I know someone that has a 97 E320 that had the same exact problem. At around 40 mph, there is supposed to be around 60-100 rpm worth of slip. Instead, there was less than 30rpm of slip which at that road speed and engine torque around 1500rpm led to a vibration. The problem was that there was not enough slip to isolate the inherent vibration that is present under those conditions, an updated computer for the tranny solved the problem in that specific case.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record