If your lawn mower won’t start, learning how to test a lawn mower ignition coil is a straightforward diagnostic process that requires a multimeter. This simple check can save you time and money by pinpointing a common failure point before you replace other parts.
An ignition coil, or magneto, is crucial for creating the spark that ignites the fuel. When it fails, your engine gets no spark and will not run. Testing it with a multimeter is a reliable way to confirm its condition.
This guide will walk you through the entire process, from safety to interpreting your multimeter readings. You’ll learn the steps to determine if your coil is good or needs replacement.
How To Test A Lawn Mower Ignition Coil
Testing the coil involves measuring its electrical resistance with a digital multimeter. You will check for continuity and specific resistance values between the coil’s terminals. A reading outside the specified range typically indicates a faulty coil that must be replaced.
Before you begin, gather a few tools. You will need a digital multimeter, a set of basic hand tools like a socket or spark plug wrench, and a clean rag. Always disconnect the spark plug wire first to prevent any accidental starts.
Safety is the most important step. Make sure the engine is completely cool before you start working on it. Remove the spark plug wire and secure it away from the plug to eliminate any chance of a spark.
Understanding The Ignition Coil And Its Function
The ignition coil is a transformer that steps up voltage. It takes the low voltage from the engine’s flywheel magnet and converts it into a high-voltage pulse. This pulse, often over 10,000 volts, travels down the spark plug wire to create a spark at the plug’s electrode.
There are two main types on small engines: coil-on-plug designs integrated with the plug boot, and traditional coils mounted near the flywheel. Both types function on the same principle and can be tested similarly.
Common symptoms of a failing ignition coil include a complete failure to start, the engine dying when it gets hot, or intermittent operation. Physical signs can be cracks, burns, or corrosion on the coil body or its terminals.
Tools And Safety Precautions You Will Need
Having the right tools makes the job easier and safer. A digital multimeter is essential for accurate resistance readings. An analog meter will not provide the precision needed for this test.
You will also need tools to access the coil, such as a socket set or screwdrivers to remove any shrouds or the coil itself. A spark plug wrench is handy for removing the plug. A service manual for your specific engine model is very useful for exact resistance specifications.
Follow these safety rules without exception:
- Disconnect the spark plug wire and ensure it cannot contact the plug.
- Work in a well-lit, dry area with the engine off and cool.
- Take pictures or notes of wire connections before disconnecting anything.
- Wear safety glasses to protect your eyes from debris.
Selecting The Correct Multimeter Setting
Set your digital multimeter to the resistance setting, marked with the Omega symbol (Ω) for ohms. For most coil tests, you will use the lowest scale, often 200 ohms or 20K ohms, depending on your meter. Consult your multimeter’s manual if you are unsure.
Before testing the coil, check your multimeter is working properly. Touch the two probes together. The display should read very close to 0 ohms, indicating good continuity. This verifies your meter and leads are functional.
Step-By-Step Guide To Testing The Coil
Now, let’s go through the testing procedure step by step. Follow these instructions carefully to get an accurate diagnosis of your lawn mower’s ignition coil.
Step 1: Locate And Access The Ignition Coil
First, you need to find the coil. It is usually located near the flywheel, which is under a blower housing on the side of the engine. Follow the thick spark plug wire back from the plug; it will lead you to the coil.
You may need to remove a plastic shroud or metal cover held by a few screws. Keep all fasteners organized. Once exposed, you will see the coil mounted with one or two screws or bolts adjacent to the flywheel.
Step 2: Disconnect Wires And Identify Terminals
Carefully disconnect the spark plug wire from the coil terminal. Some coils also have a thin kill wire that must be disconnected. This wire grounds the coil to stop the engine when you release the safety lever.
Identify the two primary terminals you will test: the high-tension terminal where the spark plug wire attaches and the ground terminal where the coil mounts to the engine. Some coils have a secondary low-voltage terminal for the kill wire.
Step 3: Test Primary Coil Resistance
This test checks the windings between the coil’s ground and the spark plug terminal. Set your multimeter to a low ohms scale, like 200Ω. Place one probe on the metal mounting leg of the coil and the other on the inside of the spark plug wire terminal.
A typical reading for a healthy primary circuit is between 0.5 and 3.0 ohms. Consult your engine manual for the exact specification. A reading of infinite resistance (OL or 1) means the primary winding is open and the coil is bad. A reading of 0 ohms indicates a short.
Step 4: Test Secondary Coil Resistance
This test checks the high-voltage windings. Set your multimeter to a higher scale, like 20K ohms. Place one probe in the spark plug wire boot and the other on the coil’s ground or mounting leg. You may need to remove the rubber boot from the coil to access the metal terminal.
Normal secondary resistance is usually between 2,500 and 10,000 ohms (2.5K to 10K ohms). Again, check your manual. An infinite (OL) or zero reading here also indicates a failed coil. The secondary winding is more prone to failure due to the extreme voltage it handles.
Step 5: Test For Ground Fault
A ground fault occurs when the coil’s internal windings short to its external metal case. Set your multimeter to its highest resistance scale or continuity setting. Place one probe on the coil’s metal body and touch the other probe to each electrical terminal.
The meter should read infinite resistance (OL) for each terminal. Any low resistance or continuity reading between a terminal and the case means the coil is shorted and must be replaced. This is a common failure after the coil overheats.
Interpreting Your Multimeter Readings
Understanding what the numbers mean is key to your diagnosis. A good coil will show specific resistance values within the expected range for both primary and secondary tests, with no connection to ground.
If any test shows infinite resistance (OL), it means the circuit is open and electricity cannot flow through the windings. This is a definitive sign of a broken coil. The internal wire has snapped, often from vibration or heat stress.
If any test shows zero or very low resistance where it shouldn’t, it indicates a short circuit. The insulation between wire winds has broken down, allowing current to take a shortcut. This also renders the coil useless.
Remember, resistance values can vary by manufacturer. Always prioritize the specifications from your engine’s service manual over general guidelines. A reading that is slightly off may still indicate a problem.
What To Do If Your Ignition Coil Is Faulty
If your tests confirm the coil is bad, replacement is the only option. Ignition coils cannot be repaired. Purchase a direct replacement part using your engine’s model number, which is stamped on the blower housing or valve cover.
Installation is the reverse of removal. Clean the mounting surface on the engine thoroughly. When installing the new coil, you must set the correct air gap between the coil armature and the flywheel magnets. This is a critical step.
To set the air gap, loosen the mounting screws slightly. Place a business card or a feeler gauge of the specified thickness between the coil leg and the flywheel. Rotate the flywheel so the magnets are directly under the coil. Tighten the screws, remove the card, and rotate the flywheel to ensure it doesn’t rub.
Recconnect the spark plug wire and any kill wires. Reinstall any covers or shrouds you removed. Your lawn mower should now be ready for a test start.
Additional Troubleshooting Tips
If you have a good coil but still no spark, other components could be at fault. The problem might not be the coil itself. Systematically check these other parts before giving up.
Inspect the spark plug wire for cracks or brittleness. Check the spark plug itself by doing a visual inspection for fouling or cracks, or test it with a spark plug tester. The kill switch or safety interlock system can also ground out the coil; ensure those wires are properly connected and not shorted.
Finally, check the flywheel key. If the key is sheared, the flywheel will be out of time, and the magnets will not align correctly with the coil, preventing spark generation. This often happens if the mower blade hits a solid object.
Frequently Asked Questions
What Are The Symptoms Of A Bad Ignition Coil On A Lawn Mower?
The most common symptom is the engine not starting at all, with no spark at the plug. Other signs include the engine running poorly, dying when hot, or backfiring. Intermittent operation, where the mower works one day but not the next, can also point to a failing coil.
Can You Test A Lawn Mower Coil Without A Multimeter?
There is a basic spark test, but it is less reliable and can be hard on the coil. Remove the spark plug, reconnect it to the wire, and ground the plug’s metal body against the engine. Pull the starter cord and look for a bright blue spark across the gap. No spark suggests a problem, but a weak spark can be misleading. A multimeter test is more definitive.
What Is The Correct Air Gap For A Lawn Mower Ignition Coil?
The air gap is typically between 0.008 and 0.012 inches, but you must check your engine’s manual. A gap that is too wide prevents a strong spark; a gap that is too close risks the coil contacting the spinning flywheel, causing damage. Using a feeler gauge or a standard business card is a common method for setting it.
Why Does My Lawn Mower Coil Keep Failing?
Repeated coil failure can be caused by excessive heat from a dirty engine, improper air gap causing physical contact and heat, vibration from loose mounting, or problems in the charging system sending too much voltage. Addressing the root cause is essential to prevent the new coil from failing prematurely.