Is there no spark in the engine of your lawn mower, snow blower or outdoor power equipment? Follow this guide to test your ignition system - including coil, switch & module - to identify any problems and troubleshoot repairs.

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If you know your specific problem, jump down to the section for step-by-step instructions.

How Do Ignition Systems Work in Small Engines & Lawn Mowers? Servicing Spark Plug Problems How to Test the Ignition Coil in Your Small Engine Testing & Replacing a Stop Switch Ignition Module Faiattract How To Inspect the Flywheel & Key

WARNING: Always read the engine and equipment manual(s) before starting, operating, or servicing your engine or equipment to avoid personal injury or property damage. See an authorized dealer or contact Briggs & Stratton if you are unsure of any procedure or have additional questions. Find all Engine Safety Warnings

How Do Ignition Systems Work in Small Engines & Lawn Mowers?


The ignition system is the starting system for your small engine. Whether you start the engine with a pull rope or the turn of a key on an electric start motor, you"re relying on the ignition system to produce a spark inside the combustion chamber.

Small Engine Ignition Parts

Flywheel with magnetsCoil or ArmaturePush Button starting or Pull Cords (depending on your engine type)Spark Plug LeadSpark Plugs

When you start your lawn mower or small engine, you turn the flywheel and its magnets pass the coil (or armature). This creates a spark. The ignition system coordinates the timing so that the spark will ignite the air-fuel mixture in the combustion chamber just as it reaches maximum compression in each engine cycle- thus, maximizing the engine’s power.

Once the engine is running, the flywheel keeps rotating, the magnets keep passing the coil and the spark plug keep firing based on a specific timing.

Types of Ignition Systems

Solid-state systems: the more modern option, these systems use a tiny transistor in the coil or armature to close the electrical circuit that travels through the spark plug lead to the spark plug(s).Breaker point systems: used on engines made before 1980, these systems use a mechanical switch instead of a transistor to close the electrical circuit used to produce a spark.

Common Flywheel Problems

If you are experiencing ignition timing issues, this is most often due to a sheared flywheel key. You can also test the flywheel magnets for any potential issues.

For information regarding this, please visit our Inspecting the Flywheel and Key FAQ.

Common Spark Plug Problems

How To Test the Ignition Coil in Your Small Engine

Safety Warnings: Stay clear of any rotating, moving parts, or other hazardous areas whenever attempting to start the engine or equipment.

The coil is probably the easiest thing to check and therefore the first thing to check when embarking upon ignition system troubleshooting.

Testing the Coil or Armature

Step 1: Clip one end of the spark tester (service part number 19368) to the ignition cable and the other grounded to the cylinder head as shown below.

Step 2: Spin the flywheel rapidly (at least 350 RPM) and watch for spark in the tester window.

If the spark jumps the tester gap, your ignition coil is working fine. If not, it needs to be replaced.

Engine quits while running? Hook the tester up between the ignition cable and the spark plug and start the engine. When it stops, monitor the window.

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Common Mistakes When Testing Coils

Be sure to unhook the coil from the equipment wiring harness as well as the engine"s wiring harness and use the spark tester. Many a technician is fooled into replacing a good coil because the coil grounding lead was shorting out against a piece of sheet metal.DO NOT attach the tester to the spark plug for this test. The engine may start. Without the grounding lead installed, you won"t be able to turn it off.

Replacing Ignition Coils or Armatures