We have a 190D that has developed a problem that I am seeking advice on.

When we accelerate up to about 40 mph, the shift from third to fourth gives the impression that the engine is now disengaged from the drivetrain. The rpm for the same position of the accelerator ramps up as if it is facing little to no resistance. If I select the '3' position, the transmission resumes operation in 3rd gear. Back to 'D' and it ramps up again. I am trying to determine if this is an issue with vacuum, the xmsn modulator or the transmission itself.

So far, I have done a bit of troubleshooting and work on the vacuum system. Many lines were cracked, so I have replaced those. The vacuum valve was not operating so I have replace that. The behavior now is showing evidence that the vacuum is getting to the transmission: the shifting between 2nd and 3rd is substantially smoother. The transmission now starts in 2nd (as I have read as a lurker ) except under heavy acceleration. The engine cutoff is starting to work better.

I have not gone down to the banjo fitting to test if vacuum is there but from the behavior improvement, I would surmise that the vacuum is getting there.

So now the question: from people's experience and knowledge, what does this symptom indicate? If there was a band that was only used in 4th, that could be the problem. Or maybe there is an aspect to the modulator that I am unaware of. Please advise.