My 1985 300SD shifts ok up to 3rd gear, but i have to do the following for it to get into 4th.... while driving, if i put the car in N, let the rpms go down a bit and then put it back into D. Hit the throttle a bit and the car is now in 4th...

How bad of a situation is this?

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1985 300SD

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did you check kick down switch... or vacuum?
when you high rev in 3rd letting a leg back a bit doesn't help ?


cheers

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w126 500SEC gen II euro, powered by OM617 turbo stolen from 84 300SD 2.88 diff,EGR blinded
next wish/project: w114 coupe OM603 powered

is the kick down switch under the pedal? or is it on the engine?
No I have not checked vacuum yet. Not sure where to check first.
I do have a mighty vac and I do have a separate vacuum gauge.
Point me to the proper DIY Vacuum check and I will try to do it.

backing off the throttle while in a high rev, in 3rd, doesn't help, but putting the car in N, watching the revs drop to about 1000, then shifting back into D works...

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1985 300SD

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yes under throttle pedal.

search PP forum will give you plenty to start with, considering
vacuum issues and modulator adjustment which may or may not be needed.
also info considering K1 kit might help....but your problem is kind of strange
so I will go with first usual suspects like kick down,vacuum...etc

cheers

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w126 500SEC gen II euro, powered by OM617 turbo stolen from 84 300SD 2.88 diff,EGR blinded
next wish/project: w114 coupe OM603 powered

It's a long one but try this thread for all the information to adjust vacuum for the transmission:
http://www.peachparts.com/shopforum/diesel-discussion/158216-its-critical-how-you-set-your-transmissions-vacuum-system-your-diesel-mbz.html

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Sam

84 300SD 350K+ miles ( Blue Belle )