1987 300SDL 200,000 miles

Hello everyone. The mighty SDL has suddenly sprung a frightening set of symptoms, out of the blue. The car ran normally on a 15-mile freeway and surface street trip in the morning. When I tried to start it mid day it made a loud unusual grinding noise and vibrated terribly when I started to crank it. It rocked the whole engine severely and sounded like something was grinding itself up. The serpentine belt, which had probably less than 10,000 on it suddenly has shredding on the edge that I had never seen before.

I had someone crank the engine while I looked at the belt. Cranking the starter made the huge noise and vibrated the engine (one or two cranks, then he stopped), but I also saw that the serpentine belt rocked slightly during the cranking, but stayed in place, perhaps getting a bit more edge shredding.

I removed the serpentine belt to see if the AC compressor was seized, but it and all other devices rotated normally. The same cranking symptoms persisted with no serpentine belt installed.

I pulled the starter and tested it in a vise. It seems fine; spins with lots of power and the solenoid pushes the gear out like it should. I don't know if there's anything else to test for, but I could tell from the original symptoms that the starter motor had plenty of torque, because it shook the engine like crazy.

Flywheel teeth looked perfect.

I inched the flywheel around at least a 1/8 of a turn with a screwdriver in the teeth, and it gave about as much resistance as you would expect a big motor to give in that situation.

Does anyone have any ideas about this?

Many thanks
Bruce S