I added a vacuum transducer out of an 85 300sd I got at a junk yard just to see how my car would shift. In my test fit I mixed up a vac line with the boost line. I swapped the "vcv" and "pres" on the transducer on accident and didn't notice. So under heavy throttle, the black line that goes to the transmission modulator was pressurized instead of just having just no or little vacuum. This caused some very nice and firm shifts when I really got on it to enter the freeway. It didn't "clunk" or "pop" into gear, but felt like it had a shift kit. I have tried turning the "t" key to increase pressure before but this always resulted in it "banging" into gear when driving easy and not very nice feeling shifts during heavy acceleration either. My question is, will sending pressure to the modulator hurt anything? I have plumbed the transducer correctly and it's just back to normal. No fun. I really like having very firm shifts when I accelerate hard and this did that and still shifted very well when taking it easy. Btw, if it is ok for the modulator to receive pressure I had an idea of getting my valve cover egr controls to only allow it to get pressure under near WOT and have a t in the line with an orfice on one side so it would only see maybe 5psi tops. Just an idea...

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83 300CD- sanden, dual p/f condensers, 160a alternator, ect
91 300TD- 722.6, #22 head, 3.5L IP, w140 manifolds, ect

Interesting. Obviously no pressure is available until the turbo is spinning up. As turbo provided increasing pressure It was kind of firming up shifts on demand.

It is a wonder nobody stumbled across this before. One has to wonder if your transmission is within the normal range. If so various smaller line orfices may allow further tuning. Actually even if not normal.

I hope you have stumbled upon a real area well worth exploring rather than just an odd coincidence with a tired transmission. Quicker firmer shifts are going to improve acceleration times.

Give some though to how the alda on the injection pump may have been responding with the vacuum feed instead of pressure. Obviously unless the pressure from the turbo already is split and I suspect it was not. Did switching the lines deny the alda a pressure feed?

You may have something to play with here. I see no issues unless the transmission is slamming in too hard. Again that may be tuneable as well though. It is seldom that any new effect is reported but when it is it is worth investigating. First it has to be observed that it might add a new approach though. We have a few guys on site that this effect might interest.

The thing I like is that it acts normally until you are in boost. Usually to have boost, you must have your foot in it. I have no alda, but you could run another line to the manifold or tee off the wastegate line for the trans or alda so they don't interfere. My concerns is the modulator itself. There must be some type of diafram inside as well as o rings and I wouldn't want this blowing anything up. I had this setup for 2 weeks before I noticed it was hooked up wrong.

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83 300CD- sanden, dual p/f condensers, 160a alternator, ect
91 300TD- 722.6, #22 head, 3.5L IP, w140 manifolds, ect

Good question if the pressure is actually making the modulator do anything. It may just be removing it from the equation. Have you tried running your car with the black line to the modulator disconnected completely? That will answer if it is pressure or just the modulator not functioning, that is giving you shift-kit shifts.

I ran two of my cars, that had well worn transmissions, with the transmission vacuum line disconnected and ended up with shifts as you describe you like. I happen to like that feel as well.

Also, in terms of making thing work as stock, have you serviced the vacuum control valve (VCV). Here is the DIY:
Transmission flaring or clunking? VCV DIY - Mercedes-Benz Forum

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

Am I remembering something wrong? What I remember is that you are supposed to drop to zero vac immediately at WOT and the transmission will up shift properly. The vac is used only for down shifting. In fact if you unplug the vac from the modulator it will upshift fine but downshift with a clunk.

Kerry - are you out there? Can you opine?

Yes, I have run with the vac line disconnected AND the modulator turned up before and it felt completely different. Yes, it shifted hard but more of a harsh pop into gear at all speeds. This felt like a shift kit or a new car with a programmer

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83 300CD- sanden, dual p/f condensers, 160a alternator, ect
91 300TD- 722.6, #22 head, 3.5L IP, w140 manifolds, ect