Replace the starter drive assembly, (the bendix; but that's a GM term). There is no accurate way to test them because you can't put the same force on it as the starter motor can. They are diagnosed based on the symptom, and you described it perfectly. The teeth on the ring gear aren't the cause either. The first half of a few teeth will wear away when the drive gear starts spinning before it meshes completely. Eventually it will get bad enough that the gear spins and grinds on those teeth without ever engaging or spinning the engine. Once it does start to crank, the worn teeth won't cause a problem because the drive gear is still meshing with the half of the ring gear teeth that are still there.
To reduce the chance of this happening again, coat the drive assembly and shaft with a light grease to prevent rust from forming that can inhibit its free movement. Heavy greases like axle grease can become hardened from heat and dirt and make the drive assembly stick and not engage smoothly.
By the way, most people would just replace the entire starter. You can save some money by replacing the drive assembly but a mechanic would never do that. The labor time involved would cost his customer more than the new starter. If he makes a mistake, he will have to start over and redo his work for free. Customers get angry when they have to wait for their car, even when it isn't costing them money, so he is going to want to get it done as quickly as possible. Also, the rest of the starter motor is just as old and tired as the drive assembly. By replacing the entire unit, the mechanic is better able to insure the quality of the repair.
Sunday, November 3rd, 2013 AT 1:24 AM