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Originally Posted by braverichard
Firstly, I am proud to announce that out of at least 12 different W126s I have driven, none shifts as smooth as my car does now. Shifts can't be felt at all anymore. You can only tell by looking at the tach and listening to the engine. Why? Not that I'm complaining, but did I heal some vacuum leaks by doing this? Why the smooth shifts all of a sudden?
Secondly, I see from your pic that the second line that comes from the firewall (from towards the fusebox) into the hood (the black line) is connected to the vacuum modulator. I left mine disconnected but the car shifts well under all driving situations. I did 2 miles in the city and 5 miles on the highway with no issues. So should I even bother connecting that black line to the vacuum modulator?
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The smooth shifts are the result of the elimination of any leaks at the 3-2 valves on the top of the valve cover. Any leakage up there will drop the vacuum to the VCV slightly and
might have resulted in reduced transmission modulator vacuum depending on the original vacuum level to the VCV. In my specific case, the vacuum to the VCV was 12". So, any leaks at the 3-2 valves would not have reduced the vacuum below 12" therefore any leaks at the 3-2 valves would not have been noticeable.
The second line that is connected to the VCV (not the modulator......it's on the side of the transmission........at the very end of the hard vacuum line from under the hood) is the vent. It's connected to the side of the VCV and it goes into the cabin. This provides a source of "clean" air for the VCV and it is less likely to be taking on soot from under the hood. You probably should reconnect it..........you won't affect the performance of the vehicle at all.
Now, you may not like the SD the way that it is..........the shifts may be too "soft". I didn't like mine and decided to firm them up a bit. If that's the case, you need a Mityvac. Just repost on this thread and I'll show you how to adjust the vacuum from the VCV to the modulator using the Mityvac.