A little more tweaking and a few more clues. Still having the downshift clunk with no change though.
I had adjusted the VCV per the FSM and put it right where in was supposed to go. It was easy to tell right where the VCV encountered resistance when you loosened the mounting bolds and twisted the VCV on the IP. No problem there, it ended right at midway in the adjustment range, which is what everybody says it where it is supposed to be. Still hard downshifts and somewhet, but tolerable, hard upshifts in all gears. 11" to the tranny at idle dropping almost immediately to zero with 1/4 pedal or greater.
So today, in a mood to experiment, I set out to see what would happen if I adjusted the VCV way out of spec, to see what effect it would have. So I twisted it all the way left (CCW) to the stop, hooked up the mity-vac to the tranny modulator line and went for a test drive.
I was now getting 12" at idle, a bit more, but instead of dropping to zero with pedal, it would go to about 5" with 1/4-1/3 pedal, dropping only to about 3" at full throttle. The upshifts were suddenly butter smooth! Even got just a bit of flaring on the 2-3 shift at times. A very noticable difference and much better! Unfortunately, I am still getting the same clunk on the downshift. When the clunk occurs, it is getting a steady 12" (11" when the VCV is set per the FSM).
So clearly, the lack of a vacuum amplifier changes the system and changes the optimal set point for the VCV. It shifts better when the VCV is set at the CCW limit of its adjustment. The tranny does better when it gets some vacuum at all times, in this case around 3-5" at most part-"throttle" conditions.
Also, I think the downshift clunk is a matter of the vacuum at idle , with 11"-12" just not enough to prevent clunking. Can someone tell me what the proper spec is for idle vacuum to the modulator for the OM602? I'm guessing it must be around 15"-17" and for some reason my VCV just can't give it more than 12" even when adjusted all the way.
Which leads to my last conclusion: that the vacuum amplifier (the "flying saucer"), which is long gone from my car, probably brings this idle vacuum up to 15" and gives the modulator enough vacuum to provide smooth downshifts. Has anybody gotten their 602 W124 to shift properly after removing the vacuum amplifier? If so, how did you do it?
Although I haven't yet solved my downshift clunk, all this tinkering sure has taught me about my car's vacuum system. It's pretty cool really. I'd sure rather have a car with vacuum controlled everything than computer controlled everything
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One more thought: I'm not even totally sure my car is supposed to have a vacuum amplifier, but I assume it did and was removed by the PO who also did the pressure wastegate conversion. Everything I've read says that all diesels from 85-96 had it, so that would include my '92, although without a vacuum diagram, I cannot confirm.