If your lawn mower won’t start, learning how to test coil on lawn mower is a crucial skill. A faulty coil will prevent spark plug firing, and checking its resistance values is a reliable diagnostic step. This guide will walk you through the entire process safely and effectively.
You don’t need to be a professional mechanic. With a basic multimeter and some simple steps, you can determine if the ignition coil is the culprit behind your engine troubles. Let’s get your mower running again.
How To Test Coil On Lawn Mower
An ignition coil is a transformer that boosts the battery’s low voltage into the thousands of volts needed to create a spark at the plug. When it fails, you get no spark, and your engine refuses to start. Testing it involves measuring electrical resistance.
Tools And Safety Equipment You Will Need
Before you begin, gather the right tools. Safety is paramount when working with small engines.
- A digital multimeter (capable of measuring ohms Ω)
- A set of basic wrenches or a socket set
- A spark plug socket
- A clean rag and a small brush
- Safety glasses and work gloves
- Your lawn mower’s service manual (highly recommended)
Preliminary Checks Before Testing The Coil
Always perform these checks first. They can save you time and confirm you’re on the right track.
Verify The Spark Plug Condition
Remove the spark plug and inspect it. Look for heavy carbon buildup, a cracked porcelain insulator, or a damaged electrode. A bad plug can mimic coil failure.
Confirm Fuel And Airflow Are Not The Issue
Ensure you have fresh fuel and the air filter is clean. A clogged fuel line or a dirty filter can also prevent starting, leading you to suspect the coil unnecessarily.
Perform A Basic Spark Test
With the spark plug removed and reconnected to its wire, ground the plug’s metal body against the engine block. Pull the starter cord and look for a bright blue spark. No spark strongly points to an ignition system fault.
Step-By-Step Guide To Testing The Ignition Coil
Now, let’s get to the core testing procedure. You will need to locate and disconnect the coil for accurate readings.
Step 1: Disconnect The Spark Plug Wire And Ground Wire
For safety, always disconnect the spark plug wire first. Then, locate the small ground wire attached to the coil (usually a single blade connector) and disconnect it to.
Step 2: Remove The Ignition Coil From The Engine
Use your wrench or socket to remove the one or two bolts securing the coil to the engine block. Carefully lift the coil away, noting its orientation. Clean any debris from the coil’s legs and the mounting area on the engine.
Step 3: Understand Coil Terminal Identification
Most lawn mower coils have two primary terminals: the primary (low-voltage) winding terminals and the secondary (high-voltage) terminal. The primary terminals are the small metal posts where the kill wire connects. The secondary terminal is the large boot where the spark plug wire attaches.
Step 4: Set Your Multimeter To Ohms (Ω)
Turn your digital multimeter on and set the dial to the ohms (Ω) or resistance setting. If you have multiple ranges, start at 20k ohms. Touch the two meter probes together; the display should read near zero, confirming the meter is working.
Step 5: Test The Primary Coil Windings
- Place one multimeter probe on one of the coil’s small primary terminals.
- Place the other probe on the other small primary terminal.
- Read the resistance value on the multimeter display.
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 (OL or “open loop”) means the primary winding is broken and the coil is bad.
Step 6: Test The Secondary Coil Windings
- Insert one multimeter probe firmly into the spark plug wire boot (the secondary terminal).
- Touch the other probe to one of the coil’s metal mounting legs or the metal core.
- Read the resistance value displayed.
A normal secondary winding reading is usually between 2,500 and 5,000 ohms (2.5k to 5k ohms). Again, an infinite (OL) reading indicates an open circuit inside the coil.
Step 7: Test For A Ground Fault (Coil To Ground)
This test checks if the coil is shorting internally to its metal casing.
- Place one probe on the metal core or a mounting leg of the coil.
- Place the other probe on one of the small primary terminals.
- Your meter should read infinite resistance (OL). Any low resistance reading here indicates an internal short.
Interpreting Your Multimeter Readings
Understanding what the numbers mean is key to your diagnosis.
- Readings Within Spec: If both primary and secondary readings fall within the typical ranges mentioned, your coil is likely functional. The problem may lie elsewhere, like the flywheel key or magnet.
- Infinite Resistance (OL): This is the most common failure. It means the wire inside the coil has broken. The coil must be replaced.
- Zero Or Very Low Resistance: A reading near zero on the secondary winding suggests an internal short circuit. The coil is faulty.
Remember, resistance specs can vary. Always check your engine’s service manual for the manufacturers specific ohm values if possible.
Other Common Ignition System Issues
A passing coil test means another component is stopping the spark. Here are two frequent culprits.
Checking The Flywheel Key
A sheared flywheel key will throw off the engine’s timing. Remove the flywheel nut and inspect the small metal key in the crankshaft slot. If it is damaged or sheared, it must be replaced for the engine to spark correctly.
Inspecting The Coil Air Gap
The gap between the coil’s legs and the flywheel magnets is critical. It’s usually set with a feeler gauge, typically between 0.008 and 0.012 inches. An incorrect gap can prevent spark generation even with a good coil.
Installing A New Ignition Coil
If your tests confirm a bad coil, replacement is straightforward.
- Set the correct air gap on the new coil using the specified feeler gauge.
- Position the coil on the engine block, aligning it with the flywheel.
- Hand-tighten the mounting bolts just enough to hold the coil in place.
- Re-check the air gap as you final-tighten the bolts, ensuring the gap remains consistent.
- Reconnect the ground wire and the spark plug wire.
- Reinstall the spark plug and try starting the engine.
FAQ Section
What Are The Symptoms Of A Bad Coil On A Lawn Mower?
The primary symptom is a complete failure to start with no spark at the plug. You may also experience intermittent starting, engine misfiring while running, or the mower dying unexpectedly and refusing to restart.
How Do You Check A Lawn Mower Coil With A Multimeter?
You check a lawn mower coil by setting a multimeter to measure resistance (ohms). Test between the two small primary terminals for a low reading (usually under 2 ohms), and test from the spark plug boot to the coil core for a high reading (usually 2,500-5,000 ohms).
Can A Lawn Mower Coil Be Intermittent?
Yes, a coil can fail intermittently. It may work when cold but fail as it heats up, or it may work only sometimes due to an internal break that makes occasional contact. This can make diagnosis tricky without a multimeter test.
What Causes An Ignition Coil To Fail?
Common causes include heat damage from a dirty engine, vibration over time, moisture corrosion, and general age. Impact damage from hitting an object can also jar the internal windings loose.
Is It Worth Replacing A Lawn Mower Coil?
Absolutely. A new ignition coil is a relatively inexpensive part compared to the cost of a new mower or a professional repair visit. Replacing a faulty coil yourself can restore your mower to full operation for many more seasons.