When your lawn mower refuses to start, a faulty ignition coil is a very common culprit. Learning how to test lawn mower coil yourself can save you time and money on a service call. This guide provides clear, step-by-step instructions to diagnose the problem with basic tools.
A faulty coil is a common reason for a mower’s lack of spark and failure to start. The coil’s job is to transform the low voltage from the battery or magnets into the high voltage needed to create a spark at the plug. If it fails, you get no spark, and your engine won’t run.
How To Test Lawn Mower Coil
Testing the ignition coil is a straightforward process that involves checking for spark and then using a multimeter to measure resistance. You will need a few simple tools and about 30 minutes of your time. Safety is paramount, so always disconnect the spark plug wire before beginning any work.
Tools And Safety Equipment You Will Need
Gathering the right tools before you start makes the job smoother and safer. You likely have most of these items already in your garage.
- A multimeter (also called a multitester)
- A basic socket or spark plug wrench
- A spark plug tester or an old, known-good spark plug
- Safety glasses
- Work gloves
- A clean rag
Step-By-Step Testing Procedure
Follow these steps in order to accurately determine if your lawn mower’s ignition coil is the source of the problem. Remember to always refer to your mower’s service manual for specific details about your model.
Step 1: Preliminary Safety And Preparation
First, ensure the mower is on a flat, stable surface and the engine is completely cool. Disconnect the spark plug wire from the plug and secure it away from the plug terminal. This prevents any accidental starting while you are working. It’s a good idea to clean any debris from around the coil and flywheel area with your rag.
Step 2: Perform A Visual Inspection
Look closely at the ignition coil, also called the armature. Check for obvious signs of damage like cracks, burns, melted areas, or corrosion on the terminals. Also inspect the wiring leading to and from the coil for any breaks or frayed insulation. Sometimes the problem is a loose mounting screw, so ensure the coil is securely fastened.
Step 3: The Basic Spark Test
This is the initial test to see if you have any spark at all. Attach a spark plug tester to the spark plug wire. Ground the tester’s clip to a clean, unpainted metal part of the engine. Alternatively, you can use an old plug; insert it into the plug boot and lay its metal body against the engine.
Now, pull the starter cord or turn the ignition key (with the spark plug removed). Look for a bright blue spark jumping across the tester’s gap or the spark plug’s electrode. A weak orange spark or no spark at all indicates a problem in the ignition system, likely the coil.
Step 4: Testing Coil Resistance With A Multimeter
If you have no spark, the multimeter test will confirm the coil’s condition. You will be measuring the primary and secondary windings resistance in ohms (Ω). Set your multimeter to the ohms or resistance setting, usually marked with the Ω symbol.
- Locate the coil’s two electrical terminals. One is the small wire from the kill switch/ignition, and the other connects to the spark plug wire.
- To test the primary winding, place one multimeter probe on the metal core of the coil (or the ground tab) and the other on the terminal where the small kill wire attaches. A typical reading should be between 0.1 and 2 ohms. Consult your manual for the exact specification.
- To test the secondary winding, place one probe on the spark plug wire terminal (inside the boot) and the other on the coil’s ground or core. This reading is much higher, often in the range of 2,500 to 5,000 ohms. An infinite reading (OL) means the winding is open and the coil is bad.
If either reading is significantly outside the specified range, the coil needs to be replaced. Remember, these values can vary widely between different mower engines.
Understanding The Results And Next Steps
Interpreting your test results correctly tells you what to do next. A failed test means replacement is necessary, but a passed test means you need to look elsewhere for the starting issue.
If The Coil Tests Bad
If your multimeter readings show an open circuit or are not within specification, you need a new ignition coil. Purchase a replacement that matches your engine model number exactly. Installation is usually the reverse of removal: disconnect the wires, remove the mounting screws, swap the coil, and reconnect. Ensure you set the proper air gap between the coil leg and the flywheel magnets, often with a business card or a feeler gauge.
If The Coil Tests Good
A good coil reading but no spark points to other issues in the ignition system. The problem could be a faulty kill switch wire that is grounding out, a broken flywheel key causing incorrect timing, damaged magnets on the flywheel, or a problem with the spark plug wire itself. You will need to continue troubleshooting these other components.
Common Mistakes To Avoid When Testing
Avoiding these common errors will give you a more accurate diagnosis and prevent damage to your mower or injury to yourself.
- Not disconnecting the spark plug wire first. This is the number one safety rule.
- Testing a dirty or corroded coil. Clean the terminals before taking readings.
- Using a multimeter with weak batteries, which can give false readings.
- Forgetting to check the air gap after installing a new coil. An incorrect gap can prevent spark.
- Overlooking the simple stuff, like a fouled spark plug or old gasoline, which can mimic coil failure.
Preventative Maintenance Tips
Regular maintenance can extend the life of your ignition coil and prevent unexpected breakdowns. Coils are generally reliable, but they operate in a harsh environment.
- Keep the engine area clean from grass clippings, oil, and moisture. Buildup can cause overheating.
- Regularly inspect the spark plug wire for cracks and ensure it is securely connected.
- Check that the coil mounting screws are tight; vibration can loosen them over time.
- Always use a fuel stabilizer if storing your mower for more than 30 days. Corrosion from old fuel system issues can affect electrical connections.
Frequently Asked Questions
What Are The Symptoms Of A Bad Lawn Mower Coil?
The primary symptom is the engine not starting due to a complete lack of spark. You may also experience intermittent starting, the engine dying while running, or backfiring. In many cases, the mower will simply not start on the first pull or crank after previously running fine.
Can A Lawn Mower Coil Be Tested Without A Multimeter?
Yes, you can perform the basic visual inspection and spark test without a multimeter. However, the multimeter resistance test provides a definitive, numerical confirmation of the coil’s health. The spark test alone can sometimes be misleading if other factors are at play.
What Causes An Ignition Coil To Fail On A Mower?
Common causes include excessive heat from a clogged cooling system, vibration loosening mounts, moisture and corrosion, old age, and voltage spikes. Physical damage from impact or improper installation can also lead to premature failure.
How Much Does It Cost To Replace A Lawn Mower Coil?
The part itself typically costs between $15 and $50, depending on your mower’s engine model. If you have a professional small engine mechanic replace it, labor could add another $50 to $100 to the total bill. Doing it yourself with this guide saves the labor cost.
Is It Worth Replacing A Lawn Mower Coil?
Absolutely. If the rest of your mower is in good condition, replacing a faulty coil is one of the most cost-effective repairs you can make. It restores reliable starting and is significantly cheaper than buying a new machine. The repair is also very manageable for most DIY owners.