Hi I just rebuilt my 1991 420sel transmission because it would not change up from second gear. The problem was caused by a sheared drive pin to the governor. A small spring from a broken plastic valve in the rear valve body found its way into the secondary pump gears causing them to lockup.These gears are driven from the end of the governor hence the sheared drive pin .without the governor spinning the tranny does not know when to shift .When it first happened I drove 5 miles and the box shifted to 3rd but after that stuck in 2nd . But check the simple things first ,chsnge the oil and filter that worked on my 300e when it was reluctant to upshift . If you do rebuild the box make sure to change the plastic valves they are cheap and if you strip the valve body be very carefull take notes or better yet photos . Steve.