Is your trusty lawn mower refusing to start? You’ve checked the fuel, the spark plug looks okay, but still nothing. It can be incredibly frustrating when your grass is getting out of control and your mower is dead in the water.
One of the most common culprits for a no-start situation is a faulty ignition coil. This small but vital component is responsible for delivering the spark that ignites the fuel-air mixture in your engine. If it’s not working correctly, your mower simply won’t fire up.
Don’t worry, though! Checking your lawn mower’s ignition coil is a task that most DIY enthusiasts can tackle. We’ll walk you through the process step-by-step, explaining what you need, what to look for, and how to interpret your findings. Get ready to diagnose and potentially fix your mower!
Understanding the Lawn Mower Ignition Coil
Before we dive into the ‘how-to,’ it’s essential to understand what the ignition coil is and what it does. Think of it as the heart of your mower’s ignition system. It’s a transformer that takes the low voltage from your mower’s battery (or magneto) and steps it up to a very high voltage (tens of thousands of volts). This high voltage is then sent to the spark plug, creating the spark that ignites the fuel and starts your engine.
Key Components of the Ignition System:
- Ignition Coil: The transformer that boosts voltage.
- Spark Plug: Creates the spark.
- Points (on older models): A mechanical switch that controls when the coil is energized.
- Condenser (on older models): Works with the points to create a strong spark.
- Magneto (on some models): Generates electricity for the ignition system.
- Ignition Switch: Turns the ignition system on and off.
When your mower won’t start, and you’ve ruled out basic issues like no fuel or a fouled spark plug, the ignition coil becomes a prime suspect. A weak or completely dead coil means no spark, and therefore, no engine operation.
Symptoms of a Bad Ignition Coil
Recognizing the signs of a failing ignition coil can save you a lot of diagnostic time. Here are some common symptoms:
- No Spark: This is the most obvious sign. The engine cranks but won’t fire up.
- Intermittent Misfiring: The mower might start and run for a bit, then sputter and die, or run rough and unevenly.
- Loss of Power: The mower might feel sluggish and lack its usual power, especially under load.
- Engine Backfiring: Unburnt fuel igniting in the exhaust can be a sign of an ignition issue.
- Difficulty Starting: The engine may crank for a very long time before eventually starting, or not start at all.
If you’re experiencing one or more of these issues, it’s time to investigate the ignition coil.
Tools and Materials You’ll Need
Before you start, gather the necessary tools. Having everything ready will make the process smoother and safer.
- Multimeter: This is your primary diagnostic tool. You’ll need one that can measure resistance (ohms or Ω) and ideally voltage (though resistance testing is usually sufficient for a coil).
- Screwdriver Set: To remove engine shrouds and the coil itself.
- Socket Set/Wrenches: For removing bolts.
- Spark Plug Gap Tool: To check spark plug gap (optional, but good practice).
- Contact Cleaner/Degreaser: To clean connections.
- Shop Rags: For cleaning up.
- Safety Glasses: Always protect your eyes!
- Gloves: To keep your hands clean and for grip.
- Service Manual (Recommended): For your specific mower model, this will provide exact resistance values and troubleshooting steps.
Safety First!
Working on any engine involves potential hazards. Always prioritize safety: (See Also: How Much Does A New Lawn Cost )
- Disconnect the Spark Plug Wire: Before doing any work, disconnect the spark plug wire from the spark plug. This prevents accidental starting.
- Allow the Engine to Cool: If you’ve recently run the mower, let the engine cool down completely.
- Work in a Well-Ventilated Area: Especially if you’re using any cleaning solvents.
- Disconnect the Battery (if applicable): For electric start models, disconnect the battery.
Step-by-Step Guide: How to Check Lawn Mower Coil
We’ll cover two main methods for checking the coil: a resistance test using a multimeter and a visual inspection. For older mowers with points and condensers, the process is slightly different, which we’ll also touch upon.
Method 1: Resistance Testing (most Common)
This is the most reliable way to determine if your ignition coil is functioning correctly. You’ll be measuring the resistance of the primary and secondary windings within the coil.
Step 1: Locate the Ignition Coil
The ignition coil is typically mounted on the engine block, near the flywheel. It often has a metal bracket that holds it close to the flywheel’s magnets. You’ll usually see the spark plug wire coming from it.
Identifying the Coil:
- Look for a black, rectangular or cylindrical component.
- It will have at least two wires coming out of it (or terminals for wires). One is usually the spark plug wire (or the wire leading to it), and the other(s) are for power and ground.
- It’s often positioned directly above or beside the flywheel.
You may need to remove the engine’s top shroud or cooling fins to get clear access to the coil. Consult your mower’s service manual if you’re unsure.
Step 2: Disconnect Wires
Carefully disconnect any wires attached to the ignition coil. Note where each wire connects so you can reassemble it correctly. You’ll typically need to disconnect:
- The spark plug wire (or the lead from the coil to the spark plug).
- Any kill switch wires or wires from the magneto/generator.
Important: Do NOT disconnect the spark plug wire from the spark plug itself yet if you plan to do a spark test later. For resistance testing, you’re just disconnecting the wire *from the coil*.
Step 3: Set Your Multimeter
Set your multimeter to measure resistance (Ω). You’ll likely have different ranges (e.g., 200Ω, 2kΩ, 20kΩ). Start with a lower range and adjust as needed.
Step 4: Test the Primary Coil Winding
The primary winding is the lower voltage side of the coil. It’s typically a few ohms. (See Also: How To Edge A Lawn With A Weed Wacker )
- Identify the Primary Terminals: On most coils, there will be two small terminals. These are where the power from the magneto (or battery) comes in and goes out to ground (or the kill switch).
- Place Multimeter Probes: Touch one probe of the multimeter to one primary terminal and the other probe to the other primary terminal.
- Read the Resistance: The multimeter should display a resistance reading.
What to Look For (Primary):
- Good Coil: Typically, you’ll get a reading between 0.5 and 5 ohms. The exact value varies significantly by manufacturer and model. Consult your service manual for the specific resistance range for your coil.
- Bad Coil (Open Circuit): If the multimeter reads ‘OL’ (Over Limit) or infinity, it means the primary winding is broken, and the coil is bad.
- Bad Coil (Short Circuit): A reading of 0 ohms or very close to it usually indicates a short circuit within the winding, meaning the coil is bad.
Step 5: Test the Secondary Coil Winding
The secondary winding is the high voltage side that goes to the spark plug. It has many more turns of wire, so its resistance will be much higher.
- Identify the Secondary Terminal: This is usually the terminal where the spark plug wire connects (or the internal connection to it).
- Place Multimeter Probes: Touch one probe to the internal terminal of the spark plug wire connection on the coil, and the other probe to a good ground point on the engine block or the coil’s mounting bracket. Sometimes, one of the primary terminals can also be used as a ground reference for the secondary test if the coil is designed that way (check your manual).
- Read the Resistance: The multimeter will display a higher resistance reading.
What to Look For (Secondary):
- Good Coil: You’ll typically see readings between 5,000 and 15,000 ohms (5kΩ to 15kΩ), but this can range much higher, sometimes up to 20,000 ohms or more. Again, your service manual is the best source for the correct range.
- Bad Coil (Open Circuit): If the multimeter reads ‘OL’ or infinity, the secondary winding is broken, and the coil is bad.
- Bad Coil (Short Circuit): A very low resistance reading (close to 0 ohms) indicates a shorted secondary winding, meaning the coil is bad.
Step 6: Reassemble and Test (if Coil Seems Okay)
If your resistance readings are within the expected range according to your service manual, the coil itself is likely good. Reconnect all wires securely. Now you can perform a spark test to confirm if the issue lies elsewhere.
Method 2: Spark Test
This test confirms if the coil is producing a spark. It’s a good follow-up if your resistance test was inconclusive or if you suspect a weak spark.
Step 1: Reconnect the Coil and Spark Plug Wire
Ensure the ignition coil is properly mounted and all wires are reconnected to it. Reconnect the spark plug wire to the spark plug. Make sure the spark plug is clean and properly gapped.
Step 2: Create a Ground for the Spark Plug
This is crucial for safety and to ensure you see the spark correctly. You need to ground the metal body of the spark plug to the engine block.
- Remove the Spark Plug Wire from the Spark Plug: You want the spark plug to be removed from the cylinder head for this test.
- Reinsert the Spark Plug into the Wire: Put the spark plug back into the end of the spark plug wire.
- Position the Spark Plug: Hold the spark plug by its insulated boot, ensuring the metal body (ground electrode) is firmly touching a clean, unpainted metal part of the engine block. DO NOT hold the metal part of the spark plug itself, as you could receive a shock.
Step 3: Crank the Engine
Have someone crank the engine using the pull cord or electric starter. Watch the gap of the spark plug.
Step 4: Observe for Spark
You should see a strong, blue spark jump across the gap of the spark plug. (See Also: How Often Can I Spray Weed Killer On My Lawn )
What to Look For:
- Good Spark: A bright, blue spark that is consistent.
- Weak Spark: A weak, yellowish, or intermittent spark. This could indicate a failing coil, a problem with the points/condenser, or a low battery on electric start models.
- No Spark: If there is no spark at all, the coil is likely the problem, or there’s an issue with the ignition circuit (kill switch, wiring, points).
Safety Note: Do not crank the engine for extended periods. A few seconds at a time is sufficient.
Method 3: Visual Inspection
While not as definitive as a multimeter test, a visual inspection can sometimes reveal obvious problems.
- Remove Engine Shrouds: Gain access to the ignition coil.
- Check for Physical Damage: Look for any cracks, burns, or melting on the coil’s housing. If the plastic casing is damaged, the coil is almost certainly bad.
- Inspect Wires and Connections: Check the spark plug wire for cracks, fraying, or a loose connection at the coil. Ensure all wires connected to the coil are secure and free of corrosion.
- Check for Gaps (Coil to Flywheel): The ignition coil needs to be very close to the flywheel’s magnets to generate a strong spark. Typically, the gap should be around 0.010 to 0.030 inches (0.25 to 0.76 mm). If the coil is bent or the mounting screws are loose, this gap could be too large, resulting in a weak or no spark. You can often adjust this gap by loosening the coil mounting screws, inserting a piece of paper or a feeler gauge of the correct thickness between the coil and flywheel, and then tightening the screws. Remove the paper/gauge afterward.
Troubleshooting Older Mowers (points and Condenser)
Some older lawn mower engines use a mechanical ignition system with points and a condenser instead of, or in addition to, a solid-state ignition module. If your mower has these:
- Access the Points: You’ll need to remove the flywheel to access the points and condenser. This is a more involved process.
- Inspect the Points: Look for pitting, burning, or corrosion on the contact surfaces. Clean them with contact cleaner and a piece of cardstock or a specialized points file.
- Check Point Gap: The points need to open and close at the correct time and with the correct gap. This is usually set with a feeler gauge when the points are fully open. Consult your service manual for the correct gap.
- Test the Condenser: Condensers can be tested with a multimeter, but they are also prone to failure. They are relatively inexpensive to replace.
- Ignition Coil Resistance Test: The ignition coil itself can still be tested for resistance as described in Method 1.
Interpreting Your Results
After performing the tests, you should have a clearer picture:
- Resistance readings out of spec (OL, 0 ohms, or far from manual spec): The ignition coil is likely bad.
- No spark during spark test (after confirming good resistance): Could be a kill switch issue, faulty wiring, or a problem with the points/condenser (if applicable).
- Weak spark during spark test (after confirming good resistance): Could be a slightly failing coil, incorrect coil-to-flywheel gap, or issues with points/condenser.
- Physical damage to the coil: The coil is bad.
If your resistance tests are good and you’re still not getting a spark, the problem might be elsewhere in the ignition system, such as the kill switch wire (which can short the coil to ground), the wiring harness, or the points/condenser on older models.
Common Pitfalls to Avoid
- Incorrect Multimeter Settings: Ensure you’re on the correct resistance (Ω) setting.
- Dirty Connections: Corroded or dirty terminals can give false readings. Clean them thoroughly.
- Inaccurate Resistance Values: Always refer to your mower’s service manual for the precise resistance specifications. Generic values can be misleading.
- Ignoring Safety Precautions: Never test for spark by holding the spark plug with your bare hands.
- Assuming the Coil is Bad Too Soon: Rule out simpler issues like a dead spark plug or bad spark plug wire first.
By following these steps, you can effectively diagnose whether your lawn mower’s ignition coil is the source of your starting problems.
When to Seek Professional Help
While checking the ignition coil is a manageable DIY task, there are times when it’s best to call in a professional:
- If You’re Uncomfortable: If you don’t feel confident working with engine components or electrical systems, it’s safer to let a qualified technician handle it.
- Complex Ignition Systems: Some modern mowers have more complex electronic ignition modules that can be harder to diagnose without specialized tools.
- Multiple Issues: If you’ve tested the coil and it seems fine, but the mower still won’t start, there might be several underlying problems that require expert troubleshooting.
- Lack of Tools or Manual: If you don’t have a multimeter or the service manual for your specific mower, it can be difficult to perform an accurate diagnosis.
Don’t hesitate to take your mower to a local repair shop if you’re stuck. They have the experience and equipment to get it running again quickly.
Conclusion
Checking your lawn mower’s ignition coil is a crucial step in diagnosing starting problems. By using a multimeter to test the primary and secondary winding resistances and performing a spark test, you can often pinpoint whether the coil is the culprit. Remember to always prioritize safety, disconnect the spark plug wire, and consult your mower’s service manual for specific resistance values.
