part

Let’s talk about the Small Engine Magneto Coil.

We’ve talked before about the starter solenoid and how it activates the electric starter motor to get the flywheel turning. We’ve talked about the spark plug and how it makes the gas go boom. So, let’s round this whole thing out by talking about that unsung little hero that allows the spark plug to keep on sparking.

This little number is called the Tale of the  Magneto and it goes a little something like this-


To find the magneto coil, just find your spark plug. Got your hands on it?

Good. Now do you see that wire coming off of the end of your spark plug? The other end of that wire is attached to a little thing that looks something like this:

Ignition coil

Glorious, ain’t it? It’s known by many names: Magneto, Magneto Coil, Armature Magneto, Ignition coil, Ignition module. Any way you slice it, there are essentially four parts to it: the spark plug connector, the lead wire, the laminate and the module itself.

Inside the module we have lots of copper wire and some other electronic bits that you (the consumer) will never have to deal with, but what it boils down to is that the ignition coil is essentially a fancy magnet.


Now, when you start your small engine (either with a pull rope or electric starter motor) you are really just turning the flywheel.

flywheel spin

And on the side of most flywheels there are magnets as well…

flywheel magnet

 So when the flywheel get’s to spinning, the magnets pass by the ignition magneto creating an electric charge.

flywheel spark

This electrical charge is generated inside of the coil. It is sent through the module, down the lead wire and into the spark plug.

The spark plug “sparks”, the gas goes “boom” and all is right with the world.

To recap:

Flywheel Magnet + Ignition Coil + Spark Plug = Boom

Clear as mud. But I digress…

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The frustrating thing about electrical components is that sometimes they just go bad. No warning, no obvious reasons, no smoke, no sparks… apparently Mars is in retrograde and all of the sudden “nuttin”! It’s confusing and it makes my head hurt. And unless you’re an electronics whiz or a Warlock, there’s nothing for it but to replace the part.

Luckily, the magneto coil is a fairly simple part to remove and install.

But before we break out the credit card, we should probably find out if the coil is bad or not.

The process for doing this is referred to as “checking the spark” and for this job, we recommend the following:

Eye protection


Spark Tester

dangerStep One: disconnect the ignition coil lead wire from the spark plug. Be careful not to yank the wires or tear the rubber boot covering the connector. Grab onto the boot and work the clip loose. Once it’s loose, attach it to the metal probe on the Spark Tester.

Next, we’ll attach the Spark Tester ground clip to a clean, solid grounding surface. Not sure what that might be? It’s time to go ask your local technician. Bring the mower and your check book with you and be prepared to leave them both.

We’ll need to get the flywheel spinning, so if you have an electric start, let’s crank her up. If not, you’ll need to position the tester so you can see the spark window when you pull the recoil rope.

If the ignition coil is still good, you will see a strong electric spark arcing between the electrode points in the spark chamber.

spark tester window

No spark means bad coil.

Let’s assume that you’ve tested the coil, and there’s no spark. As we suspected, the ignition coil is bad. So it’s time to get a new on on there!


For this job, we recommend the following:

Eye protection

Work Gloves

socket wrench

1/4 or 3/8 socket

Replacement part

A set of air gap gauges or a standard business card

dangerStep One: disconnect the spark plug from the ignition coil lead. Once again, be careful not to pull on the wire to accomplish this.

Next, remove the flywheel cover. This usually involves removing around four bolts. Once the bolts are removed, the flywheel cover should come right off. You should have pretty decent access to the ignition coil now.

Disconnect any ground wires or additional connections to the ignition coil. These will be smaller, thinner wires so be careful.

Most ignition coils are mounted to the engine by two 3/8″ bolts bolts driven through the laminate. Back the bolts out, remove them and the old coil will detach easily.

Installing the new coil is the same process in reverse, with one notable exception… you have to set the gap.


The distance between the ignition coil and the flywheel must be spaced or “gapped” precisely. Now, there are some pretty sweet gapping tools out there, but if you don’t have access to them or just aren’t inclined to invest in an entire set… here’s a little trick of the trade that’ll get you by.

Reinsert the mounting bolts through the laminates and tighten them enough to keep the coil from falling off.

Turn the flywheel  by hand until the magnets are at least two inches from the coil. If the magnets are aligned with the coil, they’ll just grab onto it and pull it close and we don’t want that.

Now, take your standard thickness business card and slide it between the end of the laminate and the flywheel. Re-position one end of the laminate until it connects (but doesn’t pinch) the business card.

ignition gap

Tighten that bolt and remove the card.

Now, repeat the procedure for the other end of the laminate.

Once the gap is set, get your socket and finish tightening up the bolts. Careful! Don’t crush the thing. Just tighten it up enough to keep it in place.

Congratulations! Your ignition coil is now gapped.

Re-connect the ground wire and any additional connectors.

Replace the flywheel cover and mounting bolts.

Re-connect the spark plug.

Now start her up for a test run. She running good?

Congratulations…you’re golden.

So there you have it. Testing and replacing a magneto coil. See how easy that was? Let that be a lesson. You can learn how to do this stuff.

I have faith in you.


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Bill Brown

Bill is the head of content creation for the LawnMowerPros Blog and DIY section. He’s been in the Outdoor Power Equipment Industry for years and he’s still learning new things everyday. You can often find him creating featured articles, DIY guides, videos, graphics and much more.