To check a lawn mower ignition coil, start by ensuring safety and disconnecting the spark plug wire. Perform a visual inspection for obvious damage or corrosion. The most reliable diagnostic method involves using a multimeter to measure the electrical resistance of the primary and secondary windings, comparing your readings to the manufacturer’s specifications. A functional spark test with an inline spark tester can also confirm if the coil is generating a spark. Common symptoms of a bad coil include no spark, intermittent spark, or engine misfires.

The ignition coil, often part of a magneto system on small engines, is a critical component that transforms the low voltage from the flywheel’s magnets into the high voltage needed to create a spark at the spark plug. When it fails, the engine won’t start or will run poorly. Diagnosing it systematically can save you time and money.

Understanding the Ignition Coil and Common Symptoms

Illustration of a lawn mower ignition system showing the flywheel, coil, and spark plug.

The ignition coil’s primary function is to act as a transformer. As the engine’s flywheel spins, magnets pass by the coil, inducing a small electrical current in its primary winding. The coil then steps up this voltage to thousands of volts in its secondary winding, which is sent through the spark plug wire to ignite the air-fuel mixture in the cylinder.

Before you begin testing, it’s helpful to recognize the common signs of a failing ignition coil. These symptoms can help confirm that the coil is the likely culprit:

  • No Spark: The most definitive symptom. If there is no spark at the spark plug, the engine will not start.
  • Intermittent Spark: The engine may start and run roughly, then stall, or may have difficulty starting consistently. This often indicates a coil that is breaking down under heat or load.
  • Engine Misfires or Runs Poorly: A weak spark can cause incomplete combustion, leading to rough idling, loss of power, or hesitation.
  • Visible Damage: Cracks in the coil’s housing, burnt or corroded electrical terminals, or oil contamination are physical red flags.

Safety Precautions and Tools Needed

Working on any engine requires safety first. Always consult your lawn mower’s owner’s manual for specific instructions and warnings.

Essential Safety Steps

  1. Disconnect the Spark Plug Wire: This is the most critical step to prevent the engine from accidentally starting while you are working on it. Pull the rubber boot firmly off the spark plug.
  2. Work on a Cool Engine: Allow the engine to cool completely to avoid burns.
  3. Work in a Well-Ventilated Area: Especially if you need to start the engine for a spark test, ensure proper ventilation to avoid exhaust fumes.
  4. Wear Safety Glasses: Protect your eyes from debris or accidental sparks.

Tools Required for Testing

Having the right tools makes the job straightforward. At a minimum, you will need:

  • Multimeter: A basic digital multimeter capable of measuring resistance (ohms) is essential for the most accurate electrical test.
  • Inline Spark Tester: This inexpensive tool is designed to safely check for spark. It connects between the spark plug wire and the spark plug (or ground) and provides a clear visual indicator.
  • Socket Set or Wrenches: To remove any engine covers or components that may be blocking access to the ignition coil.
  • Wire Brush or Electrical Contact Cleaner: For cleaning corroded terminals during inspection.

Step-by-Step Guide: Visual Inspection

Before breaking out the multimeter, a thorough visual inspection can often reveal the problem or rule out simple issues.

How to Perform the Inspection

  1. Access the Coil: On many push mowers, you may need to remove the engine cover. On riding mowers, the coil is typically mounted near the flywheel under the engine shroud. Refer to your manual for access instructions.
  2. Examine the Coil Body: Look for any cracks, chips, or signs of melting in the plastic housing. Physical damage can compromise the insulation between windings.
  3. Check the Electrical Terminals: Inspect the metal posts where the spark plug wire and any kill switch wires attach. Look for corrosion, rust, or loose connections. Clean any corrosion with a wire brush or electrical cleaner.
  4. Inspect the Spark Plug Wire: Follow the wire from the coil to the spark plug boot. Check for cracks, cuts, or brittleness in the insulation. A damaged wire can leak voltage.
  5. Look for Oil or Grease Contamination: Oil on the coil can attract dirt and create a conductive path that shorts out the high voltage.

Step-by-Step Guide: Testing with a Multimeter

If the visual inspection doesn’t reveal a clear issue, a multimeter test is the next logical step. This test measures the electrical resistance within the coil’s internal windings. A coil with an open circuit (infinite resistance) or a short circuit (zero or very low resistance) is faulty.

Important Note on Specifications

Always try to find the specific resistance specifications for your lawn mower’s make and model. These values can vary. The ranges provided below are common general guidelines, but your manual or a service guide is the authoritative source.

Testing the Primary Winding

  1. Set Your Multimeter: Turn the dial to the lowest ohms (Ω) setting, typically the 200Ω range.
  2. Identify the Terminals: The primary winding is connected to the low-voltage side. This usually includes the main terminal where the spark plug wire attaches and a smaller terminal for the kill switch wire (often a small tab or screw). Consult a diagram for your specific coil if available.
  3. Connect the Probes: Touch one multimeter probe to the main spark plug wire terminal and the other to the kill switch terminal.
  4. Read the Resistance: A healthy primary winding typically shows a very low resistance, often in the range of 0.5 to 2.0 ohms. If the meter reads “OL” (open loop) or infinite resistance, the winding is broken. If it reads zero or near zero, it may be shorted.

Testing the Secondary Winding

  1. Set Your Multimeter: Turn the dial to a higher ohms setting, typically the 20kΩ (20,000 ohms) range.
  2. Identify the Terminals: The secondary winding is connected between the spark plug wire terminal and the metal core or mounting bracket of the coil (which is grounded to the engine).
  3. Connect the Probes: Touch one probe to the spark plug wire terminal and the other probe to a clean, unpainted metal part of the coil’s mounting bracket or core.
  4. Read the Resistance: A healthy secondary winding typically shows a much higher resistance, commonly in the range of 2,500 to 5,000 ohms (2.5kΩ to 5kΩ). Again, “OL” indicates an open circuit, and zero indicates a short.

Step-by-Step Guide: Performing a Spark Test

A multimeter test confirms the coil’s electrical integrity, but a spark test confirms its functional ability to produce a spark under simulated engine conditions.

Using an Inline Spark Tester

  1. Connect the Tester: Unscrew the spark plug. Attach one end of the inline spark tester to the end of the spark plug wire. Attach the other end of the tester to the spark plug.
  2. Ground the Spark Plug: Screw the spark plug (now connected via the tester) back into the engine cylinder head to ensure it is properly grounded.
  3. Reconnect the Spark Plug Wire: Firmly push the spark plug wire boot onto the top terminal of the spark tester.
  4. Crank the Engine: With the spark plug wire disconnected from the original plug and the tester in place, pull the starter cord (for a push mower) or have someone turn the key (for a riding mower) to crank the engine. Ensure your hands and body are clear of the spark plug and tester.
  5. Observe for Spark: A healthy ignition system will produce a bright, consistent blue spark visible inside the tester’s window. No spark, a weak yellow spark, or an inconsistent spark indicates a problem with the coil, spark plug wire, or related components.

Basic Spark Test Without a Dedicated Tester

Caution: This method is less safe and less reliable. Use an inline tester whenever possible.

  1. Remove the Spark Plug: As above.
  2. Reconnect the Wire: Reattach the spark plug wire boot to the spark plug.
  3. Ground the Spark Plug: Using insulated pliers, hold the metal base of the spark plug firmly against a clean, unpainted metal part of the engine block (like a bolt head).
  4. Crank the Engine: Pull the starter cord. Watch the gap between the spark plug’s electrode and the engine metal. You should see a spark jump the gap. Do not hold the spark plug with your bare hands.

Interpreting Results and Next Steps

After completing the tests, you can determine the condition of your ignition coil and decide on the next course of action.

If the Coil Passes the Tests

If the coil shows correct resistance values and produces a strong, consistent spark, the ignition coil is likely functioning properly. The problem may lie elsewhere in the ignition system or fuel system. Consider checking:

  • The spark plug itself (for fouling or wear).
  • The spark plug wire for internal breaks (even if it looks okay externally).
  • The flywheel key (a sheared key can throw off ignition timing).
  • Fuel delivery issues (clogged carburetor, stale fuel).

If the Coil Fails the Tests

If the multimeter shows open or shorted windings, or if there is no spark during the functional test, the ignition coil is faulty and should be replaced.

  • Cleaning: If the only issue was heavy corrosion on the terminals that you’ve cleaned, re-test the coil. Sometimes this is all that’s needed.
  • Adjustment: On some magneto systems, the air gap between the coil and the flywheel magnets can be adjusted. An incorrect gap can affect spark strength. This adjustment is specific and should be done according to the service manual specifications.
  • Replacement: If the coil is internally damaged (failed resistance test) or physically cracked, replacement is the only reliable solution. Purchase a replacement coil compatible with your lawn mower’s engine model and follow the installation instructions in your manual.

By following this systematic approach—starting with safety and visual inspection, moving to precise multimeter measurements, and finishing with a functional spark test—you can confidently diagnose whether the ignition coil is the source of your lawn mower’s starting troubles.