When your lawn mower won’t start, a faulty ignition coil is a common culprit. Learning how to test a lawn mower coil with a basic multimeter can save you time and money on unnecessary repairs. This clear diagnosis helps you confirm if the coil is the problem before you buy a replacement.
A silent mower is frustrating. You pull the cord, but nothing happens. Before you assume the worst, a simple electrical test can point you in the right direction. This guide walks you through the entire process safely and effectively.
How To Test A Lawn Mower Coil
The ignition coil, or magneto, is a crucial component. It transforms the low voltage from the engine’s flywheel into a high-voltage spark at the plug. If it fails, there’s no spark, and your engine is dead. Testing it involves checking for continuity and resistance.
You only need a few tools. A digital multimeter is essential. You’ll also need a socket or spark plug wrench, some clean rags, and maybe a little sandpaper. Always disconnect the spark plug wire first for safety.
Safety First: Preparing To Test Your Mower
Never skip safety steps when working with small engines. A accidental spark can lead to injury or fire. Follow these precautions every single time.
- Disconnect the spark plug wire. Secure it away from the plug terminal.
- Allow the engine to cool completely if it was recently running.
- Work in a well-lit, dry area with good ventilation.
- Take a picture of the coil and wiring before disconnecting anything to aid reassembly.
Tools And Materials You Will Need
Gathering your tools beforehand makes the job smoother. Here is what you need to test the coil properly.
- Digital Multimeter (set to Ohms Ω for resistance)
- Socket set or wrench to remove the spark plug
- Basic screwdrivers (flathead and Phillips)
- A clean cloth or shop towel
- Fine-grit sandpaper or emery cloth
- Your mower’s service manual for specific resistance values (if available)
Locating The Ignition Coil On Your Mower
The coil is mounted near the flywheel, which is usually under a cover on the side of the engine. You’ll need to find it to perform the test.
- Locate the spark plug and follow the thick rubber wire back to its source.
- This wire connects directly to the ignition coil.
- The coil itself is a small, black module with one or two wires coming from it, fastened to the engine block with one or two bolts.
- You may need to remove a plastic engine shroud or metal cover to access it clearly.
Common Coil Configurations
There are two main types you’ll encounter. Most modern mowers use a single coil unit with an integrated stop wire.
- Single Coil with Grounding Wire: One terminal for the spark plug wire and a separate small wire that grounds to kill the engine.
- Two-Post Coil: Has two main terminals; one for the spark plug and another that connects to the flywheel magneto points (less common on newer mowers).
Step-By-Step Testing Procedure With A Multimeter
Now for the main test. We’ll check the coil’s primary and secondary windings for proper resistance. An open circuit (infinite resistance) or a short (zero resistance) means the coil is bad.
Step 1: Disconnect And Remove The Coil
- Disconnect the spark plug wire from the coil terminal.
- Unscrew the small grounding wire (if present).
- Remove the one or two bolts holding the coil to the engine block.
- Carefully lift the coil away. You might need to gently wiggle it free.
- Clean the coil’s metal armature and the engine surface it mounts to with sandpaper. This ensures a good ground.
Step 2: Testing The Primary Windings
The primary winding is the low-voltage side. To test it, you’ll measure resistance between the coil’s mounting foot (ground) and the small wire terminal.
- Set your multimeter to the lowest Ohms (Ω) setting, often 200Ω.
- Touch one probe to the metal mounting foot of the coil.
- Touch the other probe to the small terminal where the kill wire attached.
- A good primary winding typically shows a low resistance, usually between 0.1 and 2.0 Ohms. Consult your manual for the exact spec.
- If you read “OL” (Open Loop) or infinite resistance, the primary winding is broken and the coil is faulty.
Step 3: Testing The Secondary Windings
The secondary winding creates the high-voltage spark. This test checks the path from the spark plug terminal to ground.
- Keep your multimeter on the Ohms setting, but switch to a higher scale, like 20kΩ.
- Insert one probe firmly into the end of the spark plug wire (or the coil’s plug terminal if the wire is removable).
- Touch the other probe to the coil’s metal mounting foot.
- A good secondary winding will show a higher resistance, often in the range of 2,500 to 5,000 Ohms (2.5kΩ to 5kΩ).
- A reading of “OL” means the winding is open and the coil needs replaced. A reading of zero indicates a short.
Interpreting Your Multimeter Results
Understanding what the numbers mean is key. Here’s a simple breakdown of possible outcomes.
- Resistance Within Spec Range: Your coil is likely functioning correctly. The problem may be elsewhere (e.g., spark plug, fuel, carburetor).
- Infinite Resistance (OL): This indicates a break in the wire winding inside the coil. The coil cannot complete the circuit and must be replaced.
- Zero or Extremely Low Resistance: This signals a short circuit within the coil windings. The coil is faulty and needs replacement.
- Resistance Fluctuates Wildly: This points to an internal fault. The coil is unreliable and should be replaced.
What To Do If Your Coil Tests Bad
If your tests confirm a bad ignition coil, replacement is the only option. They cannot be repaired. Follow these steps to install a new one.
- Purchase the correct replacement coil for your specific mower engine model.
- Position the new coil next to the flywheel. There should be a small air gap between the coil armature and the flywheel magnets.
- Many coils have a built-in spacer; if not, use a business card or a specific feeler gauge (often 0.010 inches) to set the gap.
- Secure the coil with its bolts, but do not fully tighten them yet.
- Slide the spacer out and gently press the coil armature against the flywheel, then tighten the bolts. This sets the proper gap.
- Reconnect the kill wire and the spark plug wire securely.
- Reinstall any engine covers you removed.
Other Common Issues That Mimic A Bad Coil
A no-spark condition doesn’t always mean the coil is bad. Before you condemn the coil, quickly check these other simple possibilities.
- Fouled or Damaged Spark Plug: Remove and inspect the plug. Test it with a spark tester or by grounding it against the engine.
- Faulty Stop Switch or Wiring: A shorted kill switch wire can ground out the coil permanently. Disconnect the kill wire and retest for spark.
- Sheared Flywheel Key: If the flywheel key is sheared, the timing is off and the magnet won’t align with the coil, preventing spark.
- Dirty or Corroded Connections: Check all electrical connections, including the coil ground where it mounts to the engine.
Preventive Maintenance For Your Ignition System
Regular care can extend the life of your coil and prevent starting issues. It’s much easier than fixing a problem later.
- Keep the engine clean and free of grass clippings and debris to prevent overheating.
- Regularly inspect the spark plug wire for cracks or brittleness.
- Ensure the coil mounting bolts are tight to maintain a good electrical ground.
- Use a dielectric grease on the spark plug terminal to prevent moisture intrusion and corrosion.
- Store your mower in a dry place to minimize rust and electrical corrosion.
Frequently Asked Questions
How Do I Know If My Lawn Mower Coil Is Bad?
The most obvious sign is a complete lack of spark. If your engine has no spark when you test the plug, and you’ve ruled out the plug itself, the coil is the likely suspect. A multimeter test provides definitive proof.
Can You Test A Lawn Mower Coil Without Removing It?
You can perform a basic spark test without removing the coil, but for an accurate resistance test with a multimeter, you usually need to disconnect and remove it to isolate it from the engine’s ground. This ensures your reading isn’t skewed.
What Is The Correct Air Gap For A Lawn Mower Coil?
The air gap is critical. It is typically between 0.008 and 0.012 inches (the thickness of a standard business card). An incorrect gap can weaken or eliminate the spark. Always check your engine’s manual for the precise specification.
What Would Cause A Lawn Mower Coil To Keep Going Bad?
Repeated coil failure is often caused by excessive heat from a dirty engine, vibration from loose mounting bolts, or a underlying issue like a faulty charging system on electric-start models that sends too much voltage to the coil.
Is It Worth Replacing A Lawn Mower Coil?
Yes, replacing a faulty coil is almost always worth it. It’s a relatively inexpensive part and a straightforward repair that can restore your mower to full operation, saving you the cost of a new machine or a expensive mechanic’s bill.