transmission problem
 

My 300 SD Turbodiesel (W116, 1980) doesn't shift at all. Even if I wait for a while the gears don't engage.
I looked underneath the car and found one cable that is connected to the transmission but loose towards the engine side.
The cable is not broken just disconnected.
My question is where do I need to connect this cable on the engine side?

That is your speedometer cable. The small steel cable coming out of the back of the tranny slides up in the larger housing and it is supposed to be held up at the part in your last picture. While you are under it inspect the shift linkage on the side of the transmission good.

thanks - I just put the cable back.

My transmission problem still persists. I changed the plastic bushing at one and of the linkage but this didn't solve the problem.

If I'm in the rear gear and give full throttle then the rear gear engages.
Then I can also use the other gears.
The shifts are very soft and have a lot of slippage...

So I'm not sure what to do next. I ordered a new modulator but I don't think that solves my problem since the vacuum line is disconnected anyway.

1st thing to do is verify trans is full. If full, does it have a normal sweet odor? Is it pink in color and can you see thru it to the stick? If all thats good, then pull the trans pan and tell us if there is any residue in the pan.

Assuming that the transmission in the '80 works like my '81 300SD, you do not need to connect the vac system to the vacuum modulator to drive the car (for testing purposes). Without vacuum, the transmission should upshift fine; in fact the system is designed to bleed off the vacuum when you are upshifting. It only needs vac for downshifting smoothly; it will downshift fine without vacuum but there is an unpleasant clunk.

You can hook up a Mityvac to the line that goes to the vac modulator. Drive around with zero vacuum while you are going through the gears; when you get ready to come to a stop, just pump up the vacuum on the Mityvac and it should downshift fine.

If all this works, your problem is the vac system and not the transmission. If it does not work, then suspect the transmission. (Seems like some folks say a bad B2 is indicated if the tranny works OK in reverse but not in forward but not sure.)

How many miles? Service history on the tranny?

Reverse but no forward gears is a symptom of a b2 piston problem.

The car has 260 000 miles. I changed the transmission fluid including the filter 3000 miles before. The fluid itself looks clean.

I disconnected the vacuum because two things happened with the vacuum connected:
1. it seemed to suck out the transmission fluid.
2. I couldn't switch to P or R anymore (I had to shut off the engine to do that). Without vacuum switching was no issue.
The vacuum that comes from the pump was adjusted and follows the correct curve about 8 inch in idle and 0 with full throttle.
My reasoning is that this is a modulator problem.
I could drive around without vacuum the past months without any trouble.

However now nothing works anymore unless I put full throttle then the gears engage very slowly.

The gears were always switching very soft - almost not noticeable (up / down). The gears were shifting too early. Only in the L position it switched correctly from 1st to 2nd gear. When I selected S it immediately switched to the 3rd gear. In D it switched to the 4th gear at relatively low RPM.

Now my car is sitting in my driveway ... today I changed the bushing of the linkage underneath... but no improvement...

I don't think a modulator change will improve anything at this point.
There must be something else.

Sebastian

Sign of a bad modulator.

I think your tranny is tired.