If your lawn mower won’t start or keeps stalling, the ignition coil could be the problem. The ignition coil is responsible for generating the spark needed to ignite the air-fuel mixture in the engine. A faulty ignition coil can cause weak or no spark, preventing your lawn mower from starting or running properly. I’ve been through this issue myself, and luckily, testing a lawn mower ignition coil is a relatively simple process.
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Here’s a step-by-step guide to test your lawn mower’s ignition coil and diagnose whether it’s causing the problem.
Tools and Materials You’ll Need:
- Multimeter (for checking resistance)
- Spark plug tester (optional, for checking spark)
- Socket wrench (for removing the engine cover)
- Screwdriver
- Work gloves (for safety)
Step-by-Step Guide to Testing a Lawn Mower Ignition Coil
1. Ensure Safety First
- Turn off the lawn mower and ensure it’s on a flat surface.
- Disconnect the spark plug wire to prevent accidental starting while you work on the engine. Always wear gloves for safety.
2. Locate the Ignition Coil
- To access the ignition coil, you’ll need to remove the engine cover or shroud using a socket wrench or screwdriver. The ignition coil is typically located near the flywheel, with a wire leading to the spark plug.
- Once you’ve exposed the ignition coil, make sure the kill switch wire or ground wire is disconnected to avoid interference with the test.
3. Check for Spark Using a Spark Plug Tester (Optional)
Before testing with a multimeter, you can perform a quick spark test to see if the ignition coil is functioning.
- Reconnect the spark plug wire and use a spark plug tester between the spark plug and ignition coil wire.
- Pull the starter cord to crank the engine.
- Look for a spark in the tester window. If no spark appears, the ignition coil could be faulty, and you’ll need to test it with a multimeter.
4. Test the Ignition Coil with a Multimeter
To confirm that the ignition coil is faulty, you can check its resistance using a multimeter. Here’s how:
- Set your multimeter to the ohms (Ω) setting.
- Place one probe of the multimeter on the metal armature of the ignition coil (the metal frame or ground).
- Place the other probe on the spark plug terminal (where the spark plug wire connects to the coil).
- Read the resistance on the multimeter display. A healthy ignition coil typically shows a resistance reading between 2,500 and 5,000 ohms. If the reading shows no continuity (infinite resistance) or is far outside this range, the coil is likely faulty.
5. Test the Secondary Winding (Optional)
Some ignition coils have a secondary winding (an additional wire inside the coil). To test this:
- Place one probe on the kill wire terminal and the other on the metal armature.
- A healthy secondary winding will also show continuity. If there’s no reading, the secondary winding is likely damaged.
6. Check the Flywheel Magnet
- While testing the coil, inspect the flywheel magnet for any signs of rust or debris that could interfere with the coil’s operation. The magnet should be clean and strong.
- Use a feeler gauge to check the gap between the flywheel and the ignition coil. The gap should be about 0.010-0.015 inches. If the gap is too wide or too narrow, it could affect the coil’s ability to generate a spark.
When to Replace the Ignition Coil
If the multimeter test shows resistance outside the normal range or no continuity at all, the ignition coil is faulty and will need to be replaced. A new coil can easily be installed by:
- Removing the old coil by unscrewing the mounting bolts.
- Placing the new coil in the same position.
- Adjusting the coil-to-flywheel gap with a feeler gauge before tightening the bolts.
- Reconnecting the spark plug wire and kill switch.
Troubleshooting Other Potential Issues
If your ignition coil tests fine, but your mower still won’t start or has spark issues, consider these other possible causes:
- Bad spark plug: Check and clean or replace the spark plug.
- Dirty or damaged flywheel magnet: Clean the magnet and check the gap.
- Faulty kill switch: Ensure the kill switch wire isn’t grounding out prematurely.
- Wiring issues: Inspect for frayed or damaged wiring between the ignition coil and spark plug.
Final Thoughts
Testing the ignition coil on your lawn mower is an easy and effective way to diagnose why your mower isn’t starting or running properly. With a multimeter, you can check the coil’s resistance and determine whether it needs replacing. By following these steps, you’ll be able to quickly identify the problem and get your lawn mower back in working order.