How To Test A Coil On A Lawn Mower

When your lawn mower cranks but refuses to start, a faulty ignition coil is a common culprit. Learning how to test a coil on a lawn mower is a straightforward diagnostic task that can save you time and money. Testing the ignition coil with a multimeter can determine if it’s the reason your mower has lost its spark.

This guide provides clear, step-by-step instructions. You’ll learn to safely perform the test and interpret the results, even if you’re new to small engine repair.

An ignition coil, sometimes called a magneto, is a crucial component in your mower’s engine. Its job is to transform the low voltage from the battery (or the charge from magnets in a flywheel on older models) into a high-voltage spark. This spark ignites the fuel-air mixture in the cylinder, powering the engine.

When the coil fails, no spark is produced, and the engine cannot start. Symptoms of a bad coil often mimic other issues, making testing essential before replacing parts.

Common Symptoms Of A Faulty Ignition Coil

Before you begin testing, check if your mower exhibits these common signs of coil failure:

The engine cranks normally but will not start.
The mower starts but then stalls abruptly and will not restart.
You notice obvious physical damage to the coil, such as cracks, burns, or corrosion.
The engine runs very poorly, misfires, or lacks power under load.
The mower has difficulty starting in damp conditions but works fine when dry.

Tools And Safety Precautions You Will Need

Gathering the right tools and prioritizing safety will make the process smooth and risk-free.

Essential Tools For The Job

A multimeter (digital or analog) capable of measuring resistance (ohms, Ω).
A set of basic hand tools: usually a socket or wrench to remove the spark plug and screws to access the coil.
A spark plug tester or a known-good spark plug for a visual spark check (optional but helpful).
A clean rag and maybe a bit of sandpaper to clean connections.

Critical Safety Steps Before You Start

Disconnect the spark plug wire and secure it away from the plug. This prevents any accidental starting.
If your mower has a battery, disconnect the negative (-) terminal.
Allow the engine to cool completely if it has been running recently.
Work in a well-lit, ventilated area and keep your workspace organized.

Step-By-Step Guide To Testing With A Multimeter

This method measures the coil’s internal resistance to see if it’s within the manufacturer’s specified range. A reading that is too high or too low indicates a faulty coil.

Step 1: Locate And Remove The Ignition Coil

First, you need to access the coil. Consult your mower’s manual for the exact location. Typically, it is mounted near the flywheel, with the spark plug wire attached to it.

Remove any engine shroud or cover that blocks access to the coil.
Disconnect the spark plug wire from the coil terminal (if it’s a separate wire).
Disconnect the stop switch wire (usually a single thin wire attached to the coil).
Unscrew the one or two bolts that secure the coil to the engine block.
Carefully lift the coil away from the flywheel. You may need to gently wiggle it if it’s snug.

Step 2: Identify The Coil Terminals

Once the coil is removed, identify its terminals. Most lawn mower coils have two primary terminals to test:

The Primary Terminal: This is the smaller terminal where the stop switch wire was connected. It’s the coil’s low-voltage input side.
The Secondary Terminal: This is where the spark plug wire connects. It’s the high-voltage output side. On many coils, this is a metal tower or a rubber boot with a metal contact inside.
The Coil Armature: The laminated metal core of the coil itself often serves as the ground connection when bolted to the engine.

Step 3: Set Your Multimeter To Ohms (Ω)

Turn your multimeter dial to the resistance setting, often marked with the omega symbol (Ω). If your meter has multiple ohm ranges, set it to the lowest scale, like 200Ω or 2kΩ, for the primary test. You’ll switch to a higher scale (like 20kΩ) for the secondary test.

Touch the two meter probes together. The display should read near zero ohms (0.0 or 0.2). This confirms your meter is working properly for these tests.

Step 4: Test The Primary Coil Windings

This test checks the integrity of the inner, low-voltage winding.

Place one multimeter probe on the coil’s primary terminal (where the kill wire attached).
Place the other probe on the metal armature (the core) of the coil.
Note the resistance reading on the multimeter display.

A typical reading for a healthy primary winding on a lawn mower coil is usually between 0.1 and 2.0 ohms. Consult your service manual for the exact specification. A reading of infinite resistance (OL or “1” on the left) means the winding is open and the coil is bad. A zero reading indicates a short.

Step 5: Test The Secondary Coil Windings

This test checks the outer, high-voltage winding that creates the spark.

Switch your multimeter to a higher ohms scale, such as 20kΩ.
Insert one probe into the end of the spark plug wire boot where it makes contact, or touch it to the secondary terminal on the coil itself.
Touch the other probe to the metal armature of the coil.
Note the resistance reading.

A typical reading for a healthy secondary winding is much higher, often in the range of 2,500 to 15,000 ohms (2.5kΩ to 15kΩ). Again, an infinite (OL) or zero reading means the coil has failed. If your spark plug wire is seperate, you should test it’s continuity as well.

Alternative Test: The Spark Check Method

If you don’t have a multimeter, you can perform a visual spark check. This method is less definitive but can quickly reveal a complete lack of spark.

Reconnect the spark plug wire to a spark plug tester or a spare, known-good spark plug.
Ground the metal threads of the plug or tester firmly against a clean, unpainted part of the engine block.
With the spark plug wire connected and the stop switch in the “run” position, pull the starter cord or engage the electric starter.
Look for a bright blue spark jumping the gap of the tester or plug. A faint yellow or orange spark, or no spark at all, suggests a weak or failed ignition coil.

Warning: Never hold the spark plug or wire with your hands during this test due to the risk of a painful electrical shock.

Interpreting Your Test Results And Next Steps

Understanding what your readings mean is key to solving the problem.

Readings Within Spec: If both primary and secondary resistance values fall within the typical ranges, your ignition coil is likely functioning correctly. The no-start problem lies elsewhere (e.g., fuel system, carburetor, compression, or a faulty stop switch grounding the coil).
Readings Out Of Spec (OL or Zero): If either winding shows infinite resistance (open circuit) or zero resistance (short circuit), the coil is faulty and needs replacement. There is no way to repair a broken ignition coil.
Intermittent Failure: Sometimes a coil will test fine when cold but fail when the engine heats up. If you suspect this, you can test the coil again immediately after a failed start attempt, taking care to avoid hot surfaces.

Tips For Replacing A Faulty Ignition Coil

If your tests confirm a bad coil, replacement is you’re next step. Here’s how to ensure a successful install.

Purchase the correct replacement coil for your mower’s exact engine model number.
Before installing the new coil, thoroughly clean the mounting surface on the engine block.
Many coils require setting an air gap between the coil armature and the engine’s flywheel. Use a business card or a feeler gauge of the specified thickness (often 0.010 inches) as a spacer. Loosely install the coil bolts, slide the card between the coil and flywheel, then tighten the bolts and remove the card.
Reconnect the stop switch wire and the spark plug wire securely.
Reinstall any engine covers and reconnect the battery (if applicable).

Frequently Asked Questions

What Are The Signs Of A Bad Coil On A Lawn Mower?

The most common sign is an engine that cranks but will not start, indicating no spark. Other signs include hard starting, engine stalling, misfiring, and poor performance, especially when the engine is warm.

Can You Test A Lawn Mower Coil Without A Multimeter?

Yes, you can perform a visual spark check using a spark plug tester. However, this method only confirms a complete lack of spark and cannot diagnose a coil that is weak or has incorrect internal resistance like a multimeter can.

What Should The Resistance Be On A Lawn Mower Coil?

Resistance values vary by model. Typically, the primary winding resistance is very low, often between 0.1 and 2.0 ohms. The secondary winding resistance is much higher, commonly ranging from 2,500 to 15,000 ohms. Always check your engine’s service manual for precise specifications.

Why Does My Mower Have No Spark After Replacing The Coil?

If there’s still no spark after installing a new coil, double-check that the air gap is set correctly. Also, ensure the kill switch wire is properly connected and not shorting to ground, inspect the spark plug wire for damage, and verify that the flywheel key is not sheared, which would throw off ignition timing.