The 83 240D is one of the few trannies MB built without some form of control pressure link or cable.
To adjust the shift timing you will have to alter the relationship of the conditioned vacuum curve. The vauum is modified by a proportioned controlled leak (the white plastic device on the rear of the injection pump.
The first thing you must do is verify that the system is working by teeing into the black line that goes to the modulator. The vacuum lines are complicated but look at the modulator end and follow forward. I think I tee that model at the valves on the Valve cover.
What you are looking for is high vac at idle 20in or more. At this point the leak isn't in the system and full vacuum goes to the modulator. When you take up the slack in the linkage and engage the valve cover micro switch the leak is hooked up by the depressed micro switch. All this is to be done with the motor running. At this moment with the switch engaged the engine is still at idle (the linkage movement and switch activity happens before you get pump movement. At this point you should drop to 10-15in of vacuum.
A quick full throttle rev of the motor should drop the vacuum to close to zero. This is the activity that you need to verify before you make the adjustment. High vac - dropping to 10-15in after microswitch closes and then a smooth transition to around zero inches at full throttle.
The adjustment to this is the link between the valve (white plastic) and the throttle. If you view from the top the link goes vertically down the inside of the pump. Where the link is attached at the bottom is a adjustable ball stud that the link hooks to. The nut is 8mm (fairly hard to get to). You will need to move the ball dowm about an eighth of an inch. The easiest way to tell how much you are moving it is by turning OFF the motor and pulling the throttle to full throttle. The valve's metal lever will approach a full throttle stop. Pay close attention top this position and when your done the lever should be 1/8in further away from the stop than it was when you started.
All of this was based upon the proper vacuum curve when you started. We just rearranged the relationship between the throttle and the vacuum curve. If you have a severe drop in vacuum after the microswicth is engaged (one of the first step)then you have encured an extra leak probably when the valve cover was off. The micro swith is highly suspect here. What you will see on the vacuum gauge is high vac 20in and then a drop to 5 or less before the engine leaves idle (when the microswitch hooks the rest of the system up.
It actualy gets complicated from there.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician