Lawn mower coil resistance. How To Test A Lawn Mower Coil With A Multimeter

How To Test A Lawn Mower Coil With A Multimeter

If you winterized your lawn mower the right way, you should obviously be hopeful of an easy engine start, without any need of priming or choking the engine.

But at times, no matter how careful you have been, the lawn won’t start.

When I faced this situation, at first, it seemed like a fuel issue, or a battery issue on the lawn mower. To inspect further, I checked the spark plug to test if there’s power on the lawn mower but to my surprise, not even a flicker on the spark plug!

When the spark plug fails on a lawn mower, it means, it’s not receiving power from the ignition system. So, when there is no power on the spark plug, testing a lawn mower coil is the first thing you must do.

Ignition coil is the standard electrical part on every lawn mower. It directs power to the spark plug from the battery. That is how the spark plug keeps building spark and the lawn mower keeps running.

Over a period of time, the ignition coil on the lawn mower wears-out. In some cases, the solenoid stops working due to a sudden power-upsurge. But mostly, it is the coil which burns down due to continuous supply of high-voltage of current through it.

So, how do you test a lawn mower coil? You need a multimeter to test if current is passing through the coil. It’s quite a simple process and just takes about ten minutes.

This isn’t a very commonly occurring issue on the lawn mower and many-a-users don’t have a fair technical hand, so here I’m going to guide about how to check the coil on a lawn mower using a multimeter.

So, get your safety gloves and let’s get started!

Step-by-step Tutorial: For Testing A Lawn Mower Coil

“Get these tools first”

The ignition coil is made up of metal called, laminated magnetized iron core, surrounded by two copper wires, so it can start to become dirty and corrode after sometime, due to oil-residual vapours released by the engine.

There can be one or more ignition coils on a lawn mower, depending on the number of cylinders the mower has but the steps to test the ignition coil remain the same for each.

Here, I have outlined all the steps for testing the ignition coil on a lawn mower. So, keep scrolling and learn how to check a lawn mower coil and not just this, you shall also learn how to replace the faulty coil.

Step 1: Take out the ignition coil from the lawn mower

Ignition coil on a lawn mower is located inside the engine of the mower and is connected to the magnet of the flywheel.

  • Remove the plastic cover to access the engine

Hence, to reach the ignition coil, take off the plastic cover which sheds the engine. Use a screwdriver to loosen the screws and the cover will come off easily.

  • Spot the ignition coil right on top of the gas-tank

Next step is to remove the cover of the engine. It sits right above the fuel tank. When you unscrew the engine’s cover, you can easily spot the ignition coil.

  • Disconnect the cable which connects the coil to the spark plug

The spark plug and the ignition coil are connected by a high-tension thick black wire. Remove this connection cable and at the same time, remove the spark plug as well.

Now, you’ll see a kill switch on the ignition coil, connected by the kill wire.

This kill switch is a safety switch which turns on automatically when you get off the lawn mower and shuts down the engine to stop spinning of blades.

Using pliers, disconnect the kill wire from the ignition coil.

  • Pull the ignition coil out now

Now, the ignition coil is free from its wiring. Pull it out gently.

lawn, mower, coil, resistance

Step 2: Checking The Lawn Mower Coil

Now, take the ignition coil and the multimeter and find out how to test an ignition coil on a lawn mower.

  • Set the multimeter to 20k resistance mode of measurement. ( 20k V/A or 20 ohms)
  • Next step is to check the user manual and look at the resistance value table. Find out the resistance tolerance value of the ignition coil.
  • Testing the ignition coil
  • Stick one end of the multimeter into the spark plug cap and keep the other end on the base of the ignition coil
  • Read the ohms reading on the multimeter and match it with the resistance value range mentioned in the manual

( On most of the lawn mower engines, the ignition coil has a voltage of 15k V. Large horsepower engine coils can have 40k Voltage. In that case, increase the reading on the multimeter and then test the coil)

Now, keep one end of the multimeter on the kill wire of the coil and the other one of the base of the coil

  • Check the resistance value of the kill wire and match it with the resistance range given in the manual

Testing terminals of the kill wire: Lastly, keep one end of the multimeter on the terminal of the kill wire and instil the other end into the spark plug. Note down the resistance value.

  • When you test the ignition coil, the kill wire and its terminal using the multimeter, it should be as per the resistance tolerance range mentioned in the lawn mower manual.

After testing the coil and the kill wire, if the multimeter shows no resistance values, or the values do not match the resistance range given in the manual, the coil is defective. Replace it with a new one.

Step 3: Get a new ignition coil for the lawn mower

Since you know that the ignition coil is faulty, it has to be replaced with a new one to revive the lawn mower once again.

To get a new ignition coil, keep the following things in mind-

  • Ensure that the new coil is similar to the replaced coil
  • If there’re two coils on your lawn mower, check both and if required, replace both of them
  • Refer to the user manual, take down the engine number and then, get the coil

It’s always good to consider the model number of your lawn mower while making the purchase.

Step 4: Fix the new ignition coil on the lawn mower

At this step, rest assured, you’re almost done! Now, let’s replace the new ignition coil on the lawn mower and get the lawn mower running!

  • Connect the spark plug
  • Attach the kill wire to its terminal on the ignition coil
  • Now, Keep the screws of the ignition coil back in place and tighten them using a screwdriver.
  • (The most important step while replacing the coil)

“Keep a distance of 10 points between the flywheel and the ignition coil”

  • If you have a gauge:
  • Place the gauge between the coil and the flywheel magnet and measure a distance of 10 points.
  • Then, use the screwdriver to tighten the coil securely in place.
  • ( Pro Tip) Use a business card if you don’t have a gauge:
  • In absence of a measuring gauge, take a business card or a debit card.
  • Its length is closest to 10 points and will help you measure the right distance between the coil and the flywheel magnet.
  • Now screw the engine cover back into place to secure the connection.

Step 5: Test the replaced ignition coil

Now, you’re all set to test the new ignition coil and get your lawn mower running errands!

  • Start the ignition on the lawn mower
  • Switch the ignition ‘on’ on your lawn mower by inserting the key.
  • Connect the spark plug and enjoy mowing!

Frequently Asked Questions

Q1. Can oil-leak damage the ignition coil on a lawn mower?

Answer – Yes, if oil is leaking from the engine and keeps dripping on the ignition coil, it will cause the coil to overheat and damage both the coil and the spark plug.

Q2. Can a faulty spark plug damage the ignition coil on a lawn mower?

Answer – Defective spark plugs can cause misfire on the ignition coil. So, replace the faulty spark plug immediately to prevent it from damaging the ignition coil.

Q3. Why is there no spark on the spark plug of my lawn mower?

Answer – If the spark plug does not work after replacing a faulty ignition coil, the spark plug might be dirty or corroded. Clean its terminals using a wired brush and baking soda and check its electrodes. If the plug still doesn’t build a spark, change it.

Q4. What are the common symptoms of bad ignition coil on a lawn mower?

Answer – If the engine of your lawn mower backfires, it lacks power while running, it’s difficult to start or in worse case, the engine does not start at all, the ignition coil bears the defect. Check the spark plug before replacing the ignition coil.


With this article, you can now easily test and replace a faulty ignition coil on your lawn mower. But remember, your safety is of paramount importance to us, so perform the procedure carefully.

Never test any electrical part of the mower with wet hands, wet clothes or in a wet lawn. When you replace the new ignition coil, clean the flywheel magnet and the spark plug well.

Lastly, maintain your lawn mower on regular basis to avoid any such mishappenings from occurring and stalling the lawn mower!

How to Test a Magneto Coil With a Multimeter?

Testing a magneto coil with a multimeter is an essential part of troubleshooting any issues that you may be having with your bike. In this guide, we will walk you through the process step-by-step so that you can perform this task like a pro. We will also answer some common questions about magneto coils and multimeters, so that you have all the information you need to get started. Let’s get started!

What is a Magneto Coil?

A magneto coil is a type of electromagnet. It is an electrical conductor that has been wound into a coil so that it can create a magnetic field.

The most common use for a magneto coil is in an ignition system, where it is used to create a spark that ignites the fuel in the engine.

Magneto coils are used in a variety of applications, including generators and solenoids. They are also found in many household appliances, such as hair dryers and vacuum cleaners. [1], [2]

How Does Magneto Coil Work?

When an electric current flows through the coil, it creates a magnetic field. The strength of the magnetic field depends on the number of turns in the coil and the amount of current flowing through it.

The magnetic field can be used to create a force that can either attract or repel other objects. For example, when a magnet is placed near a piece of iron, the iron is attracted to the magnet because of the magnetic force.

Magneto coils can also be used to generate electricity. When a coil is moved through a magnetic field, it produces an electric current. This is how generators and alternators work.

In order for a magneto coil to work properly, it must have two things: a conductor and a magnetic field. The conductor is typically made of copper wire, and the magnetic field is provided by a permanent magnet or an electromagnet. [1], [2]

Symptoms of a Faulty Magneto Coil

Before you get to testing your magneto coil, it is important to know the symptoms of a faulty one. This will help you determine whether or not your coil is actually the problem. Let’s take a look at some of the most common symptoms.

Engine misfires

One of the most common symptoms of a faulty magneto coil is if your engine misfires. If your engine is misfiring, it means that the spark plugs are not firing correctly. This can cause a lot of problems, including decreased fuel efficiency and increased emissions.

Failure to start the engine

Another symptom of a faulty magneto coil is difficulty starting the engine. If your engine is having difficulty starting, it could be because the spark plugs are not firing correctly.

Loss of power

Next common symptom of a faulty magneto coil is a loss of power. This can manifest itself in a number of ways, from your engine running less smoothly to a complete loss of power. If you notice that your engine is losing power, it’s worth checking the magneto coil.

Decreased fuel economy

Decreased fuel economy can also signify a problem with the ignition coil. This is because the coil is not able to generate as much electricity, which means the spark plugs are not getting as much power. As a result, they have to work harder to ignite the fuel, which uses more fuel.


Backfiring is another symptom of a faulty magneto coil. If your engine is backfiring, it’s a good indication that the spark plugs are not working as they should. This can be caused by a number of things, but a faulty magneto coil is one of the most common culprits.

If you’re experiencing this symptom, it’s important to check the other parts of your ignition system as well. This includes the spark plug wires and the distributor cap. If everything else looks fine, then it’s likely that your magneto coil is to blame.

If you notice any of these symptoms, it is likely that your magneto coil is faulty. However, it is important to note that these symptoms can also be caused by other issues. For example, a loss of power could be due to a problem with the spark plugs. Therefore, it is always best to consult a professional before making any repairs. [3]

How to Test a Magneto Coil With a Multimeter

Now that we know the symptoms of a faulty magneto coil, let’s take a look at how to test it. The best way to test a magneto coil is with a multimeter. This is because it can measure the resistance of the coil.

A multimeter is an instrument that measures electrical voltage, current, and resistance. It can also be used to test continuity and diodes. A multimeter is a useful tool for troubleshooting electrical problems.

For this project, we will be using a digital multimeter. A digital multimeter is a type of multimeter that uses a digital display. It is more accurate than an analog multimeter and is easier to use.

The most important thing to remember is that you should never test a coil while it’s still installed on the engine. Always disconnect the spark plug wire and remove the coil from the engine before testing.

Detach the flywheel shroud

The first step is to detach the flywheel shroud. The flywheel shroud is a metal cover that protects the flywheel. It is usually held in place by screws or bolts.

Using a screwdriver or wrench, remove the screws or bolts that hold the shroud in place. Then, carefully lift the shroud off of the engine. Be careful not to drop or damage the shroud. Once the shroud is removed, you will be able to see the flywheel and magneto coil.

Remove the casing off the magneto coil

Once you locate the magneto coil, the next step is to remove the casing. The casing is usually made of plastic or metal and it covers the winding of the coil. To remove the casing, unscrew any screws that are holding it in place. Then, carefully pull the casing off of the coil. Be careful not to damage the coil while removing the casing.


Set your multimeter to ohms mode

The next step is to set your multimeter to ohms mode. On a digital multimeter, this is usually done by turning the knob until it says “Ω.” Ohms is the unit of measurement for electrical resistance. Most multimeters have a range of ohms that they can measure. For this project, we will be using the 40,000kΩ range. The correct range matters because it will give you the most accurate reading.

We suggest you turn the auto-ranging feature off when taking resistance measurements from your magneto coil. This feature is not always accurate and it can give you false readings.

Now that you have set your multimeter to the correct mode and range, let’s move on to testing the coil.

Probe your magneto coil

You probably have noticed by now that a multimeter has two probes, or leads. These probes are made of metal and they are used to touch the objects that you want to test. In order to take a resistance measurement from your magneto coil, you will need to touch both probes to the coil at the same time.

We will first begin with a primary coil. Here you shouldn’t have any confusion. Place the red lead of your multimeter on the U-shaped winding of the coil and the black lead on any nearby ground (metal surface). A good ground to use is the engine block. The U-shaped winding is where the current enters the coil.

Now, take a resistance measurement by pressing the “measure” button on your multimeter. You should see a reading in ohms on the display. This is the primary resistance of your magneto coil. Write down this number for later reference.

Next, we will measure the resistance of the secondary coil. The secondary winding is located inside of the primary one. To measure the secondary resistance, place the red probe on the U-winding again and the black probe on the rubber part of the casing.

You have now successfully measured the primary and secondary resistance of your magneto coil!

Interpreting Your Measurements

The primary and secondary resistance readings that you took in the previous section can tell you a lot about your magneto coil. Now it’s time to learn how to interpret these readings and what they mean.

The first thing you need to do is compare your readings to the manufacturer’s specifications. These specifications can be found in your owner’s manual or on the internet. If your readings are within the specified range, then your coil is most likely working correctly. However, if your readings are outside of the specified range, then there might be a problem with your coil.

Generally speaking, you want to get a resistance reading at least between 3K and 15K kilo-ohms. If your readings are lower than these numbers, then it is likely that your coil is not working correctly.

We advise against trying to fix your coil yourself. If you think there might be a problem with your coil, we recommend taking it to a professional mechanic or auto shop. They will have the tools and knowledge necessary to fix or replace your coil. If you have any particular questions, you can contact the manufacturer of your vehicle directly. [1], [2], [4]


What are the dangers of malfunctioning ignition coil?

If your ignition coil is not working properly, it can cause a number of problems with your vehicle. Ignition coils are responsible for providing the spark that ignites the air/fuel mixture in the engine, so if they are not functioning correctly, the engine will not run properly.

What is the normal voltage of a magneto coil?

This can vary, but most small engines will have a magneto that produces around 50 volts. Coil can further increase this voltage to 15K V.

How do I know if the magneto coil is damaged?

If your engine is having difficulty starting, or if it’s running inconsistently, there’s a chance that the magneto coil is damaged. Another symptom of a damaged magneto coil is if the engine backfires when starting.

To test whether or not the magneto coil is damaged, you’ll need a multimeter. This way, you can test the coil’s resistance to see if it’s within the normal range.

How do you test a magnetic coil?

To test a magnetic coil, you will need a multimeter. You can use either an analog or digital multimeter. First, set your multimeter to the ohms setting. Then, touch oen of probes of the meter to the leads of the copper wire of the coil and another one to the metal clip. If the meter reading is within 3-15K kiloohms, then your coil is working properly. If it is not, then you will need to replace the coil.

What is the resistance of a magneto coil?

The resistance of a magneto coil can vary depending on the type of coil, but it is generally should be between 3k and 15k kilo ohms.

You can test the resistance of a magneto coil with a multimeter by setting the multimeter to the “resistance” or “ohms” setting and probing the terminals of the coil. If the multimeter reading is outside of this range, then the coil is likely defective and should be replaced.


Magneto coils can go bad over time, and when they do, it can be difficult to diagnose the problem. A multimeter is a useful tool that can help you test the coil and determine if it needs to be replaced. With a little knowledge and patience, you can test a magneto coil and get your engine running again in no time. In this article we have covered all the common symptoms of a bad magneto coil and how to test it with a multimeter.

We hope you found this guide helpful! Testing your magneto coil is a great way to diagnose potential ignition problems in your engine. By regularly testing your coil, you can catch problems early and prevent them from becoming bigger, more expensive issues. Thanks for reading!


Mower Won’t Start No Spark (This Is Why)

Pulling and pulling and nothing, a mower without spark, is useless. In this post, we’ll cover all the most common ignition system failures.

Mower won’t start any spark? Common reasons a lawnmower has no spark include:

None of these tests are difficult, and twenty minutes from now, you’ll know why your mower has no spark.This post will have you covered, but if you need video help diagnosing no spark or help to fit a new coil, check out “Mower won’t start video.”

Checking Lawnmower Spark

Since you’ve checked the spark already, I’m guessing you know the procedure. However, it’s worth pointing out, getting this test wrong can lead to misdiagnosing and replacing the ignition coil or other parts unnecessarily.

Spark testing is, as you know, a simple test, you won’t need any special tools here, but a spark testing tool does make the job easier and totally foolproof.

If you need video help, check out the mower “Mower spark test video,” where I cover the whole process.

Tools needed

For these tests, you’ll need a plug spanner, insulated pliers, screwdrivers, and a spark plug is useful. You’ll also need a helper, as we’re not using a spark testing tool. It can be difficult to crank over the engine and, at the same time, check for spark. With all the tools gathered and a helper on hand, we’ll get right to it.

Spark Testing

As we’ll have a helper cranking over the engine, that means the blade will be spinning, and even though the engine’s not running, it can still remove body parts, so, you know!

You must use insulated pliers (plastic/rubber-handled pliers) to hold the plug as the voltages produced are enough to give you a jolt, which isn’t pleasant.

Tools – Plug spanner, insulated pliers, and a spark plug will be needed.

Spark test tools

Step 1 – Remove the spark plug wire by twisting and pulling, then using the plug tool, remove the spark plug.

Step 2 – Reattach the spark plug wire to the plug. Using your insulated pliers, hold the plug threads firmly against the metal of the engine. This is known as grounding. If the plug doesn’t make good contact with the metal of the engine, you won’t get a spark.

Step 3 – While you watch for spark, have the helper hold the bail lever as normal and yank on the pull cord.

If you have no spark, swap out the plug and test again.

If you still have no spark, it is most likely a failed coil, but best to check the on/off switch assembly first.

Common Spark Plug Faults

A healthy spark plug is essential for reliability, power, and smooth running. Plugs have a tough job. They carry high voltages and live at the heart of the engine where it’s hottest.

Making matters worse for the plug is its location – right out front of the engine. So getting shoved into fences and trees is all part of a spark plug’s life, and you thought you had it hard!These are the most common spark plug faults:

  • Wrong plug type
  • Dirty plug
  • Bad plug gap
  • Cracked spark plug insulator

Wrong Plug Type

Plugs areas you know are graded; each engine will have a particular plug code. So even though a plug fits, it doesn’t mean it’s correct. Plugs are graded by heat. The plug should get hot enough to burn off contaminants but not so hot that it pre-ignites. Wrong plug types can cause all types of problems, from hard starting, rough running, hot start failures, etc.

Plug type – Check your plug type with your mower engine maker.

An incorrect plug type will lead to intermittent problems.

lawn, mower, coil, resistance

Dirty Plug

Self-explanatory, it’s a plug that’s contaminated by too much gas (flooding), carbon, or oil. All of these will prevent the plug from doing its job. Flooding may be caused for a few reasons – blocked air filter, faulty choke, overuse of choke, tipping mower over on its carburetor side, and carburetor fault. Check out the video “How to fix a flooded engine.”

Carbon build-up in the engine is a normal condition. Fuel type, oil type, maintenance, and plug type all affect how quickly it builds.

Oil on the plug is also common. It’s caused by too much oil, blocked crankcase breather, head gasket fault, engine wear, and wrong plug type. Check out the video “How to clean a plug.”

Bad Plug Gap

A spark plug function is obviously to create a spark, and it can only do this if the electrode gap is correct. The coil has been designed to create a sufficient spark to jump a pre-determined spark plug gap.

  • No gap, means no spark
  • Gap too small means poor running or no start
  • Gap too big means no start and risks damaging the coil

A plug gap tool is used to set the spark plugs gap. The electrode is manipulated to the correct size by simply bending it with pliers. Check out the video “How to gap a plug.”

Plug gap – The gap is important. Too small or too big can lead to no starts or poor running.

Cracked Plug Insulator

Self-explanatory too. The insulator is the white ceramic material of the plug’s body, and as said earlier, plugs are at risk of being damaged by bumping into obstacles. If the insulator breaks or cracks, the plug stops working.

Common Spark Plug Wire Faults

A spark plug wire has a few particular problems that affect them, depending on a few variables, like how and where they’re stored.

The common faults I see again and again include:

Loose Terminal

Caused by our old friends, the trees, shrubs, and fences. The plug wire terminal that clips to the spark plug becomes loose, and that can cause no starts, poor running, and intermittent starting/running.The fix here is simple, squeeze the terminal body using pliers to tighten it.

A loose terminal will cause the engine to misfire or not start at all. The quick fix here is to squeeze the terminal until it fits snugly on the plug.

Faulty Terminal

Because this cap was loose, it created arching, which burnt the metal of the terminal cap.

Faulty terminal connector – It’s different but related to a loose connector. A loose connector will often turn into a faulty one as the spark starts to jump inside the terminal, burning it or setting up conditions for corrosion to take hold.

The outcome is the same, no spark or poor running. A replacement terminal can be purchased and fitted to solve this issue.

Damaged Plug Wire

Plug wire rubbing off the engine cover can cause the insulation to wear and the coil to ground. But more often than not, a damaged plug wire means rodents. Mice love wiring insulation, and unfortunately, our furry friends have cost us a coil.

Sure, you can wrap them with insulation tape, but it’s only a quick fix. The long-term repair is to replace.

Damaged wire – Mice love to chew on the wiring insulation.

Common Stop/StartAssembly Faults

Most mower owners are familiar with the bail lever at the handlebars, which must be held to start the mower. Most mowers will use this type of stop/start system; other manufacturers may incorporate the stop/start function with the throttle lever. But apart from this difference, all other components will be very similar.

The main components of the stop/start assembly include:

  • Bail/throttle lever
  • Cable
  • Flywheel brake assembly
  • Stop/start switch
  • Coil control wire

Bail / Throttle Lever

Common faults here include disconnected, out of adjustment, or broken levers.


The cables break and stretch, so it’s not uncommon for the bail lever to work, but because the cable has stretched, it doesn’t move the brake assembly to the start position.

Stop / start cable

Flywheel Brake Assembly

Common faults here include cable out of adjustment, meaning the bail lever doesn’t pull the brake to the off position.

Flywheel assembly

Stop/Start Switch

This is the on/off switch. It’s fitted at the flywheel brake assembly. When the bail lever pulls the assembly, it pushes on the switch removing the ground connection to the coil. This allows the mower to start.

On /off switch

Coil control – Here’s a different mower coil control switch. It’s a very simple connection; the contact points must separate before the coil and plug will create a spark.

The Coil (also known as Armature)

The control wire is connected from the stop/start switch on the flywheel brake assembly to the coil, which is fitted to the engine. The coil and plug won’t produce a spark so long as the control wire is connected to the ground (Metal of the engine).

A common fault is the chafing of the control wire on the engine (shorting to the ground); this effect is the same as releasing the bail lever – turns the engine off.

lawn, mower, coil, resistance

Check coil control wire for chafing, especially anywhere the wiring turns sharply around the engine.

Coil control wire – Coil control is a single wire with a push-on connection. Often they’ll come loose, and when they do, the mower won’t turn off.

Common Coil Faults

Coils generally work, or they don’t. Occasionally, you’ll get a coil that works when it’s cold and stops when the engine heats up. Coils are solid-state units – they can’t be repaired. Testing a coil and fitting a new one is easy; I wrote a whole post about it right here “Push mower hard to start when hot”.

Or check out the video here; it covers spark checking, diagnosing, and replacing the coil. If you need to replace the coil, check out the great deals on the Amazon link below.

Lawn mower Ignition Coil Why/How can FAIL! Know this Lawnmower during Coil Test!

Coils – Lawnmower coils give lots of problems; I replace tons of them.

Related Questions

Can a spark plug have a bad spark? Spark plugs wear out. A spark plug should be changed once every year at the start of the new season. You can check the spark plug for spark by removing it, connecting the plug wire, grounding it off the engine, and turning over the engine.

Hey, I’m John, and I’m a Red Seal Qualified Service Technician with over twenty-five years experience.

I’ve worked on all types of mechanical equipment, from cars to grass machinery, and this site is where I share fluff-free hacks, tips, and insider know-how.

And the best part. it’s free!

What Makes a Lawn Mower Coil Go Bad? ( How You Can Prevent This)

It’s very likely that you’ll need to swap out your lawn mower ignition coil at some point during its lifespan. This is a pretty normal replacement that has to be done for all types of lawn mowers, whether it’s a push mower or a ride-on mower. But if you find that your lawn mower ignition coil keeps going bad, then this definitely isn’t normal. So, let’s look at what causes multiple ignition coils to fail and what you can do to prevent it.

What Causes Repeated Ignition Coil Failure? (The Short Answer)

The most common causes of repeated ignition coil failure are engine overheating, defective coil components such as spark plugs and cables, using incorrect parts, and poor maintenance methods. Any one of these causes, or a combination of multiple causes, can lead to your lawn mower coil failing.

What Makes a Lawn Mower Coil Go Bad (6 Possible Causes)

A lawn mower coil is made up of an iron core and copper winding tucked neatly inside the lawn mower’s ignition coil. Every time the magnet attached to the flywheel passes the coil, there is a complex reaction between the iron core, the copper winding, and the magnet that produces an electrical charge. As soon as conditions for this reaction change to less than ideal, the coil suffers. So, let’s take a look at what causes an ignition coil to go bad.

Overheating of the Engine Coil

During the reaction inside the coil, a fair amount of heat is generated on the copper windings. To make sure the windings can cope with the heat, the copper winding is insulated to make sure that the single wire of the winding never touches itself. Now, this insulation is rated to cope with the reaction heat and the heat of the engine. Consequently, the insulation is not rated to take the added heat of an overheating engine.

So, if your lawn mower is low on oil or has a problem with cooling, then it’s probably overheating The result is the insulation loses its integrity, and the copper winding arcs back on itself. The final result is a change in the amount of charge created and the time of release. These changes lead to a burned out ignition coil.

Over Gapped Spark Plug

The charge that is created in the coil needs a method to discharge itself so that it can continue to process and make more charge safely. This is where the spark plug steps in. The spark plug on your lawn mower is basically a grounding point that this charge is attracted to.

So, once the charge gets to the end of the spark plug, it needs to arc over the gap. Now, if the gap is too big and the sparkplug is over-gapped, the arc can’t happen. This makes the coil think that more current is needed, so it increases the charge. Unfortunately, the coil isn’t designed to do this. The result is the coil produces more current and more heat. The one thing that the coil can’t handle is extra heat. So, if your spark plug is over-gapped, your coil is going to quickly burn out.

Faulty or Damaged Spark Plug

Other than over-gapping the spark plug, you might have a bad spark plug that needs changing. A bad spark plug is going to have the same effect as a poorly gapped spark plug. The coil is going to overheat because it is not able to discharge. The result is another burned out coil.

Incorrect Spark Plug

Yep, I’m afraid we’re still talking about spark plugs, but this is the last one. If you’re sitting there wondering, “why does my ignition coil keep burning out” it could be because you’re using the wrong spark plug.

Well, you’ll find that you can buy a spark plug that both fits into your lawn mower and that connects to the ignition cable/spark plug cable. But just because it fits doesn’t mean it’s the right one.

What happens if you use the wrong spark plug? Well, inside the spark plug is an electrode that the current reaches before it arcs to the ground. This electrode is designed with a specific resistance. So, if you have the wrong spark plug, the charge can’t pass the electrode. A wrong spark plug is the same as an over-gapped plug and a faulty plug. Once again, the coil produces more charge, more heat and burns itself out.

Damaged Ignition Cable/Spark Plug Cable

The reaction between the protons and the neutrons inside the coil produces an electron, the charge. This electron charge needs somewhere to go, so it heads for the spark plug. Time for the spark plug cable to step in.

Now, some years back, you would buy a coil and cable separately, but nowadays, they come as one item. Before, it was possible to get the wrong combination and mess up your coil. Fortunately, you don’t need to worry about a mix-up these days.

However, you do need to be aware of cable damage. Just like the overheating problems a spark plug can cause, a cable can do the same. If the cable is damaged or broken, the charge will back up in the coil and form the same heat damage.

Poor Practices

Let me ask you a question. Do you think it’s a good idea to test a spark plug that is attached to your lawn mower without it being grounded? Definitely not. Most of us, at some point, have removed the spark plug from the mower and pulled the starter cord to see if it’s working ok.

Well, if you pull the cord and the spark plug isn’t grounded to the mower’s engine, the coil begins to overheat. This is just the same as a bad spark plug or a broken cable. The charge has nowhere to go.

Signs a Lawn Mower Coil is Going Bad

When figuring out what makes a lawn mower coil go bad, you’ll probably be seeing a few symptoms with your lawn mower before the coil burns out. If you notice these symptoms quickly enough, you might be able to avoid another coil replacement. Here’s what to look out for.

Is There Anything You Can Do If Your Mower’s Ignition Coil Keeps Going Bad?

Now that we have been through what makes a lawn mower coil go bad. Let’s have a look at a few things that will help you avoid another replacement. Here are some tips to help keep the coil protected.

Check the Engine Oil Level

A common cause of a lawn mower engine overheating is low oil. A lack of oil in the engine causes the metal parts to rub together and generate excessive heat. I suggest that every time you fill the gas tap, you check the oil level.

Carry Out Regular Oil Changes

As oil is used and heated, it starts to lose its cooling and lubricating abilities. This results in overheating and potential coil damage. I suggest working out an oil change schedule and make sure that you are changing your lawn mower oil often enough.

Clean Out the Air Vane Guard

When I finish using my lawn mower, I always make a point of cleaning off all the grass. I also make sure to clean out the flywheel. Located on top of the flywheel is a fan that cools the engine. It is super important to keep this clean so that the engine can cool efficiently.

Gap the Spark Plug

Gapping a spark plug is a job that a lot of people skip. I’m guessing this is because it’s not always easy to understand why it’s important. But once you know what makes a lawn mower coil go bad, I’m pretty sure you will not skip it again. So, the simple solution is to get a spark plug gapping tool and gap your plug regularly (here’s a post that explains how to gap a mower spark plug).

Double Check the Spark Plug Specification

If you look in your lawn mower manual, you’ll find out what size spark plug your mower needs. If you can’t find it, you can look online or check with your local mower store.

Using the Mower’s Off Switch

Within the electrical circuit of your lawn mower, there is a bypass for the coil. This basically grounds the coil in a different direction than the spark plug. So, if you are doing repairs on your lawn mower that require the flywheel or engine to turn, make sure the lawn mower is switched off. Disconnecting the spark plug will stop the mower from starting, but it won’t protect the coil.

About Tom Greene

I’ve always had a keen interest in lawn care as long as I can remember. Friends used to call me the lawn mower guru (hence the site name), but I’m anything but. I just enjoy cutting my lawn and spending time outdoors. I also love the well-deserved doughnuts and coffee afterward!

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