This week a vehicle was towed into our shop. The primary complaint being that when you went to turn the key and start it, the engine would not crank and would instead make a high pitched "whirring" sound. Most often when this is seen, the main problem is the starter itself malfunctioning. Normally, when you turn the key, a gear slides into contact with the flywheel, which is attached to both the Engine and Transmission. Once the gear is in place the starter spins the flywheel, allowing the engine to start. Sometimes the gear fails to engage the flywheel and spins in place, which also results in the engine not starting.
In this particular case, the teeth on both the starter and flywheel were worn, and a few were missing. Beyond the missing teeth, most of the others on the flywheel and the teeth on the starter were worn to such an extent that they were not contacting each other. As you can see in the following photographs, there are several spots that are damaged on the flywheel. The only solution is to replace the flywheel and starter, and in order to do that you have to partially remove the transmission to gain access to the flywheel.
As you can see, the left side of these teeth are worn
Another area of the flywheel showing the damage on the left side (this is also the side that faces the front of the vehicle and the direction from which the starter engages)