How to Test a Lawnmower Coil? 20 Things You Should Know

After days of waiting, the weather has finally cleared up. You are excited to level your lawn grass, which grew quite a lot in the last few weeks. You start your lawnmower in great hopes, but it doesn’t work. You realize that the problem might be in the lawnmower coil, but you don’t know how to test it. A lawnmower coil is an essential component of a lawnmower, and if your lawnmower isn’t starting up, there is a big chance that its coil isn’t working correctly. In that case, no need to worry since this article will provide you a clear and comprehensive insight on how to test your lawnmower coil.

How to test a lawnmower ignition coil?

There are several ways to test and fix a lawnmower coil. The most common methods to test a lawnmower coil are using a multimeter or a spark plug tester. Both methods are simple and can be easily carried out by the lawnmower’s owner if instructions are followed. In the multimeter method, a multimeter is used to measure coil resistance, which is the criterion for testing in this case. In the spark tester method, the voltage developed across the spark plug electrode gap is the testing criterion.

Let’s get into the details of what an ignition coil does in a lawnmower and how to check if it is faulty.

Ignition Coil:

An ignition coil, which is generally called a lawnmower coil, is an essential lawnmower component. It transfers voltage from the battery to the spark plug, which generates the spark that starts the mower. As the name suggests, it is two set wires coiled around its central metallic core. It is mounted close to the flywheel, which spins when the starting rope is pulled. Flywheel has permanent magnets mounted on it that pass close to the coil when the flywheel spins, generating a magnetic induction voltage. A current passes through the wire, which connects the coil to the plug terminals, and the current generates a spark responsible for igniting the fuel and starting the mower.

Testing the Coil:

The lawnmower coil is also the easiest component to check, which is why we always check the ignition coil when faced with ignition system troubleshooting. Some of the most common ways of testing lawnmower coils are mentioned below, which will help solve the problem.

● Using a Multimeter:

A multimeter is a great measuring instrument with uses in many fields. A multimeter is also useful for small engines, and a lawnmower runs on a small engine, so we can use a multimeter to test a lawnmower’s coil.

– Equipment Needed:

• Digital multimeter.
• Leather gloves.
• Wrench.
• Lawnmower user manual.

– Procedure:

Step 1: Safety

Wear your leather gloves, and make sure to keep wearing them throughout the whole procedure.

Step 2: Remove the cover

The spark plug has a cover, normally a rubber cord, and can be pulled off easily with a slight tug. First of all, remove that cover. Then remove the engine cover on the top of your lawnmower if there is any since some lawnmowers do not have that cover.

Step 3: Locate the coil

First locate the flywheel of the engine. You can use your lawnmower’s user manual to see how it looks and where it is. Remove the flywheel with a wrench, and you will see ignition coils on the top of the engine. You will see that one terminal of the coil would be attached to the spark plug.

Step 4: Connect the multimeter

Now set your multimeter to measure resistance and then set it to 20k ohms setting. Insert the positive terminal of a multimeter (red probe) inside the spark plug and touch it to the metal plate. Make sure it is making proper contact. Now touch the multimeter’s negative terminal (black probe) to ignition the coil’s body again, making sure that it is making good contact. Watch the reading on the multimeter when the needle stops fluctuating.

Step 5: Check the resistance

If your reading is between 2.5k to 5k ohms, then your coil is working fine. If the reading is lower than that, it means one of the coil windings has a short, resulting in low internal resistance. The coil needs to be replaced in that case.

Step 6: Resistance is too high

If the reading is higher than 5k or around infinity, it means your coil is not working properly and should get replaced.

● Using a Spark Tester:

– Equipment Needed:

• Screwdriver.
• Leather gloves
• Spark plug tester.
• Mower owner’s manual.

– Procedure:

Step 1: Safety first

We use a 19368-spark tester for this experiment. Wear your leather gloves and keep wearing them throughout the experiment.

Step 2: Find the ignition coil

Remove the engine cover and locate the flywheel. Follow the lawnmower’s user mutual for better understanding. You’ll see the ignition coil under the flywheel. One end of the ignition coil would be leading to a spark plug.

Step 3: Use the Spark Tester

Now connect one end of the spark tester to the ignition cable and ground the other end. Spin the flywheel at around 350 RPM and look for a spark in the window.
The electrode gap in this tester is 0.166″ wide, and it has been calculated that to jump this gap, it requires around 13,000 volts. 10,000 volts is required to jump on a cold spark plug. Adding it all up, we see that we have some voltage to spare. If the spark jumps the gap, it means the said ignition coil is okay. If not, then it needs to be changed.
The same test can be done on warm engines. If your engine stops while running, connect the tester in line with the spark plug. Start the engine and watch the window when it stops; if a spark can be seen, it means your coil is working fine, and the problem is somewhere else. This test is very hard on the coil since it requires the coil to build up enough voltage to jump two gaps. So, if your engine starts and runs fine, it means your coil is very healthy.

Coil Testing Tips:

The following tips will help you avoid common coil testing mistakes:

• Make sure there is no ground connection: Ensure to detach the coil from the equipment wiring harness and engine’s wiring harness before connecting the spark tester. Only use the spark tester when you have unhooked the ignition grounding lead from the coil. Technicians often think the coil is not working properly and replace it because the coil’s grounding leads shorts out against a sheet metal piece.
• Connect with the coil: Make sure that you connect the tester to the coil and not the spark plug for this test. The engine might start if it is attached with a spark plug, but you would not be able to turn it off because there is no grounding lead installed in this case.

Why Does the Ignition Coil Break?

Long term wear and tear can cause the ignition coil to go bad over time. It also gets damaged because of poor spark plug ignition cables. An abnormal high resistance would well up in the spark plug ignition cables if they have some fault. That high resistance would cause an additional voltage to be generated by the ignition coil, which will result in excess heat. The heat melts the ignition coil’s insulation, which causes the wires to break and requires a new ignition coil. The connection between the ignition coil and flywheel can also be one reason for the coil’s breakage. If that connection wears out, the coil may stop turning and hence causing the lawnmower to stop.

Replacing a Lawnmower Ignition Coil:

If your coil comes out to be faulty after performing one of these tests, it needs to be replaced. There are proper steps to be followed while replacing the coil, as mentioned below:

• Step 1: Unhook the spark plug wire and secure it, removing the batteries if there are any. – Detach the mounting screws of the old coil from the engine.
• Step 2: There will be a stop switch wire with a flywheel brake; you have to detach it and then remove the coil.
• Step 3: Use mounting screws to attach your new replacing wire.
• Step 4: Push your new ignition coil (armature) away from the flywheel and tighten one screw.
• Step 5: Magnets should be on the opposite side of the ignition coil, so you have to turn your flywheel.
• Step 6: Place the appropriate shim between the rim of the flywheel and the ignition armature. You have to make magnets directly adjacent to the armature and in order
• Step 7: To do that, turn the flywheel while holding the shim until magnets reach that position.
• Step 8: Loosen the tight screw, so the ignition coil gets strongly pulled by magnets against the flywheel and shim. Then, tighten both mounting screws and rotate the flywheel until the shim slips free.
• Step 9: Reconnect the stop switch that you had detached earlier.

Final Remarks:

An ignition coil is an essential component of a lawnmower. It is responsible for the spark that starts the lawnmower engine. Mostly when a lawnmower does not work, an ignition coil is assumed to be the cause of it. So we test the ignition coil to find out if it is in its best condition or not. The most

Common methods to test a lawnmower coil are using a spark tester or a multimeter. Both are simple and can be easily carried out by the lawnmower’s owner if instructions are followed right.

Multimeters and spark testers are also very cheap and readily available. The spark tester method has an advantage over the multimeter method because you only have to check for a spark, which is easier than determining the coil’s condition based on its resistance. But the multimeter method is also preferred by some due to the familiarity of most people with multimeters. If your ignition coil comes out to be faulty, you have to immediately replace it with a new coil. The ignition coil can go bad over time, but you can make sure it lasts as long as possible by cleaning it once in a while and making sure the spark plug ignition cables are not damaged.