I'm thinking that the speed sensor may be working correctly, but the excessive pressure when the system is first engaged causes it to slip just a bit and lock out for the duration of the run time. Once the speed sensor opens the clutch circuitry, does the ignition have to be turned off to reset it or does it allow the clutch to reengage after the problem is no longer sensed?

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