If your lawn mower has spark but won’t start, a faulty ignition coil is a common reason. This guide will show you exactly how to check lawn mower coil with simple tools you likely already have.
You don’t need to be a mechanic. With a basic multimeter and some safety steps, you can test the coil yourself in under 30 minutes. It saves time and money compared to just guessing or taking it to a shop.
We’ll cover the signs of a bad coil, the tools you need, and clear step-by-step instructions. Let’s get your mower running again.
How To Check Lawn Mower Coil
The ignition coil, sometimes called the magneto, is a crucial component. Its job is to transform the low voltage from the engine into the high voltage needed to create a spark at the plug. When it fails, you get no spark or a weak one, preventing the engine from starting.
Checking it involves measuring its electrical resistance with a multimeter. This test tells you if the coil’s internal windings are intact or broken. It’s a definitive check that removes the guesswork.
Tools And Safety Equipment You Will Need
Before you begin, gather these items. Having everything ready makes the process smooth and safe.
- A digital multimeter (set to the ohms or resistance setting)
- A socket or spark plug wrench
- A clean rag or shop towel
- A pair of insulated gloves (for safety)
- Safety glasses to protect your eyes
- A small brush to clean debris from around the coil
Always disconnect the spark plug wire before starting any work. This is the most important safety step to prevent an accidental start.
Step-By-Step Testing Procedure
Follow these steps in order to safely remove and test your lawn mower’s ignition coil.
Step 1: Prepare Your Workspace And Mower
Park the mower on a flat, level surface. Make sure the engine is completely cool to the touch. Pull the wire boot off the spark plug to disable the ignition system. This is your primary safety precaution.
Step 2: Locate And Access The Ignition Coil
The ignition coil is mounted near the flywheel, which is usually under a plastic shroud on the top or side of the engine. You may need to remove a few screws or a cover to see it. It’s a small, often black or silver module with a thick wire leading to the spark plug.
Once you see it, use your brush and rag to clean any grass, dirt, or oil from around the coil. Debris can interfere with your test connections.
Step 3: Disconnect The Coil And Identify Terminals
Carefully disconnect the stop switch wire (usually a small single wire) from the coil. Then, unscrew the coil’s mounting bolts. Gently lift the coil away from the engine. You will see two primary terminals: one for the spark plug wire and one or two small terminals for the kill switch and ground.
Your multimeter will test the resistance between these points. Refer to your mower’s manual for the specific coil terminal layout if possible.
Step 4: Test The Primary Coil Windings
Set your multimeter to the lowest ohms setting (often marked Ω or Rx1). Touch one probe to the metal core of the coil (or the ground terminal) and the other probe to the small terminal where the kill switch wire was attached. A good primary winding typically shows a resistance between 0.1 and 2.0 ohms.
If your meter shows “OL” (open loop) or infinite resistance, the primary winding is broken and the coil is bad. A zero reading indicates a short, which also means the coil has failed.
Step 5: Test The Secondary Coil Windings
This test checks the high-voltage side. Leave one probe on the ground/coil core. Move the other probe and insert it into the end of the spark plug wire boot where it connects to the plug. A healthy secondary winding will show a much higher resistance, often between 2,500 and 5,000 ohms.
Again, an “OL” reading means the winding is open and the coil needs replacement. Readings drastically outside the common range also indicate a problem.
Step 6: Interpret Your Results And Next Steps
If both tests show readings within the expected ranges, your ignition coil is likely functioning correctly. The no-spark issue may be elsewhere, like the spark plug itself or the flywheel key.
If either test fails, you need a new coil. When installing the new one, ensure a small air gap exists between the coil leg and the flywheel magnets. A business card’s thickness is a good gauge for this gap.
Common Symptoms Of A Faulty Ignition Coil
How do you know if you should test the coil in the first place? Watch for these telltale signs.
- The engine cranks but does not start or fire at all.
- You have confirmed there is no spark at the plug using a spark tester.
- The mower runs very poorly, misfires, or loses power under load.
- The engine starts when cold but dies as soon as it warms up.
- You see physical damage on the coil, like cracks or burn marks.
These symptoms often point directly to an ignition issue, with the coil being a prime suspect. It’s a good idea to check the spark plug first, as it’s a simpler and cheaper part to rule out.
Understanding Coil Specifications And Readings
Resistance values can vary between different mower brands and engine models. The numbers provided here are common benchmarks.
Always check your engine’s service manual for the exact specifications if you can. Some coils, like those on older engines, may have different expected values. The key is to look for a consistent, stable reading, not an “OL” or a zero.
Environmental factors like heat and moisture can cause intermittent coil failure. A coil might test fine when cold but fail when the engine heats up. This is a tricky problem to diagnose.
What To Do If Your Coil Tests Good
If your coil passes the multimeter test but you still have no spark, the problem lies elsewhere. Don’t worry, you’ve successfully eliminated one major component. Here are the next parts to check.
- Spark Plug: Remove it, check the gap, and look for cracks or heavy carbon deposits. Test it with a plug tester or replace it with a known-good one.
- Flywheel Key: This small metal key aligns the flywheel. If it’s sheared, the engine’s timing is off and it won’t spark. You’ll need to remove the flywheel nut to inspect it.
- Kill Switch Wire: This wire grounds the coil to stop the engine. If it’s damaged or shorting against the engine block, it can prevent spark. Ensure it’s properly disconnected during testing.
- Armature Air Gap: The gap between the coil and flywheel is critical. If it’s too wide, the magnetic field is too weak to generate a proper spark.
Systematically checking these areas will usually lead you to the root cause of your starting problem.
FAQ: Lawn Mower Ignition Coil Questions
How Much Does It Cost To Replace A Lawn Mower Coil?
The cost for a new ignition coil typically ranges from $20 to $60 for most residential mowers. The price depends on your engine brand and model. Labor at a repair shop will add significantly to the total, which is why learning to test and replace it yourself is so valuable.
Can A Lawn Mower Coil Work Intermittently?
Yes, a coil can fail intermittently. This is often caused by internal breaks that open up when the coil expands from heat. The mower may start cold but then die once hot. It can restart after cooling down. This is a classic sign of a failing coil that may still show a passing resistance when cold.
What Causes An Ignition Coil To Go Bad?
Common causes include excessive heat from a clogged engine cooling system, vibration loosening the mounting, moisture corrosion inside the coil windings, and simple age. Physical impact or voltage surges can also damage the delicate internal components.
Is It Worth Replacing A Lawn Mower Coil?
Absolutely. If the rest of your mower is in good condition, replacing a faulty coil is a relatively inexpensive repair that restores full functionality. It’s much cheaper than buying a new mower. The repair is straightforward once you know how to check the lawn mower coil yourself.
Testing your lawn mower’s ignition coil is a straightforward DIY task that requires minimal tools. By following the step-by-step resistance tests with a multimeter, you can confidently determine if the coil is the culprit behind your no-spark condition. Remember to always prioritize safety by disconnecting the spark plug first. With this knowledge, you can avoid unnecessary repairs and get your mower back to cutting grass quickly. If your coil tests bad, replacement is usually simple, involving just a few bolts and a careful adjustment of the air gap.