I'm going to replace the flex disc and transmission yoke on my friends '79 240D(4spd manual). The disc failed and damaged the yoke a few weeks ago. With the drive shaft unattached, I took a look under the car to make sure everything looked okay, and that the yoke he purchased would fit. Everything looked good, but I was curious to see if the transmission would slip into the different gears with the engine running. Even though he was only travelling 30 mph when it failed, he said it was a pretty ugly sounding breakdown that killed the engine.
After taking the car off of it's jack stands, I started the car and put the transmission through all the gears. Unfortunately, when the car was in neutral, the speedometer started to climb as I revved the engine, meaning that the transmission is still engaged. I didn't have time to get back under the car and check the linkage, but my fear is that the flex disc failure did some serious damage to the transmission. Has anyone heard stories of flex disc failure doing serious harm to a transmission? What else should I do before I get under there and replace the flex disc and yoke? Thanks

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'81 300SD