How To Check Coil On Lawn Mower

When your lawn mower refuses to start, a faulty ignition coil is a common culprit. Learning how to check coil on lawn mower is a straightforward diagnostic skill that can save you time and money. Testing the coil with a multimeter can confirm whether it’s the source of your starting problem.

This guide provides clear, step-by-step instructions. You will learn to safely test the coil and understand the results.

With a few basic tools and some care, you can perform this check yourself.

The ignition coil, sometimes called the magneto, is a crucial component. It transforms the low voltage from the engine’s flywheel into a high-voltage spark. This spark ignites the fuel in the cylinder, making the engine run.

When a coil fails, it stops producing this spark. This leads to a no-start condition or an engine that runs poorly. Checking it involves a visual inspection and an electrical test with a multimeter.

Tools And Safety Equipment You Will Need

Gathering the right tools before you start makes the process smoother and safer. You do not need expensive equipment.

Here is what you should have ready:

A digital multimeter (capable of reading ohms for resistance)
A basic socket or wrench set to remove the spark plug and any shrouds
A spark plug tester (optional but very helpful)
A clean rag and a small brush
Safety glasses and work gloves

Always disconnect the spark plug wire before begining any work. This prevents the engine from accidentally starting.

Step 1: Prepare The Mower For Testing

Proper preparation is key to a safe and accurate test. Start by placing the mower on a flat, stable surface.

Disconnect the spark plug wire. Pull the boot off the spark plug and secure it away from the plug.
Clean the area around the coil and flywheel. Use your brush and rag to remove grass clippings, dirt, and debris. This gives you a clear view and prevents contamination.
Locate the ignition coil. It is mounted near the flywheel, which is usually under a plastic or metal shroud on the top of the engine. You may need to remove a few bolts to access it.

Take a moment for a visual inspection. Look for obvious cracks, burns, or corrosion on the coil itself or its wire. Physical damage often means the coil needs replacement.

Identifying The Coil Terminals

Most lawn mower coils have two wire terminals. One is a single wire that leads to the kill switch (often a black wire). The other is the high-tension lead that connects to the spark plug.

The coil itself will have metal contact points or spade terminals where these connect. Your multimeter probes will touch these points for testing.

Step 2: Perform A Spark Test (Initial Check)

Before using the multimeter, a simple spark test can indicate a problem. This is a quick way to see if the coil is producing any spark at all.

Reconnect the spark plug wire to the plug.
Ground the spark plug. Place the metal body of the spark plug against a clean, unpainted part of the engine cylinder head.
Pull the starter cord or engage the electric starter (with extreme caution). Watch for a bright blue spark jumping across the spark plug’s gap.

If you see no spark or a weak orange spark, the ignition system is faulty. The coil is a likely cause, but you need the multimeter to confirm it.

Step 3: Test The Coil With A Multimeter

A multimeter provides a definitive check by measuring the coil’s electrical resistance. Set your multimeter to the ohms (Ω) setting, usually marked with the omega symbol.

You will perform two primary tests: primary resistance and secondary resistance.

Testing Primary Coil Resistance

The primary circuit involves the low-voltage windings inside the coil. To test it, you need to identify the two small terminals on the coil where the kill switch and ground wires attach.

Touch one multimeter probe to each of the two small terminals on the coil.
Read the resistance value on the multimeter display.

A typical reading for a healthy coil is usally between 0.1 and 2.0 ohms. Consult your mower’s service manual for the exact specification. A reading of infinite resistance (OL or “open loop”) means the primary winding is broken and the coil is bad.

Testing Secondary Coil Resistance

The secondary circuit involves the high-voltage windings that create the spark. This test is done between one of the small terminals and the spark plug terminal (the large center tower where the plug wire attaches).

Place one probe on a small terminal.
Place the other probe inside the spark plug terminal (where the metal contact is).
Read the resistance value.

Expect a much higher reading, typically in the range of 2,500 to 5,000 ohms (2.5k to 5k ohms). Again, an infinite reading indicates an open circuit and a failed coil. Significantly higher or lower readings than spec also indicate a problem.

Step 4: Check The Coil Air Gap

Even a good coil will not produce a spark if it is too far from the flywheel. The air gap is the small space between the coil’s armature and the flywheel magnets.

An incorrect gap can prevent the magnetic field from inducing current properly. This is a common oversight after a coil is reinstalled.

Locate the mounting screws on the coil. Loosen them slightly so the coil can be moved.
Take a feeler gauge of the correct thickness. For most mowers, this is between 0.008 and 0.012 inches.
Slide the feeler gauge between the coil leg and the flywheel.
Gently press the coil against the feeler gauge, then tighten the mounting screws.
Remove the feeler gauge. The gap is now set correctly.

Interpreting Your Test Results

Understanding what your readings mean is the final step. Here is a simple breakdown:

Both primary and secondary tests show resistance within specification: Your coil is likely functioning correctly. The starting problem may lie elsewhere (e.g., fuel system, carburetor, or a faulty kill switch wire grounding out).
Either test shows infinite resistance (OL): The coil has an internal break and must be replaced.
Resistance readings are significantly out of spec: The coil is failing and should be replaced.
Good coil resistance but no spark: Double-check the air gap. Also, inspect the kill switch wire for a short to ground that could be interrupting the circuit.

If your coil tests bad, ensure you purchase the correct replacement part for your specific engine model.

Common Mistakes To Avoid

Avoiding these errors will give you a more accurate diagnosis and keep you safe.

Not disconnecting the spark plug wire first. This is the number one safety rule.
Testing a dirty or oily coil. Contaminants can interfere with electrical readings.
Using a damaged multimeter or old batteries in the meter, which gives false readings.
Forgetting to check the simple things first, like a fouled spark plug or empty gas tank.
Overtightening the coil mounting screws, which can crack the coil’s laminate core.

When To Seek Professional Help

While checking the coil is a manageable task, some situations warrant a mechanic’s expertise.

If you have tested the coil and it seems fine, but the mower still won’t start, the issue could be more complex. Problems like a sheared flywheel key, internal engine damage, or advanced electrical faults are harder to diagnose.

If you are uncomfortable with any step, especially disassembling engine shrouds, it’s wise to call a professional. They have the tools and experience to diagnose the problem quickly.

Frequently Asked Questions (FAQ)

What Are The Symptoms Of A Bad 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 very rough, misfiring, losing power under load, or stopping when it gets hot. A coil that fails when warm is a classic failure mode.

Can A Lawn Mower Coil Be Tested Without A Multimeter?

Yes, you can perform the visual inspection and spark test described earlier. However, these methods are less definitive. A multimeter provides a concrete, numerical assessment of the coil’s health, which is why it’s the recommended tool.

How Much Does It Cost To Replace A Lawn Mower Ignition Coil?

Replacement coils are generally inexpensive, often ranging from $15 to $40 for most common residential mower engines. The cost of professional replacement will include labor, typically adding another $50 to $100 to the total bill.

What Causes An Ignition Coil To Fail?

Common causes include heat damage from engine overheating, vibration causing internal wire breaks, moisture corrosion, and simple old age. Impact damage from hitting an object can also crack the coil. Sometimes, a failing spark plug with an overly wide gap can put extra strain on the coil.

Will A Bad Coil Prevent A Mower From Starting?

Absolutely. Without a strong spark to ignite the fuel-air mixture in the cylinder, the engine cannot run. A bad coil is a primary cause of a no-spark, no-start condition. It is one of the first components to check after confirming there is fuel in the tank.