I seem to recall that the set screw on the bottom of the amplifier will reduce vacuum when turned fully clockwise..........but........I haven't adjusted it in awhile.

In any case, try the amplifer with the set screw fully counter-clockwise. It takes a 4mm socket........which you probably already realize.

Also, try a test with just the Mityvac connected to the modulator. Pump up the Mityvac to 15" and leave it there. Take the vehicle for a test drive and see how it does. If the vacuum falls during the test, pump it back up to 15". See if the clunk disappears.

Tested the vcv today, and found that contrary to what I had previously thought, it loses vacuum at what Diesel Dan considers a proper rate, that is about 1 inHg per second.

Test drove the car today.

First, with mityvac teed into vcv output:
10 inHg at idle
8 inHg driving at constant speed
5 inHg with light pedal
0 inHg with 1/3 pedal

Second, with mityvac teed into vac amplifier's output to modulator on tranny:
12.5 inHg at idle
10.5 inHg driving at constant speed
5 inHg with light pedal
0 inHg with 1/3 pedal
My clunky downshift took place with about 7 inHg going to the modulator

Third, with mityvac hooked only to vac line going to modulator and with mityvac pumped up to 15 inHg as you suggested, Brian:
Tranny flared when shifting, and flared immediately before producing the clunky downshift yet again. Vac stayed at a constant 15 inHg the whole time. Tried to produce the clunk again and got it on the same downshift, this time with no flare coming first.
So, the clunky downshift is not the result of a low vacuum condition. Brilliant idea to test it this way, Brian. What does it mean that I still get the clunk? I assume that the downshift that is taking place is a 3-2 downshift. As I go from high speed to braking to 20 mph, the tranny shifts from 4 down to 3 I would guess, although I can't feel it. When I try to accelerate, the tranny kicks down to 2 unless I feather the pedal, and the shift control allows me to stay in 3 since I am not demanding much acceleration.

__________________
87 300D 162,000 miles: It's not just a car, it's an adventure!

Good job in the testing. But, the results are a bit baffling. I would have expected the downshift to be smoother with more vacuum.

Now try this:

Go to the Bowden cable adjustment screw which is down below the oil filter housing.

Adjust the cable slightly to provide more slack. Turn the adjuster inward (clockwise, aft looking forward) by two turns.

Take it for a drive and see what happens.

Brian, I turned the screw below the oil filter housing 2 turns as you suggested. Now it takes more pedal to get a downshift (when slowing down from 60mph to 20 mph then hitting the accelerator). When I do get a downshift, it is firm but not as hard as before. It feels just about normal, if a tad too firm. Also, when maneuvering the car around my driveway, it only takes a light touch of the pedal to move the car. Before I was having to push the pedal much further to do low speed parking maneuvers. I was also having to push the pedal much further when accelerating away from a stop. I assume that the adjustment you had me do somehow changed my shift point relative to throttle position? What you referred to as the bowden cable adjustment screw is called the control thrust cable in my manual. The manual states "Detach ball socket (212). Push control thrust cable (211) forward until slight resistance is felt. In this position, attach ball socket free of tension; adjust at bowden cable, if necessary". According to the manual, the bowden cable is the cable up above this control thrust cable. Which is which?
I'm not sure what you had me do, but it helped. I would love to understand more. I plan to follow my manual's procedure for adjustment of the accelerator control linkage including this control thrust cable to get things fine tuned.
My car was operating without turbo boost when I got it. Is screwing around with this control thrust cable something that people (the previous owner) might do when the car has no boost and therefore no power? I would assume that they would do this to force an earlier downshift and make the car more driveable when it is lacking power? Thanks a million for the advice, and I look forward to hearing what I did.

__________________
87 300D 162,000 miles: It's not just a car, it's an adventure!

I learn a great deal from reading Brian's trouble-shooting advice. 14,000+ posts - absolutely amazing.

I am equally impressed with your skills, Ramjensen, with helping to diagnose the problem - too many owners look for a quick & easy answer.

Finally, I apologize for not paying closer attention to your model - I thought at first reply you were in a w123.

I'll go back to 'lurking' now...

__________________
83 300d - 390k

Good job.

The adjustment that you made was to the Bowden cable. What it does is to increase or reduce throttle pressure relative to engine speed. With higher throttle pressure, the transmission will shift later and downshift earlier.

In your case, with an early downshift, the engine speed is a bit too far from the vehicle speed and you get a clunk. To avoid the clunk, we reduce throttle pressure so that it downshifts a bit later........when the engine is turning a little slower.......so the shift is not as dramatic.

I really cannot fathom how the low speed performance would be improved. We did nothing to the relative position of the rack with respect to the accelerator pedal and we did nothing to the ALDA.

It sounds like he adjusted the linkage to the rack, not the bowden cable. He's talking about adjusting a "ball socket" in his post.

The Bowden cable has a ball socket at the very end. There is no adjustment to the rack down below the oil filter housing. The only cable in that area is the Bowden cable.