How To Test a 2-Stroke Ignition Coil
By: Leon Rhodes
Every 2-stroke and 4-stroke combustion engine that powers a lawn mower, chainsaw, grass trimmer, leaf blower or other outdoor equipment has an ignition coil. Unlike coils for cars which use a distributor system, the ignition coil(s) on a small engine is responsible for providing sufficient current directly to the spark plug to generate spark.
Since most 2-stroke and 4-stroke engines have a single cylinder they also have one ignition coil. If your small engine has two cylinders it will have two ignition coils, one for each cylinder. The easy way to tell how many cylinders a small engine has is to count the number of spark plugs it has(one per cylinder).
The Most Common Way to Check for Spark
The most common method used to test for spark (and a good ignition coil) is to remove the spark plug and, with the ignition coil attached, ground the plug to the engine and visually detect spark. Alternatively an inline spark testing tool can be used.
The good thing about these methods is that if the engine has spark you will know the ignition coil is good. But what if the engine doesn't have spark or you want to test the replacement before installing it?
The problem with the most common testing methods are that they don't narrow an electrical problem down to the coil itself. They test the entire system including the plug, coil, wires and flywheel together but any single component can be faulty.
How to Properly Test a 2-stroke Ignition Coil
#1 - Remove the coil assembly from your small engine.
The ignition coil is held in place with two bolts, a stop wire(kill switch) and the main cable with a boot which connects to the spark plug(See diagram below).
To gain access to an ignition coil you will typically need to remove the engine's shroud and/or remove the starter cover.
#2 - Inspect for signs of coil damage such as a torn boot or frayed wires.
When checking for frayed wires pay attention to signs of rodent chewing on any wire cladding(rubber wire covering) because damaged cladding can lead to moisture corroding the wire inside.
I recommend you temporarily connect the boot to a spark plug to see if the connection is secure. If it's loose electricity will not pass as easily which might cause intermittent loss of spark. Disconnect the spark plug before continuing.
#3 - Measure coil winding resistance using a multimeter.
I made this simple ignition coil resistance chart to help you better understand where to connect your multimeter's leads and which readings to look for.
Start by setting your multimeter to measure resistance(Ohms) and select the 20k resistance range.
Next, connect your multimeter's negative(black) lead to the base of the coil(point A on the diagram) and connect the positive(red) lead to the stop switch(Point B on the diagram). A reading of between 2k and 18k Ohms is generally good.
After that move the positive(red) lead to the connector inside the boot(Point C on the diagram). You should get a similar 2k to 18k Ohm reading afterwards.
If everything is good so far connect your negative(black) lead to the stop switch(B on the diagram) and touch points A and C with the other lead. Neither should give a reading, if they do the stop switch is faulty(grounded).
Important: As you can see on the chart, the stop switch is directional. You will not get a reading when measuring from B to C but reversing the leads and testing from C to B does result in a high Ohm reading. Check with your manufacturer's specifications for exact readings.
For safety purposes never run a small engine without the stop wire connected, you may not be able to stop the engine if you do.
- Frequently Asked 2-stroke Ignition Coil Questions
How can I find exact ignition coil resistance specs?
To find manufacturer specs for any engine component find the engine model number stamped on the engine itself and look that up. Engine model numbers are different than the model number of the equipment itself.
How can you tell if an ignition coil is bad?
The easiest way determine ignition coil condition is to test with a multimeter. If you get a reading of no resistance(short) when resistance is expected or, alternatively, you get an infinite resistance measurement when only some was expected the coil is likely defective.
How does a 2 stroke small engine ignition system work?
A typical 2-stroke engine has magnets mounted to one side of the flywheel which are aligned with the 'top dead center' position of the combustion stroke. As these magnets pass in front of the ignition coil an induction charge is produced and sent to the spark plug.