If your lawn mower won’t start, a faulty ignition coil can prevent spark, and testing it with an ohmmeter can confirm if it’s the culprit. Learning how to test an ignition coil on a lawn mower is a straightforward process that can save you time and money. This guide will walk you through the steps using a basic multimeter.
You don’t need to be a professional mechanic to perform this diagnostic. With a few common tools and some safety precautions, you can determine if the coil is the reason your engine refuses to turn over.
We’ll cover everything from understanding what the coil does to the exact resistance values you should look for. By the end, you’ll know exactly how to check this critical component.
How To Test An Ignition Coil On A Lawn Mower
The ignition coil is a simple transformer that boosts the low voltage from the battery (or magneto) into the high voltage needed to create a spark at the plug. When it fails, no spark occurs, and your engine remains silent. Testing it involves measuring its electrical resistance with a digital multimeter set to the ohms (Ω) function.
Before you begin, gather your tools and ensure the mower is in a safe state. You will need a digital multimeter, a socket or spark plug wrench, and possibly a pair of gloves. Always disconnect the spark plug wire before starting any work to prevent accidental starting.
Tools And Safety Equipment You Will Need
Having the right tools makes the job easier and safer. You likely have most of these items already in your garage.
- A digital multimeter (with an ohmmeter function)
- A socket wrench or spark plug socket to remove the spark plug
- A clean rag or brush
- Safety glasses to protect your eyes
- Work gloves (optional but recommended)
- The engine’s service manual for specific resistance values (if available)
Remember, safety first. Always disconnect the spark plug lead and secure it away from the plug terminal. This ensures the engine cannot start while you are handling the ignition components.
Step-By-Step Testing Procedure
Follow these steps carefully to get an accurate reading. The process is similar for most push and riding mowers, though coil location may vary.
Step 1: Locate And Remove The Ignition Coil
First, you need to find the coil. It’s usually mounted near the flywheel, with a thick wire running to the spark plug. You may need to remove a plastic shroud or engine cover to access it. Consult your mower’s manual if you’re unsure.
Once located, disconnect the spark plug wire and the kill wire (a smaller wire that grounds the coil to stop the engine). Then, unscrew the one or two bolts holding the coil to the engine block. Carefully remove the coil, taking note of any spacers or washers.
Step 2: Identify The Coil Terminals
With the coil in hand, identify its terminals. A typical lawn mower ignition coil has two primary terminals: one for the kill wire/ground and one that connects internally to the spark plug terminal. The spark plug terminal is the large, central tower where the plug wire attaches.
Some coils have the primary terminals as small metal tabs or spade connectors. Others might have them as screw posts. Clean any dirt or corrosion from the terminals with your rag for a better connection with the multimeter probes.
Step 3: Set Your Multimeter To Ohms
Turn on your digital multimeter and set the dial to the resistance setting, marked with the Omega (Ω) symbol. For primary coil testing, you’ll usually use the lowest scale, often 200Ω. For secondary (spark plug terminal) testing, you may need a higher scale, like 20kΩ.
Touch the two meter probes together. The display should read close to 0.0 ohms. This confirms your meter is working correctly before you test the coil. If it doesn’t, check the probes or the meter’s battery.
Step 4: Test The Primary Coil Windings
The primary winding is the low-voltage side of the coil. To test it, place one multimeter probe on one of the small primary terminals and the other probe on the second primary terminal. You are measuring the resistance across these two points.
A typical reading for a healthy primary winding is between 0.1 and 2.0 ohms. Consult your manual for the exact specification. A reading of infinite resistance (often displayed as “OL” or “1”) means the winding is open and the coil is bad. A reading of zero ohms indicates a short.
Step 5: Test The Secondary Coil Windings
This tests the high-voltage side. Place one multimeter probe on one of the primary terminals. Place the other probe inside the spark plug boot or on the metal terminal inside the coil tower where the spark plug wire connects.
Resistance here is much higher. Expect a reading between 2,500 and 5,000 ohms (2.5kΩ to 5kΩ) for most mower coils. Again, an “OL” reading indicates an open circuit and a failed coil. Consistency with your engine’s specified range is key.
Step 6: Test For A Ground Fault
Finally, you need to check if the coil is shorted to ground. This is a common failure. Place one multimeter probe on the metal mounting bracket of the coil (not a painted area). Touch the other probe to the spark plug terminal, then to each of the small primary terminals.
Each of these tests should show infinite resistance (“OL”). Any low resistance reading here means the coil’s insulation has broken down and it is shorting to ground. This coil must be replaced.
Interpreting Your Test Results
Understanding what the numbers mean is crucial. If all your readings fall within the expected ranges and show no ground fault, your ignition coil is likely functioning correctly. The problem may lie elsewhere, such as with the spark plug, flywheel key, or fuel system.
If any one test fails—showing infinite resistance, zero resistance, or a short to ground—the ignition coil is defective. It cannot produce the necessary spark and needs to be replaced. There is no repair for a faulty coil; replacement is the only option.
Keep in mind that resistance can vary with temperature. A coil that tests okay cold might fail when hot. If your mower has an intermittent starting problem, this could be the cause.
Common Symptoms Of A Bad Ignition Coil
Testing is the definitive method, but these symptoms often point to a failing coil. If you notice any of these, it’s time to get your multimeter out.
- The engine will not start or is very difficult to start.
- The mower starts but then stalls out quickly and won’t restart.
- You notice obvious physical damage on the coil, like cracks or burns.
- The engine runs rough or misfires under load.
- There is no visible spark when checking the spark plug (perform a spark test safely).
It’s important to note that these symptoms can also be caused by other issues. A spark plug fouled with oil or carbon, a sheared flywheel key, or a faulty kill switch can mimic a bad coil. Testing eliminates the guesswork.
Tips For Replacing A Faulty Ignition Coil
If your tests confirm a bad coil, replacement is your next step. Here are some tips to ensure a smooth installation.
- Purchase the correct replacement coil for your specific engine model and number.
- Before installing the new coil, clean the mounting area on the engine thoroughly.
- Many coils require a precise air gap between the coil armature and the flywheel. Use a feeler gauge (often the thickness of a business card) to set this gap as specified in your manual.
- Do not overtighten the mounting bolts, as this can crack the coil’s housing.
- Recconnect all wires securely and double-check your work before attempting to start the engine.
A proper installation is just as important as the testing. A new coil installed with the wrong gap or loose connections will not work correctly and could lead to further problems.
Frequently Asked Questions
What Is The Most Common Way To Test A Lawn Mower Coil?
The most common and reliable way is to measure its primary and secondary resistance with a digital multimeter, as outlined above. This provides a definitive electrical diagnosis. A visual inspection for cracks or burns is also a good first step.
Can You Test An Ignition Coil Without A Multimeter?
You can perform a basic spark test, but it is less definitive and requires caution. 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. No spark suggests a problem with the coil, plug, or kill circuit. However, a coil can sometimes produce a weak spark outside the engine but fail under compression, so a multimeter test is more accurate.
What Should The Resistance Be On A Lawn Mower Ignition Coil?
Resistance values vary by manufacturer. Typically, primary winding resistance is very low, often between 0.1 and 2.0 ohms. Secondary winding resistance (from a primary terminal to the spark plug terminal) is much higher, usually in the 2,500 to 5,000 ohm range. Always check your engine’s service manual for the exact specifications, as some coils can have secondary resistance up to 15kΩ.
Why Does My Mower Coil Keep Failing?
Repeated coil failure is uncommon and points to an underlying issue. Common causes include excessive engine heat due to cooling fins being clogged with grass, a faulty charging system sending too high a voltage, vibration from loose mounting, or moisture ingress due to a cracked casing. Address the root cause to protect your new coil.
By following this guide, you can confidently diagnose your lawn mower’s ignition system. Remember, working with small engines requires patience and attention to detail. With a simple multimeter, you’ve taken a major step towards becoming more self-reliant in your lawn care maintenance.