How To Test The Coil On A Lawn Mower

Your lawn mower refusing to start is frustrating, especially when the grass is screaming for a trim. A faulty ignition coil is a common culprit. Fortunately, you can often test the coil yourself, saving time and money on repairs. Here's how:

Safety First

Before you even think about touching your lawn mower, disconnect the spark plug wire. This prevents accidental starting and potential injury. Remove the key if your mower has one. Allow the engine to cool down completely.

Gather Your Tools

You'll need a few basic tools:

• A multimeter (analog or digital)
• Screwdrivers (various sizes)
• Spark plug wrench
• Rag or paper towel
• Possibly a small engine repair manual for your specific mower model (optional but helpful)

Locate the Ignition Coil

The ignition coil is usually a small, rectangular component located near the engine flywheel. It's often attached to the engine block with screws or bolts. It will have a wire running to the spark plug and possibly a ground wire. Consulting your owner's manual or a repair manual specific to your mower model can be incredibly helpful here, as coil placement varies.

Visual Inspection

Before you start electrical testing, give the coil a thorough visual inspection.

• Look for cracks, chips, or other physical damage to the coil body.
• Check the wires connecting to the coil. Are they frayed, corroded, or loose?
• Examine the metal laminations (if visible) for rust or damage.

If you see any significant damage, the coil likely needs replacement, regardless of the electrical test results. Clean any dirt or debris from the coil and surrounding area with a rag.

Testing the Ignition Coil with a Multimeter

A multimeter allows you to measure the electrical resistance within the coil. This helps determine if the internal windings are intact and functioning correctly. There are generally two resistance tests you'll perform: a primary resistance test and a secondary resistance test.

Primary Resistance Test

This test measures the resistance of the primary winding, which is the low-voltage side of the coil.

1. Set your multimeter to the lowest resistance setting (usually ohms - Ω). Some multimeters have an auto-ranging feature; if yours does, select that.
2. Touch the two multimeter probes together. The reading should be close to zero (this checks the meter's accuracy). If it's not, adjust the meter if possible or make a note of the reading for later subtraction.
3. Disconnect the wire that runs from the coil to the points (or electronic ignition module) and the ground wire, if present. You want to isolate the coil for testing.
4. Place one probe on the terminal where the wire was connected (the primary terminal) and the other probe on the metal frame of the coil (ground).
5. Note the resistance reading.

Understanding the Results: The acceptable resistance range varies depending on the specific coil. A typical range is between 0.5 and 3 ohms. Consult your repair manual or search online for the specific resistance range for your mower's coil. If the reading is significantly higher than the specified range (e.g., infinity or several hundred ohms), the primary winding is likely open (broken). If the reading is significantly lower than the specified range (e.g., zero ohms), the primary winding is likely shorted. Either way, the coil needs replacement.

Secondary Resistance Test

This test measures the resistance of the secondary winding, which is the high-voltage side of the coil that connects to the spark plug.

1. Set your multimeter to a higher resistance setting (typically in the kilo-ohms or mega-ohms range - kΩ or MΩ).
2. Place one probe on the spark plug terminal of the coil (where the spark plug wire connects).
3. Place the other probe on the metal frame of the coil (ground).
4. Note the resistance reading.

Understanding the Results: Again, the acceptable resistance range varies. A typical range is between 6,000 and 30,000 ohms (6 kΩ to 30 kΩ). Refer to your repair manual or online resources for the correct range for your coil. A reading significantly higher than the specified range indicates an open secondary winding. A reading significantly lower suggests a shorted winding. In either case, the coil needs to be replaced.

Air Gap Adjustment (Critical!)

Even if your coil passes the resistance tests, it might still not function correctly if the air gap between the coil and the flywheel is incorrect. The air gap is the small distance between the coil's metal core and the magnets on the flywheel. This gap must be precise for the coil to generate enough voltage to fire the spark plug.

1. Loosen the screws or bolts that hold the ignition coil in place. Just loosen them enough so you can move the coil slightly.
2. Use a business card or a specific air gap tool (available at auto parts stores) as a spacer. The thickness of a business card is usually within the acceptable range for most lawn mowers.
3. Place the business card between the coil's metal core and the flywheel magnets.
4. Gently push the coil towards the flywheel until it touches the business card.
5. Tighten the coil's mounting screws securely, being careful not to move the coil while tightening.
6. Remove the business card.

Adjusting the air gap correctly is essential for proper ignition. If the gap is too large, the coil won't generate enough voltage. If the gap is too small, the coil can overheat and fail prematurely.

Reassembly and Testing

After testing the coil and adjusting the air gap, reassemble everything in the reverse order of disassembly.

• Reconnect the spark plug wire to the spark plug.
• Reconnect any other wires you disconnected during testing.

Now, try starting your lawn mower. If it starts and runs smoothly, you've successfully diagnosed and potentially resolved the ignition coil issue. If it still doesn't start, there might be other problems, such as a faulty spark plug, a clogged carburetor, or a problem with the fuel system.

When to Replace the Coil

Replace the ignition coil if:

• The visual inspection reveals significant damage.
• Either the primary or secondary resistance readings are outside the acceptable range.
• The mower still doesn't start after adjusting the air gap and performing other basic troubleshooting steps.

Practical Application: Knowing how to test your lawn mower's coil allows you to quickly diagnose starting problems. This is especially valuable during peak mowing season when time is precious. It also saves you the cost of unnecessary repairs by confirming whether the coil is actually the problem. If you maintain a small engine repair business, this knowledge is essential for efficient and accurate diagnostics.

Quick Checklist/Guideline

1. Safety First: Disconnect the spark plug wire!
2. Gather Tools: Multimeter, screwdrivers, spark plug wrench.
3. Locate Coil: Consult your manual if needed.
4. Visual Inspection: Check for damage, frayed wires.
5. Primary Resistance Test: Measure resistance between primary terminal and ground. Compare to spec.
6. Secondary Resistance Test: Measure resistance between spark plug terminal and ground. Compare to spec.
7. Air Gap Adjustment: Use a business card as a spacer.
8. Reassemble and Test: Reconnect everything and try starting the mower.
9. Replace if Necessary: Replace coil if tests fail or visual damage is evident.