Test a small engine ignition coil by removing the engine's cover, attaching one end of the tester to the ignition cable, grounding the other end to the cylinder head and manually spinning the flywheel at high speed. If a visible spark forms in the spark tester, the coil works.
When no spark forms in the spark tester's gap, the most common assumption is that the ignition coil is faulty and requires replacement. While coils do sometimes fail, there are several common testing mistakes that can cause you to unnecessarily replace a perfectly good ignition coil. Knowing how to avoid these common mistakes saves time, money and hassle when repairing a small engine.
To form a spark in the tester's gap, the flywheel must be spinning quickly enough to generate electricity. For most engines, the flywheel has to spin at least 350 revolutions per minute or no spark forms, even if the coil is in perfect working order. If you do not see a spark, try spinning the flywheel as fast as possible before replacing the ignition coil.
Before connecting the tester, disconnect the coil from both the engine wiring harness and the equipment wiring harness. If the coil is touching either of these pieces of metal, its spark can short out between the gaps. Therefore, even though the coil is generating a spark, it never gets to the tester.