Hello, This is my first time posting here, I have read many threads in the past year trying to solve a mystery problem with my 300SD trans. I hope this can help others who may have the same issues.
For the past year, my '85 300SD would not shift into third or fourth gear (drive). First and second gear shifted perfectly, and also reverse. I began by reading many websites, and talking to many people. Some thought the problem was internal, some thought it was vacuum related or a defective modulator. And yet others said a bad K1 spring in the valve body, or a broken B2 piston. I spent hours reading, applying, and trying to understand Steve Brotherton's thesis on exact vacuum adjustments regarding various parts of the complex vacuum system puzzle. Everything seemed to check out, and I did make the engine run better by messing with the vacuum line running to the intake manifold. But it still wouldn't shift.
I tried adding Trans-X, or some other additive, thinking that there may be a stuck valve somewhere. That yielded no results either, so I changed the fluid and filter, thinking maybe a clogged filter was inhibiting fluid pressure. After putting it back together, still no shifting into third gear. It had to be a modulator adjustment. So after research, I cranked it up about 3 or 4 clicks (slots). Still no upshift. Maybe the Bowden cable?
I basically ruled out the B2 piston, due to the fact that the car shifted into first, second, and reverse perfectly. But I did price one, just in case. The only thing I could think of next was the K1 spring in the valve body. I ordered 2 kits from the dealer, drained the system (again), and installed a new K1 spring kit. This HAD to be the problem, or so I thought. The old K1 spring looked fine. After re-assembly and re-fill (again), still no shift.
Next was the Bowden Cable. Repreated attempts to adjust it to proper shifting again yielded NO proper results. At this point, after a year of on- and-off efforts to fix this vehicle, I was on the verge of either driving it off of a cliff, or parting it out. I refused to pay a dealer or shop $2000-$3000 for a new transmission.
Just as I was about to give up, I read an obscure posting in a thread that I haven't seen before. It mentioned a small plastic switch underneath the accelerator pedal, called the kickdown solenoid. If this was broken or stuck, would the car be stuck in second gear for eternity? YES !!!!!!
After locating it, unplugging it, and removing it, IT WAS STUCK IN THE CLOSED POSITION ! This lousy $15 plastic switch had decommissioned my vehicle for A YEAR ! I test drove the car, and it shifted into third and drive perfectly. It obviously dosen't downshift properly with the switch removed (I ordered one from ******** for $15), but upshifts perfectly. Thank God.
I hope this can help someone who may have tried everything else. If you have this problem, and haven't given up hope, CHECK THE CHEAP PLASTIC SWITCH UNDER THE PEDAL FIRST !!!!! This can save you hundreds or even thousands of dollars, and dozens of hours of labor and frustration. Good luck, and remember to CHECK THE SWITCH FIRST !!!