Another 603 vacuum adjustment/shifting thread... help Brian, Dave, etc.!

[AlexTheSeal]

Hi guys,

I have a hard-shifting transmission on my '87 300D. I've been doing a lot of diagnostic work and parts replacement and have read just about every thread here on the issue, but my symptoms aren't quite the same as any I've read about.

My vacuum pump, vacuum control valve, and transmission vacuum modulator are all new. I have been through literally every vacuum line and connector on the car and replaced quite a few that tested bad; there are no leaks to speak of. In particular, the lines to the EGR and ARV are capped, and the ACC vacuum subsystem is tight (tested at the manifold behind the glove compartment). The ALDA could stand a tweak, but off-the-line performance is adequate so I haven't done it yet. The transmission fluid and filter are fairly new (10K mi or so) and I think the level is correct, although I always have a hard time reading the dipstick.

The new vacuum pump is putting out a steady 25 inches of mercury. When I replaced the VCV, I adjusted it to output 14 inches at idle. I have a tranny pressure test gauge, and I adjusted the new modulator correctly (with the vacuum line disconnected, and to the pressure from the TDM [I forget what the spec is now]).

I've been driving the car with my MityVac teed into the line from the top of the vacuum amplifier to the transmission modulator, and am seeing pretty consistent behavior:

• At a warm start, the vacuum amp puts out about 5 in. If I leave the car idling, this slowly (over the course of a few minutes) rises to about 12-13 in.
  
• Any accelerator input, even a very light touch, drops the output back to 5 in. For example, the amount of "throttle" needed to keep the car going a steady 25 mph on a flat road puts the vacuum amp output at a steady 5 in.
  
• Pushing the accelerator down any further causes the MityVac gauge to drop to zero almost immediately. When I let up on the pedal, it slowly (over ten seconds or so) rises back to 5 (or higher if the car is just sitting and idling).

From my reading of how the vacuum amp is supposed to work, this doesn't seem right. I expected output at idle to be nearly as high as the reading directly from the VCV (even allowing for the MityVac bleeding off some vacuum). And output above idle, I think, should be inversely proportional to accelerator position, in other words gradually tending towards zero as the pedal goes down. Last, I would think that response to letting off the accelerator would be quicker, so as (for example) to smooth out an upshift after accelerating to cruising speed and letting off the pedal.

Any thoughts here? If it's just that the vacuum amp is bad, that's fine---I've replaced practically everything else, and I wouldn't be surprised if the flying saucer were bad too! But I want to make sure there isn't anything else I'm overlooking.