With vacumn on spec with old modulator it would shift 1-2 VERY quick then 3 and 4 all too soon. Removing vacumn helped some but 1st shift was very hard. Put in new modulator Hard shift to 2 (bark tires) then not shift to 3rd at all. Have tried some adjustment to modulator but no help. If I disconect vacumn , nothing changes.

Thank You
Phil

This link should give you some kind of idea what you got yourself into. Pay attention to the links in post #7.
A new modulator is not a drop in replacement. Though many people have done just that without problems.

MB manual for 123 vacuum modulator replacement

Danny

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1984 300SD Turbo Diesel 150,000 miles

OBK member #23

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Since you say your MBZ is an '81 300CD, this means it is a W123 chassis and has a 617.912 engine... which means this should be the vacuum diagram for your engine/tranny shift control system:
http://www.peterschmid.com/vacuum/1977_1985/616_617_91/616_617_91.jpg

I think you will find your modulator adjusts over a range of ~100 "clicks" [ 8"clicks" = one 360 deg. turn] or ~12 turns. If I were you I would first make certain that the various OTHER components of your vacuum syste are functioning and that you repair any leaks... and then put your modulator at about mid-range of its adjustable range.

Q1 - Have you read the Steve Brotherton article? If not check it out at:http://www.continentalimports.com/ser_ic20242.html
Q2 - Do you need a combo vacuum hand pump & gauge? You'll need one.

Now we anxiously await your questions and IF you don't have several to many, then we probably will not be able to talk you through this! Seriously, it's a bit abstact but certainly doable !!