How to Test a Coil on a String Trimmer

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The ignition coil on a string trimmer conducts the strong electrical current that fires the spark plug. This in turn ignites the fuel in the cylinder. If your ignition coil stops working, the string trimmer will either slow down or stop running altogether. However, it can sometimes be difficult to determine if the coil is what's causing the problems with your garden tool. Testing the ignition coil with an ignition system tester will reveal if that part of the string trimmer is causing the problem.


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How the Ignition Works

Three major parts comprise the ignition system on a string trimmer: a magnet, which is located on the flywheel, a spark plug and the trimmer's ignition coil. When it's time to start cutting weeds, you pull the cord and the flywheel rotates. A current is induced once the flywheel spins past the ignition coil, and this current moves through the wire to the spark plug. The traveling of this current to the spark plug creates a spark that ignites the fuel-air mixture in the chamber. This process is what makes the string trimmer start.


Ignition coils tend to burn out over time or develop increased electrical resistance. This can lead to misfires or a weakened, less-efficient spark. If the spark starts having problems, your trimmer will too.

Testing the Trimmer's Ignition Coil

The easiest way to test your string trimmer's ignition coil is to use an ignition system tester. These testers are fairly inexpensive, simple to use and can be bought online or at your local hardware store. To test your string trimmer's ignition coil, connect the tester between the spark plug boot and the engine.


Then, when you pull the starter cord, watch for a spark in the tester window. The good news is that his type of tester can be used on any tool with a small engine, including a string trimmer, a chainsaw or a snowblower — making your purchase even more worthwhile.

Weak Spark vs. Strong Spark

When testing your string trimmer with the system tester, you'll need to watch for a spark in the test window. If a weak spark appears in the window, this means you'll need to replace the ignition coil. If you see a strong spark but the string trimmer still fails to start, you can try cleaning, regapping or replacing the spark plug, and then try the test again to see if the spark is stronger.


Always Practice Safety

The ignition systems in this type of string trimmer can carry a lethal charge. This means you must take the necessary safety precautions when working on them. Make sure to set the trimmer on the ground. Wear leather work gloves. Be sure that you disconnect the HT lead wire from the tip of the spark plug when troubleshooting. And if you're not comfortable working with equipment that requires safety precautions, remember that there are alternatives to powered lawn tools. For weed trimming, you could go with a rotary lawn edger, a step edger or a flat-head spade.