How do You Check a Lawn Mower Coil? - Plant Care Guide

When your lawn mower cranks but simply won't start, or runs roughly and then dies, many frustrated homeowners quickly suspect fuel issues or a fouled spark plug. However, a less obvious but equally critical component in the ignition system, the ignition coil (often just called the coil), can also be the silent culprit. This seemingly simple part is responsible for transforming low-voltage electricity into the high-voltage spark needed to ignite the fuel-air mixture in the engine.

Diagnosing a faulty coil requires a systematic approach, moving beyond the easy checks to delve into the heart of the ignition system. While it's not always the first thing to test, understanding how to confirm its functionality can save you time and money, ensuring your lawn mower is ready for action. This guide will walk you through the essential steps to determine if your coil is delivering the spark needed for a healthy engine.

What is an Ignition Coil and What Does it Do in a Lawn Mower?

An ignition coil is a vital component of a lawn mower's engine ignition system, acting as an electrical transformer that converts the low-voltage electricity from the battery or flywheel into the extremely high voltage required to create a spark at the spark plug. Without a functioning ignition coil, your lawn mower simply won't start or run. It's often referred to simply as the "coil" or "magneto."

• How it Works (The Science):
  1. Low Voltage In: When the engine's crankshaft rotates, it passes a magnet attached to the flywheel near the ignition coil. This induces a low-voltage electrical current in the coil's primary winding.
  2. Magnetic Field Creation: This low-voltage current creates a magnetic field around the coil.
  3. Field Collapse and High Voltage Out: At the precise moment the spark is needed (timed by the engine's ignition system), the low-voltage current is interrupted. This causes the magnetic field around the primary winding to rapidly collapse.
  4. Voltage Transformation: This collapsing magnetic field induces a very high voltage (thousands of volts) in the coil's secondary winding.
  5. Spark Delivery: This high-voltage surge travels through the spark plug wire to the spark plug, where it jumps the gap between the electrodes, creating a powerful spark that ignites the fuel-air mixture in the combustion chamber.
• Key Functions:
  • Voltage Amplification: Transforms 12 volts (from the battery) or induced low voltage into 15,000 to 30,000+ volts needed for a spark.
  • Spark Generation: Delivers the electrical punch required for combustion.
  • Timing: Works in conjunction with the flywheel magnet and engine's timing to deliver the spark at the precise moment for optimal engine performance.
• Location:
  • In most small engines, the ignition coil is located near the flywheel, often mounted on the engine block. The spark plug wire directly connects to it.

A faulty ignition coil prevents this crucial high-voltage spark from reaching the spark plug, leading to common problems like a lawn mower that cranks but won't start, or a sputtering engine.

What Are the Symptoms of a Bad Ignition Coil in a Lawn Mower?

The symptoms of a bad ignition coil in a lawn mower often manifest as common starting or running problems, but they specifically point to a lack of proper spark delivery. Recognizing these signs can help you narrow down your troubleshooting efforts and suspect the ignition coil as the culprit.

• Engine Won't Start (No Spark):
  • Cranking but No Start: The most common symptom. The engine turns over vigorously when you try to start it (you hear the starter motor engaging), but it never actually fires up and runs. This indicates fuel is getting to the engine, but there's no spark to ignite it.
  • No Spark at Spark Plug: When you remove the spark plug and test for a spark (see troubleshooting section), you see no spark, or a very weak, inconsistent orange spark (instead of a strong, blue-white spark).
• Engine Starts But Dies Quickly:
  • The engine might briefly sputter to life, especially if cold, but then quickly dies, often after just a few seconds. This can indicate an inconsistent spark or a coil that's failing intermittently.
• Rough Idling or Misfiring:
  • The engine runs unevenly, sputters, backfires, or misses cycles. This suggests the spark is inconsistent or weak, causing incomplete combustion.
• Loss of Power/Poor Performance:
  • The lawn mower might run, but it lacks its usual power, struggles under load (e.g., cutting thick grass), or surges. This is due to inefficient combustion caused by a weak spark.
• Overheating Engine (Less Common, but Possible):
  • Inconsistent or poor ignition can lead to unburnt fuel or inefficient combustion, which can contribute to the engine running hotter than normal.
• No Fuel-Related Issues:
  • You've checked the fuel tank (full of fresh fuel), the fuel filter (clean), and the carburetor (appears to be getting fuel), ruling out most common fuel-related starting problems.
• Spark Plug Fouling (Secondary):
  • A weak or inconsistent spark can lead to incomplete combustion, causing fuel and carbon deposits to build up on the spark plug, fouling it more quickly.

When these symptoms of a bad ignition coil in a lawn mower are present, especially after you've ruled out fuel and a simple spark plug issue, investigating the ignition coil becomes a logical next step in your repair process.

What Safety Precautions Should I Take Before Checking a Lawn Mower Coil?

Before you attempt to check a lawn mower coil or any part of the ignition system, safety precautions are paramount. Working with electrical components and moving parts carries inherent risks, and neglecting safety can lead to severe injury. Always prioritize your well-being.

1. Disconnect the Spark Plug Wire (Crucial!):
   • The absolute first and most important step. Pull the spark plug boot off the spark plug and move it away from the spark plug terminal. You can also ground it to the engine block if preferred, but ensuring it's completely disconnected is safest.
   • Why: This prevents the engine from accidentally starting while you are working on it, which could cause the blades to engage or other parts to move unexpectedly.
2. Ensure Engine is Cool:
   • Allow the lawn mower engine to cool completely before working on it. Hot engine parts can cause severe burns.
3. Remove Battery (if applicable for electric start):
   • If your lawn mower has an electric start and a separate battery, it's a good practice to disconnect the negative battery terminal first. This completely cuts off power to the electrical system, eliminating any risk of accidental short circuits or sparks.
4. Work in a Well-Ventilated Area:
   • If any fuel spills or fumes are present, ensure you are working in an open, well-ventilated area to avoid inhaling harmful fumes.
5. Wear Protective Gear:
   • Gloves: Wear work gloves to protect your hands from sharp edges and grease.
   • Safety Glasses: Protect your eyes from debris or sparks.
6. Stabilize the Mower:
   • If you need to tilt the mower to access the underside (e.g., to inspect the flywheel), ensure it is stable and secure. Block the wheels to prevent rolling. Tilt it so the carburetor side is up to prevent fuel/oil spills.
7. Have a Fire Extinguisher Nearby:
   • Working with fuel and electrical components always carries a small risk of fire. Have a Class B fire extinguisher readily accessible.
8. Read Your Owner's Manual:
   • Always consult your lawn mower's owner's manual for specific safety precautions and component locations related to your model. It may have specific instructions or warnings.

By diligently following these safety precautions, you minimize the risks associated with checking your lawn mower coil, ensuring a safe and successful troubleshooting process.

How Do You Check a Lawn Mower Coil for Spark?

To check a lawn mower coil for spark, you will primarily perform a spark test, which involves removing the spark plug and observing if a strong, visible spark occurs when the engine is cranked. This is the most direct way to diagnose if the ignition coil is functioning correctly.

Tools You'll Need:

• Spark Plug Wrench: To remove the spark plug. A spark plug socket wrench is ideal.
• Spark Tester (Recommended): A specialized tool that plugs into the spark plug wire and provides a visual indication of spark. A lawn mower spark tester is safer and more reliable than a manual method.
• Screwdriver or Pliers: If removing engine shrouds.
• Gloves, Safety Glasses.

Step-by-Step Guide to Checking for Spark:

1. Perform All Safety Precautions:

   • Crucial: Disconnect the spark plug wire first. Ensure the engine is cool. (Refer to the previous section).

2. Remove the Spark Plug:

   • Use your spark plug wrench to carefully unscrew and remove the spark plug from the cylinder head.
   • Inspect the spark plug itself. Is it wet with fuel (indicating no spark/over-choking)? Is it fouled (black, oily)? If so, clean it or replace it, then retest.

3. Connect Spark Plug to Wire (Manual Test - Use Caution!):

   • Option A (Less Safe, Visual Only): Reconnect the spark plug to its wire (the rubber boot should fit snugly).
   • Ground the Plug: Hold the metal body (hex nut) of the spark plug firmly against a clean, unpainted metal part of the engine block. Ensure good metal-to-metal contact. You should use an insulated glove or pliers to hold it.
   • Crank the Engine: Have a helper (or use a remote starter, if available) crank the engine (pull the starter cord for a pull-start, or turn the key for an electric start).
   • Observe Spark: Look at the gap between the spark plug's electrodes. You should see a strong, bright blue-white spark. A weak orange or no spark indicates a problem with the ignition system.
   • Caution: Holding the spark plug directly can result in a powerful electrical shock. Use insulated tools.

4. Use a Spark Tester (Safer and Recommended Method):

   • Connect Tester: Disconnect the spark plug wire from the spark plug. Insert the metal tip of the spark tester into the spark plug boot (the end of the wire).
   • Ground Tester: Clip the other end of the spark tester to a clean, unpainted metal part of the engine block.
   • Crank the Engine: Crank the engine (pull the starter cord or turn the key).
   • Observe Spark: Most spark testers have a small window or gap. Look for a strong, consistent blue-white spark jumping the gap inside the tester.
   • Benefits: This method is much safer and easier to confirm a strong spark.
   • No Spark or Weak Spark: If the spark tester shows no spark, or a very weak, inconsistent spark, it strongly suggests a problem in the ignition system, which could be the ignition coil.

5. Further Investigation (If No Spark/Weak Spark):

• If you've confirmed no spark with a good spark plug and spark tester, and your safety interlocks are all met (seat switch, PTO, brake, neutral), the issue likely lies with the ignition coil, the spark plug wire (less common), or the flywheel's magnet/gap.

By methodically performing a spark test, ideally with a dedicated spark tester, you can effectively check a lawn mower coil for spark and determine if it's the root cause of your starting problems.

How Do You Test a Lawn Mower Coil for Resistance with a Multimeter?

To test a lawn mower coil for resistance with a multimeter, you'll measure the electrical resistance across its primary and secondary windings. This advanced diagnostic step helps determine if the internal coils are open-circuited (broken) or short-circuited, indicating a faulty ignition coil. This requires basic knowledge of using a multimeter.

Tools You'll Need:

• Multimeter (Digital is preferred): Capable of measuring resistance (Ohms Ω). A digital multimeter auto ranging is ideal.
• Screwdriver, Wrench, or Pliers: For removing engine shrouds and the coil itself.
• Owner's Manual/Service Manual: Crucial for finding the specific resistance specifications (Ohm readings) for your lawn mower's ignition coil. These values vary significantly between manufacturers and models.
• Gloves, Safety Glasses.

Step-by-Step Guide to Testing Coil Resistance:

1. Perform All Safety Precautions:
   • Crucial: Disconnect the spark plug wire first. Ensure the engine is cool. Disconnect the battery if applicable. (Refer to the safety section).
2. Locate and Access the Ignition Coil:
   • The coil is usually mounted on the engine block, near the flywheel. You'll likely need to remove the engine shroud (plastic cover) to access it.
   • Disconnect the spark plug wire from the coil.
   • Identify the low-voltage wire(s) that connect to the coil (often a ground wire and a kill switch wire). Disconnect these.
3. Set Your Multimeter:
   • Turn your multimeter to the Ohms (Ω) setting, usually the lowest range (e.g., 200Ω or auto-ranging).
4. Test the Primary Winding (Low Voltage Side):
   • Identify Terminals:
     • Place one multimeter probe on the low-voltage terminal of the ignition coil (where the kill switch wire connects).
     • Place the other multimeter probe on the coil's ground (usually the metal mounting bracket of the coil, or a clean metal part of the engine block if the coil is still mounted and grounded).
   • Read Resistance: Note the resistance reading.
   • Compare to Specifications: Compare this reading to the primary winding resistance specification in your owner's or service manual. This is usually a very low Ohm value (e.g., 0.3-2 Ohms).
5. Test the Secondary Winding (High Voltage Side):
   • Identify Terminals:
     • Place one multimeter probe inside the spark plug wire boot (where it connects to the spark plug, ensuring contact with the metal terminal inside the boot).
     • Place the other multimeter probe on the coil's ground (metal mounting bracket or clean engine block).
   • Read Resistance: Note the resistance reading.
   • Compare to Specifications: Compare this reading to the secondary winding resistance specification in your manual. This is usually a much higher Ohm value (e.g., 5,000-15,000 Ohms or 5k-15k Ohms).
6. Interpret the Readings:
   • Open Circuit (Infinite Resistance): If your multimeter reads "OL" (overload) or infinity (very high resistance), it means there's a break in the coil's winding, and the coil is bad.
   • Short Circuit (Too Low Resistance): If the resistance reading is significantly lower than the specified value (especially for the secondary winding), it indicates a short circuit within the coil, and it is bad.
   • Within Specification: If both readings are within the manufacturer's specified range (typically ±10-20%), the coil is likely good, and the problem lies elsewhere in the ignition system (e.g., flywheel magnet gap, kill switch wiring).

Important Considerations:

• Coil-to-Flywheel Air Gap: A critical specification is the air gap between the ignition coil's laminations and the flywheel magnet. If this gap is too wide, the magnetic field won't be strong enough to induce proper voltage. You adjust this with a feeler gauge, typically to 0.010-0.014 inches.
• Kill Switch Wiring: A faulty kill switch or its wiring can prevent the coil from firing. Check for continuity.

Testing a lawn mower coil with a multimeter provides a precise way to diagnose its electrical health. If the readings are outside the manufacturer's specifications, replacement of the ignition coil is usually required.

How Do You Replace a Lawn Mower Ignition Coil?

When you need to replace a lawn mower ignition coil, it typically involves a few key steps: safely accessing the coil, carefully noting its wiring and placement, installing the new one, and correctly setting the crucial air gap between the coil and the flywheel. This repair ensures your ignition system can deliver a strong spark for reliable starting.

Tools You'll Need:

• New Ignition Coil: Ensure it's the correct part number for your specific lawn mower make and model.
• Wrench/Socket Set: For bolts holding the coil and engine shroud.
• Screwdriver: For wiring connections.
• Feeler Gauge: Essential for setting the air gap (typically 0.010-0.014 inches, check manual). A feeler gauge set is useful.
• Gloves, Safety Glasses.

Step-by-Step Guide to Replacing an Ignition Coil:

1. Perform All Safety Precautions:
   • Crucial: Disconnect the spark plug wire first. Ensure the engine is cool. Disconnect the battery (if electric start). (Refer to the safety section).
2. Access the Ignition Coil:
   • The ignition coil is usually located near the flywheel, often under a plastic engine shroud.
   • Use appropriate wrenches and screwdrivers to remove any covers or parts necessary to gain clear access to the coil.
3. Disconnect Wires:
   • Carefully disconnect the spark plug wire from the coil.
   • Disconnect any other wires attached to the coil (e.g., kill switch wire). Take a photo of the wiring before disconnecting to ensure correct reassembly.
4. Remove the Old Coil:
   • Using a socket or wrench, unscrew the bolts that hold the old ignition coil to the engine block.
   • Carefully remove the old coil.
5. Install the New Coil (Do NOT Tighten Yet):
   • Position the new ignition coil in place.
   • Loosely install the mounting bolts, but do NOT tighten them down yet. The coil needs to be able to move slightly for the next step.
6. Set the Air Gap (CRITICAL STEP):
   • Rotate Flywheel: Rotate the engine's flywheel by hand until the magnet on the flywheel is directly aligned beneath the coil's laminations (the two metal "legs" of the coil).
   • Insert Feeler Gauge: Take a feeler gauge of the correct thickness (typically 0.010-0.014 inches, refer to your mower's owner's manual for the exact specification).
   • Position: Slide the feeler gauge between the coil's laminations and the flywheel magnet.
   • Tighten Bolts: While holding the feeler gauge firmly in place, push the ignition coil firmly against the flywheel (with the gauge in between) and then tighten the mounting bolts securely. This ensures the correct air gap.
   • Remove Gauge: Once the bolts are tight, carefully slide out the feeler gauge. The coil should now be properly spaced.
7. Reconnect Wires:
   • Connect the kill switch wire(s) to the new coil, referring to your photo or manual.
   • Connect the spark plug wire to the new coil.
8. Reassemble Shrouds:
   • Reinstall any engine shrouds or covers you removed.
9. Reconnect Spark Plug Wire to Spark Plug:
   • Finally, reconnect the spark plug wire to the spark plug.
10. Test the Mower:
    • Attempt to start the lawn mower. With a new, properly installed ignition coil and a correct air gap, it should now start reliably.

Replacing a lawn mower ignition coil is a precise repair, and correctly setting the air gap is paramount for proper spark generation. Following these steps carefully will restore your lawn mower to full functionality.

What Are Other Common Reasons a Lawn Mower Won't Start?

If your lawn mower still won't start after you've checked the ignition coil, it's helpful to remember that many other common reasons can prevent an engine from firing up. A systematic troubleshooting approach is key, addressing the most frequent culprits first.

• Fuel System Issues (Very Common):
  • Stale Fuel: Gasoline can go bad in as little as 30 days, especially if it contains ethanol. Old fuel loses its volatility and can clog the carburetor.
    • Solution: Drain old fuel, refill with fresh, stabilized 87 octane unleaded gasoline.
  • No Fuel: Check the fuel tank level.
  • Clogged Fuel Filter: A dirty lawn mower fuel filter restricts fuel flow. Replace it annually.
  • Clogged Carburetor: The carburetor's tiny jets and passages can get gummed up by old fuel.
    • Solution: Clean the carburetor with a carburetor cleaner spray, or if severely clogged, disassemble and clean/rebuild.
  • Fuel Shut-off Valve: Ensure any fuel shut-off valve is in the "ON" or "OPEN" position.
• Spark Plug Issues (Very Common):
  • Fouled Spark Plug: A spark plug covered in black, oily, or carbon deposits won't produce a strong spark.
    • Solution: Clean the spark plug with a wire brush or replace it with a new one.
  • Incorrect Gap: The gap between the electrodes needs to be set precisely (check your manual).
  • Damaged Spark Plug: Cracks in the porcelain can short out the spark.
• Air Filter Issues:
  • Clogged Air Filter: A dirty or clogged lawn mower air filter starves the engine of air, leading to a rich fuel mixture that prevents starting.
    • Solution: Clean or replace the air filter. Try starting without it (briefly) to diagnose.
• Safety Interlocks (Extremely Common):
  • Most lawn mowers have safety switches that prevent starting if conditions aren't met.
  • Blade Engagement: Blades must be disengaged.
  • Parking Brake: Parking brake must be set.
  • Operator Present: You must be seated in the operator's seat.
  • Transmission: Transmission must be in neutral (riding mowers).
  • Solution: Methodically check each interlock.
• Battery Issues (for Electric Start Mowers):
  • Dead Battery: A common problem, especially after winter storage.
    • Solution: Test the battery with a multimeter or try charging it with a lawn mower battery charger.
  • Corroded Terminals: Clean any corrosion from battery terminals for a good connection.
  • Faulty Starter/Solenoid: If you hear a click but no cranking, the solenoid might be bad. If it whirs but doesn't engage, the starter motor might be failing.
• Engine Oil Level:
  • Low Oil: Some modern mowers have a low-oil sensor that will prevent the engine from starting or running to prevent damage.
    • Solution: Check the oil level with the dipstick and fill with appropriate small engine oil.

By systematically checking these various components, you can often diagnose why your lawn mower won't start, allowing you to get it back to work. If all else fails, a trip to a small engine repair shop is in order.

When your lawn mower refuses to fire up, knowing how to check a lawn mower coil becomes an invaluable skill in your troubleshooting arsenal. While not always the first component to suspect, a faulty ignition coil can certainly be the silent saboteur, preventing that crucial spark from igniting your engine. By methodically testing for spark, understanding resistance, and carefully performing a replacement if needed, you can restore your mower's vitality and keep your lawn looking pristine.