I had the same problem in a '85 300D until I swapped out the tranny and resolved an issue with the kick down switch. The kick down switch underneath "fuel" pedal was not getting energized to power the kick down solenoid at the passenger side rear of the tranny. Remove the floor mat and look for the switch behind the pedal. You will see two tabs that have to be squeezed in to pull it out. Make sure it has 12 volts on one wire coming to it and insure that it makes up when depressed. When it makes up it is supposed to pull the kick down solenoid in. If it is not getting power it will not work. You will have to troubleshoot why there is no power to it. In a properly working system all you should have to do is adjust the Bowden cable. I have not torn into the tranny I pulled out of the car to try to ascertain what is going on inside of it yet but I had replaced the Bowden cable and that did not do any good.

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Jim