1981 300d turbo diesel, pre bowden cable.

I posted about this problem previously, but the situation has since developed and it was suggested that I start a new topic.

I purchased the car some weeks ago, it had shifting issues which I resolved, it was due to vacuum leaks. It would shift up from 2-3 only at very high RPMs. To fix this problem I isolated the vacuum controls by bypassing everything else, tested VCV and modulator, adjusted the VCV (I have NOT touched the actual modulator). I bypassed the entire switchover valve part, cutting out the EGR entirely. Now my routing goes directly from vacuum pump to modulator, with the VCV in between, and NOTHING else. The vacuum bleed i get from the VCV is quite good and the modulator holds vacuum. After these adjustments the trans shifted very well.

That was, worked for a few days. Suddenly, it began slipping whenever I push more than 1/4 throttle from 2-3 (it feels like someone is pressing down on the clutch when I accelerate). Then I lay off the pedal as it slips and slowly depress throttle again, and it runs more or less smoothly, if it doesn't slip/flair again, albeit slowly. If I just try to floor the pedal, it pretty much slips infinitely... so of fear to burn something I lay off the throttle. I have to manually alternate between L,S and D controls to really drive safely.

Now, even as I disconnect the vacuum controls entirely, it still slips, so it can't be related to the vacuum pressure. It just seems strange that the actual transmission would crap out on me two days into fixing the vacuum system... I mean, coincidence? I wanna try anything possible before giving up... is it possible that the modulator is stuck? The behavior feels similar as it did to when I had too much vacuum going to the modulator... Perhaps the pushrod inside the modulator got stuck after years of not being in use? Perhaps if I blow into the modulator (or give it a bit of pressure with a bike pump) it might release the rod? Is it possible to disconnect the modulator entirely (not just the vacuum line, but to physically remove it from the transmission) and drive the car? At least I could test to see if it stops slipping without the modulator? Maybe I should give a couple clockwise turns to the modulator adjustment screw?

What else could cause a transmission to slip/flair so violently?

A Trannie shop would hook it up and test the pressure of the trannies functions.I had a torq converter bad once before on a 300cd ,its symptoms was not exactly the same ,wouldnt shift over 2nd.Yours sounds to be pressure related so I would start their 1st before you sink alot of money into a non working trannie.Have you tried changing the fluid/ filter as a start to get a better trannie function.In another issue I bought a 190d that would only get final 4th gear after warming up.Around 5 different fluid and filter changes turned things around in about 6 months of ownership.That one was a dirty valve body most likley..Never opened the trannie up ,alot of itey ditey parts ready to spill out gives me caution.

So last night I crawled underneath and adjusted the modulator, tightened it 1.5 turns, drove around, felt it shifted better immediately, so turned it another .75 turns, a little better... so today I guess I'll tighten it maybe another 1.5-2 turns!

The way it is now, it shifts all right with the VCV and modulator getting fed zero vacuum / unplugged. So I'll tighten the modulator until it's a bit on the harder side, then plug the vacuum system back in to get it back to normal.. should work and would feel pretty satisfied.

Is there any limit to how much it can be tightened?

I would do the whole precise adjustment with a pressure gauge / banjo bolt thing, but probably I'll try that some other time when I have more time/resources. I feel it's so far off now that even just tighten to test drive and tighten some more will be sufficient.

Check, double check the fluid levels when hot and running on level ground, idling, after one minute and before two have elapsed. Remove excess fluid with a vacuum pump, via the dip stick. Buy a new modulator switch and re fit it (the one on the rocker cover). Return the modulator (on the box) to original position, before you break the trans. What fluid do you have in the trans? No dx fluids for w123s! I have just been recommended Transmax F in the absence of Transmax M. You may have cracked the modulator diaphragm in the trans. Check for leakage. See how you go.

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