The ignition module on a John Deere lawn tractor is responsible for creating the electricity sent to the engine cylinder where it ignites the fuel and oxygen, causing the engine piston to turn. Under heat or wear, the windings inside the coil can become damaged, causing the engine to run erratically or not run at all. You can easily test the coil in as little as 10 minutes.
Park the lawn tractor on a hard, level surface and turn the ignition key to the "Off" position. Place the forward/reverse pedals into the neutral position and set the parking brake. Place wheel blocks under the right front tire.
Raise the engine hood. Locate the spark plug wire at the front of the engine, coming out from the engine shroud. Remove the spark plug wire from the spark plug. Connect the end of the spark plug wire into the spark tester. Attach the alligator clip from the spark tester to the metal tip on the end of the spark plug.
Adjust the knob on top of the spark tester until the gap is set to 4.2 mm. Turn the ignition switch to the "Run" position and watch the window in the spark tester. If the spark is strong, steady and blue, the ignition module is fine. If the spark is weak or there is no spark at all, the ignition module needs replacement.