There are at least two ways I know they can go bad. The first is where the disc actually physically starts to come apart or unraveled. The other is where the rubber just hardens and makes the drivetrain less able to absorb the jolt of shifts and putting it into gear. Mine are not coming apart, they are just getting old and hard.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles