Lawn mower coil pack. Ignition Coil Packs: A Simple Guide

Ignition Coil Packs: A Simple Guide

An ignition coil pack takes the low voltage from the battery and boosts it to the level where it can cause a spark to bridge the gap in a spark plug. This then ignites the fuel and air in an engine’s cylinder. Hence, ignition.

So, if your car has trouble starting, runs rough, has a misfire, or is getting thirstier, it may be that there is a problem with the ignition coil, or one of the coil packs.

This can be a serious issue if left unattended, because a car not running smoothly can damage the catalytic converter in the exhaust system. However, the good news is that an igntion coil pack replacement is usually pretty cheap if caught early.

What is an ignition coil?

The coil is the part of a car’s ignition system that takes the battery’s 12-volt output (called low-tension current) and transforms it into as much as 45,000 volts (called high-tension current) before then supplying it to the engine’s spark plugs. It is typically just a wire-wound transformer filled with an insulator.

Why does an engine need one?

Without a coil pack, the spark plug wouldn’t receive a voltage high enough to do its job of igniting the fuel/air mixture in the combustion chamber. The pressures are so high in the cylinder that the voltage has to be extremely high for the spark to be effective. A coil that is going bad can deliver a voltage that only fires the plug under certain conditions, which is what causes an intermittent misfire.

What does the coil look like?

If your car is an older vehicle with traditional distributor ignition, it’ll look like a small metal cylinder (in fact, it’s often called a canister-type coil) with wires sprouting out of it, one connecting it to the battery, another to the distributor. Some systems, like the GM HEI distributor, mount the coil directly in the distributor cap, and look like #1 below.

On a modern car, coils typically are mounted directly on top of the spark plugs, so the high voltage does not have far to travel. Others mount boxy coil packs to the fender, firewall, or motor, with short leads to the spark plugs. Some coils are constructed in series and called cassette or sequence coils, or coil rails. The engine computer sends 12 volts to each coil in turn to fire the plugs when needed.

Where is the coil?

In older vehicles, the coil pack is typically mounted to the firewall or the engine near a distributor that ‘distributes’ the high voltage it produces to each spark plug, via thick rubbery spark plug wires.

On modern cars with an electronic ignition system that uses a computer rather than a distributor to fire the spark plugs at the right time, the coil is likely to be mounted directly to the spark plug, or close by, without the need for high-voltage spark plug wires. For this reason, a modern engine can have multiple coils.

How does the coil work?

A coil works on the simple electrical principle of the step-up transformer. It does this using two separate coils of wire, both coiled around a central core, all contained within an insulated body. One wire, called the secondary, is made up of thousands more windings than the other one, called the primary. The difference in the number of windings (imagine a spool of thread) determines the level of voltage that comes out for a given input. If you’d like, the Wikipedia article on transformers goes into great detail. The primary wire receives the low voltage from the battery which generates a magnetic field around it. However, the instant that flow is interrupted by the ignition system or electronic control unit (ECU), the magnetic field collapses, creating or inducing a higher voltage in the secondary wire that travels to the spark plug.

Why does a coil fail?

Ignition coil packs hate heat and vibration, so a hot engine bay is a challenging place for it to live. Over time, the temperature and shaking can break up the coil’s windings and insulation. If the internal insulation breaks down, it can cause a short in the winding, limiting the amount the voltage gets stepped up. Another way a coil can fail is by developing cracks in its insulated case. These cracks can then allow moisture in to short out the windings intermittently, causing rough running. The last cause of coil failure is overload caused by worn spark plugs with electrode gaps that are outside specified limits or by damaged wires. In time, the needed voltage can rise to levels, causing the coil to overheat and short circuit.

How do you diagnose a faulty coil?

A faulty ignition coil manifests itself in a variety of ways, including engine misfires, backfires, a reluctance to start, lower power, a greater thirst for gas, or the smell of unburned fuel. If your car has a distributor-based ignition system, all the spark plugs will be affected, but if it’s a modern car with computer-triggered ignition, only one plug could be affected, or two if they share the same coil. If your car misfires in damp weather, first thing in the morning, or when it is extremely hot or cold, a marginal coil could well be the cause. If your car was built after 1996 it should have an OBD II (on-board diagnostic) port with misfire detection, and will light the check engine light if there is a repeated misfire. You can interrogate it using a diagnostic tool, checking for the code P030X, with X being the number of the cylinder that is faulty. Of course, a faulty cylinder can be caused by all manner of ignition and fuel supply problems, not just a faulty coil. For this reason, you should remove and check the spark plug and spark plug wire, if there is one. Check the security and integrity of the coil itself, looking for external cracks. On most cars with multiple coil packs, you can swap the coil on the misfiring cylinder for a different, good one. If the coil is bad the misfire code should now move to the other cylinder.

What to do if there is a bad coil?

Coil packs are easy to change for the DIY mechanic. Once you locate the coil or coil packs, which will be fairly easy once you remove the engine cover, there is usually just one screw or a small bolt holding it down. Remove the bolt, pull the coil and it will pop off. There is no way to fix a defective unit.

Lawn Mower Ignition Coil Test With Multimeter

There are many helpful uses of multimeters around your house and while many of these are typically used on your vehicle or in your home, it is also very useful for smaller engines you use including your lawnmower.

If you have ever tried starting your lawn mower and pulled that cord over and over without any luck, it could mean many different things including a fault with fuel/oil, the induction coil, or the spark plug.

Checks before testing the ignition coil

After eliminating things like oil and gas being present and that the cord is connected to the spark plug, wait 10 minutes after you’ve tried starting it and then try starting it again as an initial troubleshooting.

If it still won’t start, before you begin testing the ignition coil, it’s important to first check the spark plug to eliminate that as an issue.

Before you start the next steps, it’s important that you wear leather work gloves to protect your hands from different parts of the engine that may be sharp or spark.

Spark plugs

Often get dirty and worn down over time. If you’ve been using the same spark plug for a while, you may want to pull it out and check for different signs of problems with your lawnmower. Removing the spark plug is easy by using a socket wrench and twisting it out.

If your spark plug has some light-brown deposits on it, it means it’s normal. If you see black carbon deposits then it’s a sign that the fuel-to-air mixture is too rich and that your carburetor has an issue. Safe to say if you know you’ve had the same spark plug for the past year in your lawn mower, it’s time to change it.

You can also use a spark plug tester (purchased in most auto stores) which you can replace the spark plug with and try to pull the cord of the lawnmower to start it.

If you see a spark appear in the tester, you know that the lawnmower is in good condition but the spark plug was faulty and it’s time to replace it. A spark plug tester is an easy way to check that.

What is an affordable multimeter to test this on?

There are many great multimeters on the market and if you’re looking for the best brand, then the Fluke brand is definitely the one to go for. The Fluke 116, Fluke 117, and Fluke 87V are some of the best on the market.

If you are looking for a good multimeter that’s a bit more affordable, then I would instead suggest the Innova 3340 which is a much better choice for DIY and homeowners.

Testing your ignition coil by using a multimeter

After you have ensured the spark plug is not an issue, it’s time to check on the ignition coil.

Make sure you continue to wear your leather gloves through this part.

Here are the steps to follow to check the ignition coil:

  • Remove the spark plug cover which is usually a rubber cord that pulls right off the spark plug easily with a little tug.
  • Remove the engine cover which is on the top of your lawn mower. Depending on the type of lawnmower you are using, there may not be a cover (in the case of a riding lawnmower, the hood may already be the cover).
  • Locate the engine’s flywheel which looks like a circular piece with multiple blades going around in a circle. Each engine is different so if you are unsure you should check your lawn mower’s user manual to determine what/where it is.
  • Remove the flywheel using a wrench and then inspect the coils. The ignition coils are situated on top of the engine beside the flywheel with two ends touching the flywheel rim. One of the coil’s terminals should be shown leading to the spark plug.
  • Now, set your multimeter to test resistance (ohms) by first placing the red lead into the socket which has this symbol: Ω. Then set your multimeter to measure ohms(Ω).If you need more information on measuring resistance, check this articlehere.
  • Touch the positive(red) probe to the metal connector inside the spark plug housing. The negative(black) probe should then be placed against the metal that extends out of the second terminal on the ignition coil.
  • Allow your multimeter to measure for a few seconds until it stops fluctuating. If the meter is showing 0, your coil is no longer working and you will need to bring it in for repairs.
  • On a working ignition coil, you should get a reading of between 0.5 and 1.3 ohms. If it’s below 0.5 it needs to be replaced as it will cause damage to your engine.

Why does the ignition coil break?

Typically, the ignition coil can go bad over time due to either long-term wear and tear or due to bad spark plug ignition cables.

If the spark plug ignition cables are getting faulty, it will have a much higher than normal resistance which causes additional voltage to be generated by the ignition coil which causes excess heat.

This, in turn, melts the coil’s wire insulation causing the wires to break requiring a new ignition coil.

Wrap up

The things you can do with a multimeter is endless and you can find so many different benefits from using it that it’s no doubt one of the best devices to keep around the house.

I often talk about many of these tasks that you should do yourself at home not only save you time and effort, but also to save you money.

Doing the above test may have caused you up to 50 at a small engine shop which you could do yourself. Start saving time and money by learning these awesome multimeter life hacks.

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.

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.”

lawn, mower, coil, pack, ignition

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:

How To Check A Simple Lawn Mower Ignition Coil

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.

lawn, mower, coil, pack, ignition

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.

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.

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.

lawn, mower, coil, pack, ignition

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.

Simple Trick To Diagnose A Bad Ignition Module (Coil) On A Lawnmower, Chainsaw, Weedeater Etc.

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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