When your lawn mower refuses to start, a faulty ignition coil is a common culprit. Learning how to check a coil on a lawn mower is a straightforward diagnostic task that can save you time and money. A faulty ignition coil will prevent the spark plug from firing, stopping the engine from starting. This guide provides clear, step-by-step instructions to safely test your mower’s coil with basic tools.
How To Check A Coil On A Lawn Mower
An 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 magnets in a flywheel on a battery-less system) into a high-voltage pulse. This pulse, often exceeding 20,000 volts, travels down the spark plug wire to create the spark at the plug’s electrode. Without this strong spark, the fuel-air mixture in the cylinder won’t ignite, and your engine will not run. Checking the coil involves verifying that it can produce this essential spark under pressure.
Safety First: Preparing To Work On Your Mower
Before you touch any tools, your safety and the mower’s integrity are paramount. Ignition systems carry a high voltage, and the engine’s moving parts pose risks. Following these preparation steps is non-negotiable.
- Disconnect the Spark Plug Wire: Always remove the spark plug wire from the plug itself. Secure it away from the plug terminal to prevent any accidental sparking.
- Allow the Engine to Cool: Work on a cold engine to avoid burns from the muffler, cylinder head, or other hot components.
- Clean Your Work Area: Clear away grass clippings, debris, and any flammable materials from around the mower before you begin.
- Gather Your Tools: You will typically need a socket or spark plug wrench, a set of feeler gauges, a basic multimeter, and possibly a screwdriver or wrench set to remove the coil.
Tools You Will Need For The Test
You don’t need a professional mechanic’s toolbox to perform a reliable coil test. Here are the essential items:
- A Multimeter: This is the key tool for an electrical test. An inexpensive digital model with ohms (Ω) setting is perfect.
- Feeler Gauges: These are essential for setting the correct air gap between the coil arm and the flywheel, a critical step for spark generation.
- Spark Plug Wrench or Socket: To remove the spark plug for testing.
- Basic Hand Tools: Screwdrivers and wrenches to remove the shroud covering the flywheel and to loosen the coil mounting bolts.
- A Known-Good Spark Plug (Optional): Helpful for a spark test, but not always required.
Understanding The Two Main Types of Coil Tests
There are two primary methods to check your lawn mower’s ignition coil: a visual and physical inspection, and a more precise electrical test using a multimeter. We will cover both. The spark test is a classic method, but the multimeter test provides a more definitive, numerical result.
Step-By-Step Guide To Checking The Ignition Coil
Follow these steps in order to systematically diagnose your lawn mower’s ignition coil.
Step 1: Initial Visual Inspection
Start with a thorough visual check. Look for obvious signs of damage that would immediately indicate a problem.
- Locate the coil. It is mounted near the flywheel, usually under a metal or plastic shroud on the top or side of the engine.
- Remove the shroud or blower housing. This often involves taking out a few screws.
- Inspect the coil body for cracks, carbon tracking (black lightning-bolt-like lines), melted areas, or corrosion on the terminals.
- Check the spark plug wire for brittleness, cracks, or any signs that its insulation is compromised.
If you see any of this damage, the coil likely needs replacement. If it looks physically intact, proceed to the next steps.
Step 2: Performing A Spark Test (The Quick Check)
This test checks if the coil is generating a spark. It is a functional test but must be done carefully.
- Reconnect the spark plug wire to the spark plug.
- Using insulated pliers, hold the spark plug’s metal body against a clean, unpainted part of the engine cylinder head. Ensure the plug’s electrode is not touching anything.
- With the spark plug wire securely attached and the plug grounded, pull the starter cord or engage the electric starter (if safe to do so).
- Look for a bright blue spark jumping across the spark plug’s electrode gap. A faint yellow or orange spark, or no spark at all, indicates a weak or failing coil.
Important Caution: Do not hold the spark plug wire or plug with your bare hands during this test. The voltage is extremely high and can deliver a painful shock.
Step 3: Testing Coil Resistance With A Multimeter
This is the most accurate way to check the coil’s health. You will be measuring resistance in ohms. Consult your mower’s service manual for the exact specifications, as values vary. A common range for primary winding resistance is 0.1 to 2 ohms, and for secondary winding (including the spark plug wire) is 2,500 to 5,000 ohms.
- Disconnect the spark plug wire and any stop or kill wires connected to the coil terminals.
- Set your multimeter to the ohms (Ω) setting, usually the lowest scale (like 200Ω or 2kΩ).
- Primary Winding Test: Touch one multimeter probe to the metal arm of the coil (where it faces the flywheel) and the other probe to the ground terminal or the coil’s metal mounting point. Note the reading.
- Secondary Winding Test: Insert one probe into the end of the spark plug wire boot (you may need a small pin to make contact) and touch the other probe to the coil’s ground terminal or arm. Note this reading.
If either reading is infinite (OL or “open loop” on the meter) or zero, the coil has an internal break or short and must be replaced. A reading within the manufacturer’s specified range indicates a good coil. If you don’t have a manual, a secondary reading in the thousands of ohms is typically a good sign.
Step 4: Checking And Setting The Air Gap
Even a good coil won’t spark if the air gap is incorrect. The air gap is the space between the coil’s armature legs and the flywheel magnets. It’s usually between 0.008 and 0.012 inches (0.2 – 0.3 mm).
- Loosen the two coil mounting bolts just enough so you can move the coil.
- Select the correct feeler gauge blade (e.g., 0.010 inch).
- Slide the feeler gauge between the coil leg and the flywheel magnet.
- Gently move the coil inward until it lightly pinches the feeler gauge. The gauge should slide in and out with a slight drag.
- While holding the coil in this position, tighten the mounting bolts in a criss-cross pattern to ensure it stays square.
- Remove the feeler gauge and rotate the flywheel to ensure it doesn’t contact the coil at any point.
Common Symptoms Of A Bad Lawn Mower Coil
How do you know if you should check the coil in the first place? Watch for these telltale signs:
- The engine will not start at all, and you have confirmed you have fresh fuel and a clean air filter.
- The mower starts but runs very rough, misfires, or lacks power under load.
- The engine starts when cold but dies as soon as it warms up (a classic symptom of a failing coil).
- You notice the mower is harder to start than usual, requiring many more pulls.
- Physical damage, as mentioned in the visual inspection, is visible on the coil body or wire.
What To Do If Your Coil Tests Bad
If your tests confirm the ignition coil is faulty, replacement is the only option. Coils cannot be repaired. Here’s the process:
- Purchase the correct replacement coil. Your mower’s model number and engine model number (found on a sticker on the engine shroud) are crucial for getting the right part.
- Disconnect the spark plug wire.
- Remove the old coil, noting exactly how the kill wire (if present) is attached.
- Before installing the new coil, clean the mounting surface on the engine.
- Install the new coil but do not fully tighten the bolts. Set the air gap using your feeler gauges as described in Step 4.
- Once the gap is set, tighten the bolts, reconnect the kill wire and spark plug wire, and reinstall the engine shroud.
Preventative Maintenance Tips
While coils can fail without warning, you can take steps to prolong their life and ensure reliable starts.
- Keep It Clean: Regularly clean grass clippings and debris from around the engine shroud and coil area to prevent overheating and moisture retention.
- Check Connections: Periodically ensure the spark plug wire is snug on the plug and that any kill switch wires are securely fastened to the coil terminals.
- Use Dielectric Grease: A small dab of dielectric grease inside the spark plug wire boot can prevent corrosion and ensure a good connection.
- Proper Storage: Store your mower in a dry place to minimize condensation and corrosion on electrical components.
Frequently Asked Questions
Can A Lawn Mower Run With A Bad Coil?
It is very unlikely. A completely failed coil will provide no spark, preventing the engine from starting at all. A weak or intermittently failing coil may allow the engine to start and run poorly for a short time, often stalling when it gets warm.
How Much Does It Cost To Replace A Lawn Mower Coil?
The cost varies by engine model, but a typical replacement ignition coil for a push mower engine ranges from $20 to $60. For larger riding mower engines, the cost can be higher, sometimes up to $100 or more. The repair is straightforward if you do it yourself.
What Causes An Ignition Coil To Go Bad?
Common causes include excessive heat from a dirty engine, vibration loosening mounting bolts, physical impact damage, moisture corrosion, and simple age. Electrical overload from a faulty component elsewhere in the system can also rarely cause failure.
Can You Test A Coil Without A Multimeter?
Yes, the spark test described earlier is a common method. However, it is less definitive than a multimeter test. A weak coil might still produce a visible but inadequate spark, leading to misdiagnosis. A multimeter provides a concrete measurement for better accuracy.
Is It The Coil Or The Spark Plug?
Both cause similar symptoms. Always check the spark plug first, as it is cheaper and easier to replace. Remove the plug, inspect it for fouling, cracks, or a worn electrode, and test for spark. If a new, properly gapped spark plug does not produce a strong blue spark, then the coil is the likely problem.