How To Check Ignition Coil On Lawn Mower

Learning how to check ignition coil on lawn mower is a key skill for any DIY repair. A faulty ignition coil can prevent a spark, and testing it on your lawn mower requires just a few tools. This guide will walk you through the entire process, from safety to diagnosis, with clear steps.

You can save time and money by confirming if the coil is the problem before buying a new part. We’ll cover the tools you need, how to locate the coil, and two reliable testing methods.

Before you start testing, it’s crucial to understand what an ignition coil does. Often called a magneto, it’s a transformer that converts the low voltage from the battery (or magnets in the flywheel) into a high-voltage spark. This spark jumps the gap on the spark plug to ignite the fuel. When it fails, you get no spark, and your mower won’t start or will run poorly.

Safety First: Preparing To Work On Your Mower

Always prioritize safety when working with small engines. A few simple steps will protect you and prevent damage to the mower.

First, disconnect the spark plug wire. This is the most important step to prevent accidental starting. Gently pull the boot off the spark plug and secure it away. Next, if your mower has a battery, disconnect the negative terminal. This eliminates any chance of electrical shorts.

Allow the engine to cool completely if you’ve just been using it. Work in a well-lit, clean area so you can see small parts and keep track of your tools. Having the right mindset for safety makes the whole job smoother.

Tools And Equipment You Will Need

Gathering your tools beforehand streamlines the process. You likely have most of these already.

A basic set of socket wrenches or a multi-tool
A screwdriver (flathead and/or Phillips)
A spark plug tester or an inline spark tester (the easiest method)
A multimeter (for a more advanced electrical test)
A clean rag and some isopropyl alcohol for cleaning connections
A small container for holding screws and small parts

Locating The Ignition Coil On Your Engine

The ignition coil is always mounted near the flywheel, which is the large, finned wheel under the engine cover. To access it, you usually need to remove a plastic or metal shroud.

Consult your mower’s manual for the exact location. If you don’t have the manual, the process is generally the same for most push and riding mowers. Look for a black or metal box with a thick wire leading to the spark plug and a thinner kill wire attached. It will be secured with one or two bolts.

Common Ignition Coil Placements

On top of the engine, under a rectangular metal or plastic cover.
On the side of the engine block, adjacent to the flywheel.
It is often positioned so its armature (leg) sits very close to the flywheel’s magnets.

Visual Inspection: The First Diagnostic Step

Before any electrical testing, perform a thorough visual inspection. Look for obvious signs of damage that could indicate a problem.

Check the coil body for cracks, carbon tracking (black lines), or burn marks. These are clear signs of failure. Examine the spark plug wire for brittleness, cracks, or cuts. The wire’s insulation must be intact.

Inspect the mounting area for rust or debris. Sometimes, rust buildup on the coil’s mounting legs can affect the magnetic gap. Clean any corrosion you find with a rag and a bit of isopropyl alcohol. A simple visual check can often reveal the issue without further testing.

Method 1: Using A Spark Plug Tester (The Easiest Way)

This is the most straightforward method to check for spark and is highly recommended. A spark plug tester is an inexpensive tool that provides a clear visual indicator.

Ensure the spark plug wire is disconnected from the spark plug.
Connect the spark plug tester between the spark plug wire and the spark plug itself. Alternatively, some testers clip directly to the engine block.
Ground the tester’s clip to a clean, unpainted metal part of the engine.
Attempt to start the engine. Have someone pull the starter cord or turn the key while you observe the tester.
Look for a bright blue spark jumping the gap inside the tester. A strong blue spark means the ignition coil is likely working. A weak orange spark or no spark at all suggests a faulty coil or another ignition system issue.

Method 2: Testing With A Multimeter (Ohms Test)

For a more precise electrical test, you can use a multimeter set to measure resistance (Ohms, Ω). This tests the coil’s internal windings. You will need to find the correct specifications for your coil, usually in the service manual.

First, disconnect all wires from the coil. You may need to remove it from the engine for better access. Identify the two terminals: one for the spark plug wire and one for the kill switch wire.

Set your multimeter to the Ohms (Ω) setting, usually the 20k range.
Place one probe on the metal terminal where the spark plug wire connects (inside the coil).
Place the other probe on the coil’s metal mounting leg or the ground terminal.
Read the resistance on the multimeter display. Compare this reading to your engine’s specification. A typical primary coil reading might be between 2.5 and 5 Ohms. A reading of zero (short) or infinity (open) means the coil is bad.
For a secondary winding test, place one probe in the spark plug wire boot and the other on a ground. This reading will be much higher, often in the thousands of Ohms (kΩ).

If your readings are significantly outside the specified range, the ignition coil needs to be replaced. Remember, a multimeter test checks for internal opens or shorts, but a spark test under load is the ultimate proof.

Checking And Setting The Ignition Coil Air Gap

Even a good coil won’t produce a spark if it’s too far from the flywheel. The air gap is a critical adjustment. It’s the tiny space between the coil’s armature and the flywheel magnets.

An incorrect gap can prevent the magnetic field from inducing enough current. This is a common oversight after reinstalling a coil. The gap is usually specified in your manual, often between 0.008 and 0.012 inches.

Loosen the coil’s mounting bolts just enough so you can move the coil.
Cut a piece of business card or use a feeler gauge of the correct thickness.
Place the card or gauge between the coil leg and the flywheel.
Gently push the coil against the flywheel so it pinches the card, then tighten the mounting bolts evenly.
Carefully pull the card out. The gap is now set correctly.

Interpreting Your Test Results And Next Steps

After testing, you’ll have a clear idea of the coil’s condition. Here’s what to do next based on your findings.

If you got a strong blue spark with a tester, your ignition coil is functioning. The no-start problem lies elsewhere—check the fuel system, carburetor, or the spark plug itself. If the multimeter reading was within spec and the gap is correct, the coil is likely good.

If you got no spark or a weak spark, and the visual check and multimeter test confirm a fault, you need a new ignition coil. When purchasing a replacement, have your engine model number ready to ensure an exact match. Installation is the reverse of removal, and remember to reset the air gap.

Common Mistakes To Avoid When Testing

Not disconnecting the spark plug wire first. This is a major safety risk.
Assuming a new spark plug will fix a bad coil. Always test the coil directly.
Overtightening the coil mounting bolts, which can crack the coil housing.
Forgetting to check and set the proper air gap after installation.
Testing for spark without properly grounding the tester or spark plug.
Ignoring the kill switch wire. A shorted kill wire can mimic a bad coil.

FAQ: Frequently Asked Questions

What Are The Symptoms Of A Bad Ignition Coil On A Lawn Mower?

The most common symptom is the engine not starting at all, with no spark at the plug. Other signs include the engine running very rough, misfiring, losing power under load, or stopping when it gets hot. Intermittent starting problems can also point to a failing coil.

Can You Test A Lawn Mower Ignition Coil With A Screwdriver?

While an old method involves holding a screwdriver near the coil to see if it sparks, this is not recommended. It is unsafe, can damage the coil, and provides an unreliable diagnosis. It’s much better to use an inexpensive spark tester for a safe and clear result.

How Much Does It Cost To Replace A Lawn Mower Ignition Coil?

The cost varies by engine model, but a typical replacement ignition coil for a push mower costs between $15 and $40. For a riding lawn mower engine, they may range from $30 to $70. The repair is straightforward if you do it yourself, saving on labor costs.

Will A Bad Ignition Coil Cause No Spark?

Yes, a failed ignition coil is a primary cause of having no spark. However, other issues like a faulty kill switch, broken flywheel key, or damaged spark plug wire can also prevent a spark. Systematic testing, starting with the coil, will identify the true culprit.

Is It Hard To Change An Ignition Coil On A Lawn Mower?

No, replacing an ignition coil is generally considered one of the easier DIY lawn mower repairs. It typically involves removing a cover, taking out a couple of bolts, swapping the old coil for the new one, and setting the air gap. Basic tools and about 30 minutes are usually all that’s needed.