Learning how to check a lawn mower ignition coil is a straightforward task that can save you time and money. A failing ignition coil can prevent your mower from starting, but testing it requires only a basic multimeter. This guide will walk you through the entire process, from safety first steps to interpreting your multimeter readings.
You don’t need to be a professional mechanic. With a few simple tools and about 30 minutes, you can confidently diagnose this common problem. Let’s get your mower running again.
How To Check A Lawn Mower Ignition Coil
The ignition coil, sometimes called a magneto, is a crucial component. Its job is to transform the low voltage from the engine’s flywheel magnet into a high-voltage spark at the plug. If it fails, no spark means no start. Testing involves checking for two types of electrical failures: a break in the primary winding and a short to ground.
Before you start, gather your tools. You will need a digital multimeter, a socket or spark plug wrench, a clean rag, and maybe a small brush. Always disconnect the spark plug wire before beginning any work for safety.
Safety Precautions Before You Begin
Your safety is the most important part of this repair. A lawn mower engine can cause serious injury if it starts unexpectedly. Follow these steps every single time.
- Disconnect the spark plug wire. This is non-negotiable. Pull the boot off the spark plug and secure it away from the plug terminal.
- Allow the engine to cool completely if you have just been using the mower. This prevents burns.
- Work in a well-lit, clean area where you can see all the components clearly.
- If you must tip the mower, always ensure the air filter and carburetor side face upward to prevent oil and fuel from flooding the engine.
Tools And Materials You Will Need
You likely have most of these items already. A digital multimeter is the key tool, and they are very affordable if you don’t own one.
- A digital multimeter (set to the Ohms or resistance setting, shown as Ω).
- A set of basic wrenches or sockets to remove any shrouds or the coil itself.
- A spark plug socket or wrench.
- A small brush or cloth for cleaning.
- A piece of sandpaper or emery cloth (fine-grit) for cleaning contact surfaces.
Locating The Ignition Coil On Your Mower
The ignition coil is mounted near the flywheel, which is typically under a plastic or metal shroud on the side of the engine. You may need to remove a few screws or bolts to access it.
Look for a small, black or metallic module with a thick wire leading to the spark plug and a thinner kill wire (often connected to a grounding tab or the mower’s safety switch). It will be positioned so its armature legs are close to the rotating flywheel.
Common Engine Types And Coil Placement
Most walk-behind mowers use a Briggs & Stratton, Kohler, or Honda engine. While designs vary, the coil placement is generally similar.
- On vertical shaft engines, the flywheel and coil are usually at the top, under a cover.
- On some models, the coil is mounted directly to the engine block just beside the flywheel.
- Refer to your mower’s manual for the exact location if you are unsure. A quick online search for your engine model number can also provide a diagram.
Step-By-Step Testing Procedure
Now for the main process. We will perform two primary tests: a resistance test of the coil windings and a test for a short to ground. For accurate results, you may need to fully remove the coil from the engine to isolate it.
Step 1: Disconnecting And Removing The Coil
- Disconnect the spark plug wire from the coil tower by firmly pulling it off.
- Disconnect the thin kill wire. It may be a spade connector or a small screw.
- Remove the one or two bolts that secure the coil to the engine block.
- Carefully lift the coil away. You might need to gently wiggle it if it’s snug.
- Use your brush or cloth to clean any dirt or debris from the coil’s metal legs and the mounting area on the engine. This ensures a good ground path.
Step 2: Setting Up Your Multimeter
A digital multimeter is your best friend here. Turn it on and set the dial to the Ohms (Ω) or resistance setting. If you have multiple ranges, set it to the 20kΩ (20,000 Ohms) range. Touch the two meter probes together. The display should read near 0.0, confirming the meter is working.
Step 3: Performing The Primary Resistance Test
This test checks the internal primary winding of the coil. Locate the two electrical contact points on the coil: the metal tab where the kill wire attached and the inside of the spark plug wire tower.
- Place your multimeter’s black probe on the metal kill wire tab.
- Insert the red probe firmly into the spark plug wire boot socket (where the plug wire connects).
- Read the resistance value on the multimeter display.
A typical reading for a healthy coil is between 2.5 and 5.0 Ohms. A reading of infinite resistance (often shown as “OL” or “1” on the left side) indicates an open circuit—the coil is faulty and needs replacement.
Step 4: Testing For A Short To Ground
This test checks if the coil’s windings are shorting out against the metal frame of the coil itself, which would kill the spark.
- Leave your multimeter on the Ohms setting.
- Place the black probe on one of the coil’s bare metal mounting legs.
- Touch the red probe to the kill wire tab. Note the reading.
- Now, touch the red probe to the inside of the spark plug tower. Note this reading.
In both cases, the meter should show infinite resistance (OL). Any low resistance reading here, especially under 1.0 MΩ (1,000,000 Ohms), indicates the coil is shorted to ground and is faulty.
Interpreting Your Multimeter Readings
Understanding what the numbers mean is the key to your diagnosis. Don’t panic if the numbers aren’t perfect; focus on the extremes.
- Good Coil: Primary resistance is within spec (2.5-5Ω) and both ground tests show infinite resistance (OL).
- Open Coil: Primary resistance test shows OL. This means the internal wire is broken. Replace the coil.
- Shorted Coil: Either ground test shows a low or zero resistance reading. The insulation inside has failed. Replace the coil.
If your coil fails either test, it cannot produce a sufficient spark. Replacement is the only option, as coils cannot be repaired.
What To Do If The Coil Tests Bad
If your tests confirm a faulty ignition coil, you need to install a new one. The process is essentially the reverse of removal, with one critical adjustment.
- Purchase the correct replacement coil. Your engine’s model number is essential for this. Find it on the engine’s valve cover or blower housing.
- Before installing the new coil, you must set the air gap. This is the space between the coil’s legs and the flywheel magnets.
- Use a business card or a feeler gauge set to the specification in your manual (common gaps are 0.010″ or 0.3mm). Loosen the coil mounting bolts just enough so you can move it.
- Slide the business card between the coil leg and the flywheel. Move the coil until it gently pinches the card, then tighten the bolts. Pull the card out.
- Reconnect the kill wire and the spark plug wire. Reinstall any engine shrouds you removed.
Additional Troubleshooting Tips
Sometimes, a weak spark can be caused by issues other than a completely failed coil. Consider these points if your tests were inconclusive.
- Check the spark plug wire for cracks or damage. A damaged boot can allow the spark to arc to ground.
- Inspect the flywheel key. If the mower hit a solid object, the flywheel key can shear, throwing the ignition timing off. This can mimic a weak coil.
- Ensure the kill wire circuit is not grounding out. Disconnect the kill wire from the coil and test for spark again. If spark returns, the problem is in the safety switch or wiring.
- Always test with a known-good spark plug. A plug can be fouled or cracked, preventing a start even with a good coil.
Preventative Maintenance For Ignition Systems
A little care can extend the life of your mower’s ignition components. It’s not complicated and takes just minutes.
- Keep the engine area clean. Regularly blow out grass clippings and debris from around the flywheel and coil.
- Apply a dielectric grease inside the spark plug boot to prevent moisture ingress and corrosion.
- Check that the coil mounting bolts are tight at the start of each mowing season. A loose coil can vibrate and fail.
- Store your mower in a dry place. Prolonged moisture is the enemy of electrical components.
Frequently Asked Questions
Here are answers to some common questions about lawn mower ignition coils.
What Are The Symptoms Of A Bad Ignition Coil On A Lawn Mower?
The most obvious symptom is the mower failing to start. When you pull the cord, you get no ignition at all. Sometimes, a weak coil may cause the engine to run poorly, misfire, or lose power under load. A simple spark test can confirm a lack of spark.
Can You Test A Lawn Mower Coil Without A Multimeter?
You can perform a basic spark test. Remove the spark plug, reconnect it to the wire, and ground the plug’s metal body against the engine block. Pull the starter cord and look for a bright blue spark jumping the plug gap. A weak orange spark or no spark suggests a problem. However, this test is less definitive than a multimeter test and should be done with caution.
How Much Does It Cost To Replace A Lawn Mower Ignition Coil?
Ignition coils are generally inexpensive. A replacement coil for a common engine model typically costs between $15 and $40. If you have a professional small engine shop do the replacement, labor might add another $50 to $75 to the total, depending on your location.
What Causes An Ignition Coil To Fail?
Heat and vibration are the primary causes. Overheating from a dirty engine or running with low oil can damage the coil’s internal insulation. Excessive vibration from a loose mounting or unbalanced flywheel can also lead to premature failure. Physical damage from impact or corrosion from moisture are other common culprits.
Is It Worth Replacing An Ignition Coil On An Old Mower?
In most cases, yes. If the rest of the mower is in good mechanical condition—the deck is solid, the engine has compression, and the transmission works—then a $30 coil is a worthwhile repair. It is often much more cost-effective than buying a new mower. However, if the mower has multiple other issues, it might be time to consider an upgrade.