How to test and repair small engine ignition system problems? 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 Engines - » Basic Tractor wiring diagram For example, the new garden tractors that have and electic PTO and a mow in reverse bypass switch. This setup requires the safety wiring system to run through the PTO switch and the introduction of a relay or two to allow the tractor to move backwards without killing the mowing deck or the engine. The thing in the middle is the Ignition Swtich.