This is for everyone out there that has had a 300dt that wouldn't shift into 4th or was very hard shifting and high rpm's to even shift. I found out today that my kickdown switch that is located behind the accellerator pedal was stuck in the down position. I happend accross this by trial and error, but I solved my problem. One day, my tranny was shifting fine, except that is was always a hard shift, but the next day, my wife drove it to work and home and let it sit for about 2 hours, then got back in it to go to the store and she couldn't get it to shift out of 1st gear. I got into it the next day and drove it and I got it to shift out of first gear... at about 4000 rpm! I looked at post on this site for over 6 hours and did everything that was suggested, but nothing fixed my problem. Finally, I went out to the old beast and looked at the kickdown switch. I tried to physically push down on the butten, but it wouldn't move, so I pulled on the butten...really hard and it came up. The button was stuck in the down position! I took it out for a test run and my problem was solved! In fact, the car shifts better than it ever did. I would say that it is even smooter than my 420 SEL and my 450SL. So anyone with this same problem should look at the kickdown first, then look at the tranny. This should save you alot of time and money. And by the way, the two vacuum valves on top of the valve cover under the black box do nothing at all. My arms on these are completly worn out and do not even touch the plunger on these valves and my 84 300dt runs like it was brand new and this one has 285,000 miles on it.