When your lawn mower refuses to start, learning how to test the coil on a lawn mower is a fundamental skill. Determining if the coil is generating a spark is a key step in electrical troubleshooting. This guide will walk you through the process with clear, step-by-step instructions.
You don’t need to be a certified mechanic. With a few basic tools and some safety precautions, you can diagnose a faulty ignition coil yourself. This can save you time and money on repairs.
How To Test The Coil On A Lawn Mower
The ignition coil, sometimes called the magneto, is a crucial component. Its job is to transform the low voltage from the engine’s flywheel magnet into a high-voltage spark. This spark ignites the fuel in the cylinder. If the coil fails, you get no spark, and your mower won’t run.
Before you begin testing, it’s essential to gather the right tools and ensure your safety. Working on small engines involves flammable fuel and electrical components.
Safety First And Tools You Will Need
Always prioritize safety when working on machinery. A simple mistake can lead to injury or further damage to your mower.
- Disconnect the spark plug wire and secure it away from the plug.
- Allow the engine to cool completely before starting any work.
- Work in a well-ventilated area, away from sources of ignition.
- Wear safety glasses to protect your eyes from debris.
For this test, you will need a few specific tools. Most are common in a basic tool kit.
- A spark plug tester (the in-line type is best and safest).
- A multimeter capable of reading resistance (ohms).
- A set of wrenches or sockets to remove the spark plug and possibly the coil.
- A clean cloth and a small piece of fine sandpaper.
Initial Visual Inspection
Before using any tools, perform a thorough visual inspection. Many problems can be spotted with a careful eye. Look for obvious signs of damage or wear that could be causing the issue.
- Check the spark plug wire for cracks, brittleness, or burns. The insulation must be intact.
- Inspect the coil itself for physical cracks, carbon tracking (black lines), or melted areas.
- Look at the coil mounting area. Ensure it is clean and the coil is securely fastened to the engine.
- Examine the flywheel and the magnet for rust or debris that could interrupt the magnetic field.
If you see significant physical damage, the coil likely needs replacement. If everything looks good visually, proceed to the spark test.
Common Visual Faults To Identify
Cracks in the coil’s epoxy casing are a clear sign of failure. Carbon tracking looks like thin, lightning-bolt lines and indicates the spark is arcing where it shouldn’t. Also, check for corrosion on the metal laminations or the kill wire terminal.
Performing The Spark Test
The spark test is the most direct way to check for ignition output. It tells you if the coil is creating the high-voltage spark needed for combustion. A spark plug tester is highly recommended over the old “ground the plug” method, which can be unreliable and dangerous.
- Remove the spark plug wire from the spark plug.
- Connect your in-line spark plug tester between the spark plug wire and the spark plug itself.
- Ground the tester’s clip to a clean, unpainted metal part of the engine cylinder head.
- With the ignition switch in the “on” position, pull the starter cord rapidly or use the electric starter.
Observe the tester’s window. You should see a bright, blue spark jumping the gap inside the tester. A weak orange spark or no spark at all indicates a problem with the ignition system, likely the coil.
If you get no spark, double-check your connections and ensure the kill switch wire is disconnected from the coil if your mower has one. Sometimes a faulty safety switch can interrupt the circuit.
Testing Coil Resistance With A Multimeter
If the spark test fails, using a multimeter to measure resistance is the next step. This test checks the internal windings of the coil for opens or shorts. You will need to know if your mower has a two-terminal or a three-terminal coil, as testing procedures differ slightly.
First, you must disconnect the coil from the engine to get accurate readings. Remove the mounting screws and carefully disconnect any wires, noting thier positions for reassembly.
Testing A Two-Terminal Coil
Most common lawn mower engines use a two-terminal coil. One terminal connects to the kill switch wire, and the other is the ground through the mounting points.
- Set your multimeter to the ohms (Ω) setting, usually the 20k range.
- Touch one meter probe to the metal laminations of the coil (the ground).
- Touch the other probe to the spark plug wire terminal inside the coil where the boot attaches.
You should get a reading. Consult your engine’s service manual for the exact specification, but a typical primary resistance reading for a small engine coil is between 2,500 and 5,000 ohms. A reading of zero (short) or infinity (open) means the coil is faulty.
Testing A Three-Terminal Coil
Some coils have three terminals: one for the kill switch, one for the spark plug wire, and a separate ground terminal.
- Set your multimeter to ohms.
- Test between the spark plug terminal and the ground terminal. This is the secondary resistance.
- Test between the kill switch terminal and ground. This is the primary resistance.
Again, compare your readings to the manufacturer’s specs. A significant deviation indicates a bad coil.
Checking The Air Gap
The air gap is the tiny space between the coil armature and the flywheel magnet. If this gap is too wide or too narrow, the magnetic field cannot induce the proper voltage in the coil. An incorrect air gap is a common cause of a weak or absent spark.
To check and adjust the air gap, you will need a feeler gauge of the correct thickness. This specification is critical and varies by engine, so check your manual. A common gap is 0.010 inches.
- Loosen the coil mounting screws just enough so the coil can be moved.
- Place the correct feeler gauge between the coil leg and the flywheel magnet.
- Gently press the coil against the feeler gauge and flywheel, then tighten the mounting screws.
- Remove the feeler gauge and rotate the flywheel to ensure it doesn’t scrape the coil.
Interpreting Your Test Results And Next Steps
After completing your tests, you will have a clear picture of the coil’s condition. Here is how to interpret the results and decide on the next course of action.
- Strong Blue Spark: Your ignition coil is functioning correctly. Look for other issues like fuel problems or compression.
- No Spark or Weak Spark with Correct Resistance: The coil may be failing under load, or there could be an issue with the flywheel magnet losing its strength. The kill switch circuit could also be faulty.
- Incorrect Resistance Reading: The coil’s internal windings are damaged. The coil must be replaced.
- Spark After Adjusting Air Gap: The problem was a simple adjustment. Your mower should now start.
If you need to replace the coil, ensure you purchase the correct part for your specific engine model. Installation is the reverse of removal, paying close attention to setting the proper air gap.
Preventative Maintenance Tips
Regular maintenance can extend the life of your mower’s ignition coil and prevent unexpected failures. A few simple habbits can make a big difference.
- Keep the engine clean and free of grass clippings and oil buildup, especially around the coil and flywheel area.
- Always use a fuel stabilizer if storing your mower for more than 30 days. Old fuel can cause corrosion and varnish that affects the entire engine.
- Store your mower in a dry place to prevent moisture-related corrosion on electrical components.
- Check and tighten the coil mounting screws periodically, as engine vibration can loosen them.
Frequently Asked Questions
What Are The Symptoms Of A Bad Lawn Mower Coil?
The primary symptom is the engine not starting due to a lack of spark. You may also experience intermittent operation, where the mower runs for a while then dies, or backfiring as unburned fuel ignites in the exhaust. A visibly cracked or burnt coil is a definite sign.
Can You Test A Lawn Mower Coil Without A Multimeter?
Yes, you can perform the spark test using an in-line spark tester without a multimeter. This will tell you if the coil is producing a spark. However, a multimeter provides a more definitive diagnosis by checking the coil’s internal electrical resistance, which the spark test cannot do.
How Much Does It Cost To Replace A Lawn Mower Ignition Coil?
The cost varies by engine model. A replacement coil typically ranges from $20 to $60. If you have a professional replace it, labor will add to the total cost. Doing the replacement yourself involves mainly the part cost.
What Causes A Lawn Mower Coil To Fail?
Common causes include overheating from a dirty engine, vibration loosening the mount, moisture intrusion leading to corrosion, and simple old age. Physical impact or damage from debris can also cause immediate failure. A weak flywheel magnet can also mimick coil failure.
By following this guide, you can confidently diagnose a faulty ignition coil. Remember to work safely, methodically follow the steps, and use the correct tools for an accurate diagnosis. With a little patience, you’ll have your mower running smoothly again.