300sd transmission trouble - PeachParts Mercedes-Benz Forum

dalpil · #1 12-23-2006, 04:20 PM

hi, have an 84 300sd that started acting up today. it upshifts as if it were under full throttle. it downshifts ok, but only upshifts at the full throttle shift points. this means that i dont get 4th gear till about 70 mph and that poses a problem on most roads where i live. any suggestions? thanks.

jshadows · #2 12-23-2006, 05:12 PM

I'll jump in here as I've been diagnosing tranny problems after a rebuild.

What if anything has been changed over time? b2 piston? fluid/filter change.

Any adjustments to bowden cable and or vacuum modulator?

If you let off the pedal will it eventually shift or do you have to take it to high rpms to get there?

Does this occur on all gear upshifts? Could be the sticky elements in the valve body and/or worn K1 and K2 clutches and/or pistons not releasing the b1 and b2 brake bands which are associated which the Kx clutch packs.

dalpil · #3 12-23-2006, 05:42 PM

Nothing has been changed. The car was purchased after being in a collision. I repaired all the bodywork, the frame, steering, engine etc. was all ok, just cosmetic damage. I have put about 2-3 thousand miles on the car and it has has been fine. We started out in it today and it downshifted on its own from 4th to 3rd at about 50 mph and has been behaving this way since. Not sure where to start so I posted the message.

Brian Carlton · #4 12-23-2006, 07:13 PM

Look under the accelerator pedal. You'll see a small round button switch there. Find the wires that go to this switch and unplug the switch.

Drive the vehicle.

Report back with results.

mplafleur · #5 12-23-2006, 08:12 PM

Quote:

Originally Posted by Brian Carlton

Look under the accelerator pedal. You'll see a small round button switch there. Find the wires that go to this switch and unplug the switch.

Drive the vehicle.

Report back with results.

This worked for mine, and several others. Including one who was about to spend $2000 an a trans rebuild, and another who was about to sell the car.

dalpil · #6 12-24-2006, 10:22 AM

brian you were exactly right. i unplugged the switch and the car shifts at it's less agressive shift points under any throttle position. what do i need to replace to restore the original condition of the car, is it this switch or is there another somewhere else? this message board is very helpful, i might never have known about or looked at that switch. thanks alot.

Brian Carlton · #7 12-24-2006, 10:32 AM

Quote:

Originally Posted by dalpil

brian you were exactly right. i unplugged the switch and the car shifts at it's less agressive shift points under any throttle position. what do i need to replace to restore the original condition of the car, is it this switch or is there another somewhere else? this message board is very helpful, i might never have known about or looked at that switch. thanks alot.

Yep, that's the kickdown switch. It lives in a nasty environment and frequently gets stuck in the down position. You only need to replace it and the vehicle will function properly again.

Phil can get you one if it's not listed in the forum parts lists.