When your lawn mower engine cranks but won’t start, a faulty ignition coil is a common culprit. Learning how to test ignition coil lawn mower systems is a straightforward diagnostic task you can do yourself. Checking the ignition coil with an ohmmeter helps diagnose an engine that has no spark, saving you time and money on unnecessary parts.
This guide provides clear, step-by-step instructions for testing your mower’s coil using a basic multimeter. We’ll cover the tools you need, safety precautions, and how to interpret the results to determine if your coil is good or needs replacement.
How To Test Ignition Coil Lawn Mower
The ignition coil, sometimes called a magneto, is a crucial component in small engines. Its job is to transform the low voltage from the battery (or the magnetic charge from the flywheel) into a high-voltage spark that ignites the fuel-air mixture in the cylinder. When it fails, the spark plug doesn’t fire, and your engine remains silent.
Testing the coil involves measuring its electrical resistance with a multimeter set to the ohms (Ω) function. This resistance check tells you if the internal windings are intact or broken. Before you begin, ensure you have the following tools and items ready.
Tools And Safety Equipment You Will Need
Gathering the right tools before you start makes the process smoother and safer. You don’t need professional mechanic’s equipment for this job.
- A multimeter (digital or analog) capable of measuring resistance (ohms).
- A basic socket or wrench set to remove the spark plug and possibly the coil mounting bolt.
- A spark plug wrench or socket.
- A clean rag or shop towel.
- Safety glasses to protect your eyes from debris.
- Work gloves to protect your hands.
Always disconnect the spark plug wire and secure it away from the plug before any work. This prevents the engine from accidentally starting. If your mower has a battery, disconnecting the negative terminal is also a wise precaution.
Preparing Your Lawn Mower For Testing
Proper preparation ensures an accurate test and keeps you safe. Follow these steps to get your mower ready.
- Park the mower on a flat, level surface and allow the engine to cool completely if it was recently running.
- Disconnect the spark plug wire by firmly grasping the boot (not the wire) and twisting it gently as you pull it off. This is the most critical safety step.
- Remove the spark plug using your spark plug socket and wrench. This relieves compression, making it easier to pull the starter cord later if needed for a different test.
- Locate the ignition coil. It is typically mounted near the flywheel, which is often under a plastic or metal shroud on the side of the engine. You may need to remove a few screws or a cover to access it.
- Identify the coil’s terminals. You will see a primary terminal where the kill wire (usually a single wire) connects and the secondary terminal where the thick spark plug wire connects internally. The coil’s metal armature must also be clean where it faces the flywheel.
Once the coil is accessible and the spark plug wire is disconnected, you can proceed with the actual resistance tests. Make sure the coil’s mounting area is clean and free of grass clippings or oil.
Step-By-Step Resistance Test With A Multimeter
This is the core diagnostic procedure. You will be taking two primary resistance measurements: one for the primary windings and one for the secondary windings. Consult your mower’s service manual for exact specifications, as values can vary, but general ranges are provided here.
Setting Up Your Multimeter
First, configure your multimeter correctly. Turn the dial to the lowest setting for resistance, usually marked “200Ω” or a similar low range. If your meter is not auto-ranging, you may need to adjust this later. Touch the two meter probes together; the display should read near 0 ohms, indicating a good connection.
Testing The Primary Windings
The primary windings are the first set of coiled wire inside the unit. To test them, you’ll measure between the coil’s ground and the terminal where the kill wire attaches.
- Touch the black (negative) multimeter probe to the metal body or mounting leg of the ignition coil. This is the ground.
- Touch the red (positive) probe to the small terminal where the kill wire was connected. You may need to disconnect this thin wire for a clean contact.
- Read the measurement on the multimeter display. A typical reading for a good primary winding is usually between 0.1 and 2.0 ohms. A reading of infinite resistance (often displayed as “OL” or “1”) means the primary winding is open and the coil is faulty.
Testing The Secondary Windings
The secondary windings create the high-voltage spark. This test measures the resistance between the spark plug wire terminal and ground.
- Keep the black probe on the coil’s metal ground.
- Insert the red probe firmly into the end of the spark plug boot where the spark plug would normally connect. If you can’t get a good connection, you may carefully insert the probe into the boot to touch the metal terminal inside.
- Read the measurement. A typical reading for a good secondary winding is much higher, often in the range of 2,500 to 5,000 ohms (2.5k to 5k ohms). An infinite “OL” reading here also indicates a broken coil.
If either test shows an infinite reading, the coil has failed and needs to be replaced. Compare your readings to your engine manufacturer’s specifications for the most accurate assesment. Sometimes a coil can pass a static test but fail under load, which leads us to a secondary check.
Performing A Spark Gap Test
A spark gap test is a functional test that checks if the coil can actually produce a visible spark under a simulated load. This is a good verification if your resistance tests were inconclusive.
- Reconnect the spark plug wire to the coil if you detached it.
- Obtain a known-good spark plug or a dedicated spark tester from an auto parts store.
- Connect the spark plug wire to the tester or to the spare spark plug.
- Ground the metal body of the tester or the spare spark plug against a clean, unpainted metal part of the engine cylinder head. You must see a solid metal-to-metal contact.
- With the ignition switch in the “on” position, quickly pull the starter cord or engage the electric starter (with the spark plug removed from the engine, as prepared earlier).
Observe the gap on the spark tester or between the electrodes of the spare plug. You should see a bright, blue spark jumping the gap consistently. A weak, orange spark or no spark at all confirms the ignition coil or another part of the ignition system is not working properly. Remember, a spark outside the engine does not guarantee proper compression-timing, but it strongly indicates the coil is functioning.
Interpreting Test Results And Next Steps
Understanding what your tests mean is key to solving the problem. Here is a simple breakdown of possible outcomes.
- Both Resistance Tests Are Within Spec and You Have a Strong Blue Spark: Your ignition coil is likely fine. The no-start issue may be caused by a faulty spark plug, fuel problems, or compression issues.
- One or Both Resistance Tests Show Infinite (OL) Reading: The ignition coil is faulty and must be replaced. Internal windings are broken.
- Resistance Tests Are Okay But You Have No Spark or a Weak Spark: The coil may be failing under load, or there could be a problem with the kill switch wire (which may need to be disconnected temporarily during testing), a faulty engine ground, or a damaged flywheel key shearing and altering ignition timing.
If you need to replace the coil, ensure you purchase the correct part for your specific engine model number. Installation is typically the reverse of removal, but pay close attention to the air gap between the coil armature and the flywheel magnets; a business card’s thickness is often used as a feeler gauge to set this critical distance.
Common Mistakes To Avoid
Even simple tests can go wrong if you overlook a few details. Avoid these common errors for a reliable diagnosis.
- Not disconnecting the spark plug wire first. This is a major safety hazard.
- Testing a dirty or corroded coil. Clean contact points before testing.
- Having poor probe contact during resistance tests, leading to false readings.
- Forgetting to check the simple things first, like a fouled or damaged spark plug, which is easier and cheaper to replace.
- Ignoring the kill switch circuit. A shorted kill wire can prevent spark even with a good coil.
FAQ About Lawn Mower Ignition Coils
What Are The Symptoms Of A Bad Ignition Coil On A Lawn Mower?
The most common symptom is an engine that cranks but does not start. You may also experience intermittent operation, the engine dying under load, or backfiring. A visual inspection won’t usually reveal anything, so testing is required.
Can You Test A Lawn Mower Coil Without A Multimeter?
You can perform the spark gap test described above without a multimeter. However, the resistance test with a multimeter provides a more definitive, numerical diagnosis of the coil’s internal condition and is the recommended method.
What Causes An Ignition Coil To Fail On A Small Engine?
Common causes include excessive heat from a clogged cooling fin, vibration loosening the mount, general age and wear, moisture corrosion, and voltage overload from a failing charging system (on battery-equipped engines). Physical damage from impact can also ruin a coil.
How Much Does It Cost To Replace A Lawn Mower Ignition Coil?
Ignition coil prices vary by engine model but typically range from $20 to $60 for the part. If you have it replaced by a small engine repair shop, expect to pay for labor, which could bring the total to between $75 and $150 depending on the accessibility of the coil.
Is It Worth Repairing A Lawn Mower Ignition Coil?
Ignition coils are not repairable; they are sealed units. If testing confirms it is faulty, replacement is the only option. Given the relativly low cost of the part and the straightforward installation on most models, it is usually a worthwhile repair that extends the life of your mower.