How To Test Lawn Mower Coil With Ohm Meter

Alright, let's talk about troubleshooting a lawn mower that's giving you fits. One of the most common culprits for a no-start or poor-running engine is a faulty ignition coil. The coil's job is to amplify the low voltage from the mower's charging system into a high voltage surge – enough to create a spark at the spark plug and ignite the fuel-air mixture. Fortunately, testing a lawn mower coil with an ohm meter is a straightforward process. This guide will walk you through it, assuming you've already got some basic mechanical and electrical experience.

Understanding the Ignition Coil

Before we dive into testing, let's briefly review what the coil actually *does*. Think of it as a transformer. It takes the relatively low DC voltage (Direct Current) supplied by the stator or magneto and transforms it into a high-voltage pulse, typically thousands of volts. This is achieved through electromagnetic induction. The coil consists of two windings: a primary winding, which has relatively few turns of heavy wire, and a secondary winding, which has thousands of turns of very fine wire. When current flows through the primary winding, it creates a magnetic field. When this current is interrupted (by the points or the electronic ignition module), the magnetic field collapses rapidly. This collapsing field induces a voltage in both the primary and secondary windings, but because the secondary winding has so many more turns, the voltage induced in it is much, much higher. This high voltage is then sent to the spark plug, creating the spark that ignites the fuel.

Key components of the ignition system we'll be focusing on are:

Coil: The transformer, as described above.
Spark Plug: Creates the spark to ignite the air/fuel mixture.
Spark Plug Wire: Conducts the high voltage from the coil to the spark plug.
Ignition Module (or Points): Controls when the primary current is interrupted, triggering the spark.

Safety First!

Before you even think about grabbing your meter, remember safety is paramount. Disconnect the spark plug wire from the spark plug. This prevents accidental starting and electrical shock. Also, remove the key from the ignition or disconnect the battery (if equipped). If your mower has a fuel shut-off valve, close it to prevent fuel spillage. Always work in a well-ventilated area, especially when dealing with gasoline. And of course, wear appropriate personal protective equipment (PPE), such as safety glasses.

Tools You'll Need

Here's a list of the tools you'll need for this procedure:

Digital Multimeter (DMM): An essential tool for any DIY mechanic. Make sure it's capable of measuring resistance (ohms).
Screwdrivers: Likely a flathead and Phillips head, depending on your mower.
Wrench Set or Socket Set: To remove the coil, if necessary.
Wire Brush or Sandpaper: To clean connections.
Pen and Paper: To record your readings.
The Lawn Mower's Service Manual (Optional, but Highly Recommended): This will provide the exact resistance specifications for your coil.

The Testing Procedure

Now for the main event! Follow these steps carefully:

1. Accessing the Coil

The first step is to locate and access the ignition coil. It's usually found mounted near the engine's flywheel. You may need to remove a cover or other components to gain access. Consult your mower's service manual for the exact location and removal procedure.

2. Visual Inspection

Before you start poking around with your meter, give the coil a thorough visual inspection. Look for any signs of damage, such as:

Cracks or chips in the coil's housing.
Burn marks or melted plastic.
Corrosion or rust on the terminals.
Loose or damaged wiring.

If you see any of these signs, the coil is likely bad and needs to be replaced, regardless of what your meter readings say.

3. Setting Up Your Multimeter

Set your digital multimeter (DMM) to the ohms (Ω) setting. You'll need to know the expected resistance range for both the primary and secondary windings of your coil. This information is typically found in the mower's service manual. If you don't have the manual, you can often find this information online by searching for your mower's model number and "ignition coil resistance." If you still can't find the exact specs, a general rule of thumb is that the primary winding should have a very low resistance (typically less than 1 ohm), and the secondary winding should have a much higher resistance (typically several thousand ohms).

4. Testing the Primary Winding

The primary winding is the low-voltage side of the coil. To test it:

Connect one of the DMM's probes to the coil's primary terminal. This is usually the terminal where the wire from the ignition module (or points) connects.
Connect the other DMM probe to the coil's metal core (ground).
Observe the resistance reading on the DMM. It should be very low, typically less than 1 ohm. A reading of infinity (OL or open loop on the DMM) indicates an open circuit in the primary winding, meaning the coil is bad. A significantly higher-than-expected reading suggests a short circuit or increased resistance, also indicating a problem.

5. Testing the Secondary Winding

The secondary winding is the high-voltage side of the coil. To test it:

Connect one of the DMM's probes to the spark plug terminal on the coil.
Connect the other DMM probe to the coil's metal core (ground).
Observe the resistance reading on the DMM. It should be much higher than the primary winding resistance, typically several thousand ohms. Again, a reading of infinity (OL) indicates an open circuit, and a significantly different reading from the expected range suggests a problem.

6. Interpreting the Results

Here's a summary of how to interpret your ohm meter readings:

Correct Resistance (within the specified range): The coil *might* be good, but this doesn't guarantee it's functioning perfectly under load. Proceed with other troubleshooting steps if the problem persists.
Open Circuit (OL or infinity): The winding is broken, and the coil is definitely bad. Replace it.
Short Circuit (very low resistance on the secondary winding): The windings are shorted together, and the coil is bad. Replace it.
Significantly Different Resistance (outside the specified range): The winding is likely damaged or has increased resistance due to corrosion or internal breakdown. Replace the coil.

Important Considerations

Temperature: Resistance can vary slightly with temperature. Perform the test in a stable environment.
Meter Accuracy: Ensure your multimeter is accurate and calibrated correctly.
Wiring and Connections: Before condemning the coil, double-check all wiring connections to and from the coil. A loose or corroded connection can cause similar symptoms to a faulty coil. Clean the terminals with a wire brush or sandpaper and ensure they are securely connected.
Other Potential Issues: A bad spark plug, a faulty ignition module, or a broken spark plug wire can also cause a no-start or poor-running engine. Don't immediately assume the coil is the problem; systematically troubleshoot the entire ignition system.

Final Thoughts

Testing a lawn mower coil with an ohm meter is a valuable troubleshooting technique that can save you time and money. By understanding how the ignition coil works and following these steps carefully, you can accurately diagnose whether the coil is the culprit behind your mower's engine problems. Remember to consult your mower's service manual for specific resistance values and always prioritize safety. Good luck, and happy mowing!