Testing off the engine is not a valid test. If the bushings are worn and the armature is dragging, it will draw excessive current and usually will crank too slowly. That will not show up on a test bench because without the normal drag of spinning the engine, the starter will draw about one quarter of its normal current and will spin fine. A screeching or whining noise is caused by something with the drive gear / over-running clutch, or the teeth on the flex plate. Neither of those are in use when the starter is off the engine.
As long as the starter is off anyway, try to spin the drive gear. You should be able to turn it one way but not the other way. If it will turn the second way at all, it is slipping. The drive assembly can be replaced but a mechanic won't do that for you because the cost of his time is almost as much as the cost of a rebuilt starter. The replacement starter comes with a warranty too.
Also look in where the starter drive meshes with the ring gear on the flex plate. If the teeth are chewed up, the transmission will have to be removed so the flex plate can be replaced. If you already have the starter reinstalled, just grab the serpentine belt and use it to rotate the engine in the normal direction a little. Engines tend to coast to a stop in the same place every time so wear to the teeth on the ring gear occurs in just one spot. Rotating the engine will bring up unworn teeth. Only half of a few teeth will wear away. The grinding occurs because the starter has started spinning before those teeth have fully engaged. Once the engine does start cranking, the drive gear will move all the way out, so when the worn teeth come around, half of them are still there and no grinding will occur.
Tuesday, October 14th, 2014 AT 9:34 PM