W124 Transmission Upshifts too quickly
 

I have a 1990 300E 2.6 (W124) and the transmission started upshifting too quickly (only from 3rd to 4th gear). It shifts into 4th gear at about 20MPH. Also, it will not downshift when I depress the accelerator to the floor.
I can manually shift up and down and everything is OK - just will not do the automatic thing any more. Any ideas?

Loose bowden cable ? either on the engine or transmission side

Check your kickdown switch underneath the gas pedal, could be faulty or just loose.

Thanks - I'll check the bowden cable and the kickdown switch and let you know.

May be two separate problems. Did this happen suddenly?

Look at the kickdown switch for a stuck mat, if not test it or replace. For the stacked upshifting check the vacuum hose at the modulator and while in that area the wires to the kickdown solenoid. They are in the same area, wondering if something from the road damaged both.

Good luck!!!

kickdown switch/solenoid has nothing to do with early upshifts. It's only activated at full throttle. But I agree that there could be an issue with it as well.