The ignition system of a vehicle has several main components. Cranking of the engine is performed by the starter. As the starter ages, several problems may occur, one of which is that the starter will continue to run after the engine has started.
The ignition switch determines when the starter is to engage the vehicle's flywheel. This switch is a basic electronic component that senses when the key is turned to the different selections. A malfunctioning switch may allow current to flow continuously to the starter and allow it to run while the engine is started.
If the starter fails mechanically, the gear may not be able to retract from the flywheel and back into the starter housing. When this happens, the starter will stay connected to the flywheel and turn with the engine.
If the flywheel becomes damage, the starter gear may not engage the flywheel properly. Typically this will cause a non-starting condition, but it can cause the starter gear to become jammed in the flywheel teeth. This will cause the starter to be spun by the engine after it is started.
