Ok, this morning, I connected the Mityvac directly into the line running to the tranny, bypassing the T-junction. I pumped it to about 14 mm Hg vacuum, and it has remained there - no leaking over the short drive, so I didn't even need to pump the Mityvac at all.

The up-shifts were somewhat smoother although not totally smooth, but the downshifts were definitely less so - they seemed to lurch a bit. The shift points were fine, though - responding to the gas pedal in the same manner I am used to.

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84 300SD - bought in April '06 with 237,000 mi

Good tests.

You've confirmed that we cannot be successful with any vacuum adjustments to smooth the shifts. The only viable alternative is to attempt an adjustment to the modulator itself. I'm hesitant to do this as the modulator is typically set by using a gauge and setting it to a specific pressure. However, you're out of options.

The modulator adjustment is found at the point where the vacuum line enters the transmission. The rubber cap is removed and you'll be able to pull out a T handle for the adjustment. Rotate the handle exactly three turns counter-clockwise..........fold up the handle..........connect up the vacuum line............and take it for a drive. See if the shifts are noticeably softer. If the do soften up..........we can tune it with vacuum.

Here's an article on modulator adjustment:

http://www.peachparts.com/Wikka/Trans722VacMod

Let us know what happens.

Scott

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Scott
1982 Mercedes 240D, 4 speed, 275,000
1988 Porsche 944 Turbo S (70,000)
1987 Porsche 911 Coupe 109,000 (sold)
1998 Mercedes E300 TurboDiesel 147,000 (sold)
1985 Mercedes 300D 227,000 (totaled by inattentive driver with no insurance!)
1997 Mercedes E300 Diesel 236,000 (sold)
1995 Ducati 900SS (sold)
1987 VW Jetta GLI 157,000 (sold)
1986 Camaro 125,000 (sold - P.O.S.)
1977 Corvette L82 125,000 (sold)
1965 Pontiac GTO 15,000 restored (sold)

Would it be of any use to try the varied levels of vacuum supplied to the modulator? I used ~ 14 mm Hg, but I could also try to make separate runs with 12.5, 13, and 13.5 to see if there's any sensitivity in shift quality to that level...

I think I don't want to adjust the modulator blindly - I'll have to rig up the gauge contraption described in the Wiki and get some numbers out of it.

I had my car at the Benz dealer this week, and they told me the tranny is leaking around the B1 band. Could that have any bearing on the internal tranny pressure and shift quality?

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84 300SD - bought in April '06 with 237,000 mi

No, I don't think you need to make further tests. If it won't smooth the shifts at a steady 15" vacuum level, then vacuum adjustments won't cure the problem.

If you can properly rig up the gauge, that would certainly be the better approach. Hopefully, the pressure is higher than desired, however, if not, I'd still take a shot at reducing it and testing to see how it performs. You can always return it to the starting point if you are careful in noting the adjustment.

Don't have enough knowledge of the internals to state whether the leak can cause these issues.

Brian, theres one thing I am not clear on. I actually had set up a remote vac gage at one time on my 84D teeing off the line going to the modulator and drove the car while reading the gage. As I went up in speed the vac dropped to zero vac. It was my understanding that it is suppose to be that way. Did I get it wrong?

My symptoms are similar to whats described in this thread. I had adjusted the modulator just like you had suggested here with slight improvement in downshift clunking - I'm guessing from 3-2.

__________________
the sooner you start... the sooner you'll get done If it ain't broke, don't fix it.. Its always simpler to tell the truth...
2007 Honda Accord EX
2007 Honda Accord SE V6
96 C220
97 Explorer - Found Another Home
2000 Honda Accord V6 - Found Another Home
85 300D - Found Another Home
84 300D - Found Another Home
80 300TD - Found Another Home
Previous cars:
96 Caravan
87 Camry
84 Cressida
82 Vanagon
80 Fiesta
78 Nova
Ford Cortina
Opel Kadet
68 Kombi
Contessa

If you T off the line going to the modulator with the VCV in the system, the VCV will properly bleed more vacuum as you apply more pedal. At full pedal, the VCV will bleed enough vacuum so that the resulting vacuum at the modulator will be close to zero.

If you send vacuum straight to the modulator from the Mityvac, there will be no bleeding and the vacuum should basically remain at the same level that you originally pump up on the Mityvac. The VCV is not in the system at this point.

The first point of attack for hard shifts and clunking downshifts is vacuum. If you can increase the vacuum and soften the shifts, that's the way to go. But, sometimes, it's not going to be successful, as I'm inclined to believe in the present situation as posed by the OP.

What would you suggest the next step should be? Also, say I simulate a steady vac to the mod with the Mytivac resulting in corrected shifting, how do you introduce vac to the mod from the system after the Mytivac is removed?

__________________
the sooner you start... the sooner you'll get done If it ain't broke, don't fix it.. Its always simpler to tell the truth...
2007 Honda Accord EX
2007 Honda Accord SE V6
96 C220
97 Explorer - Found Another Home
2000 Honda Accord V6 - Found Another Home
85 300D - Found Another Home
84 300D - Found Another Home
80 300TD - Found Another Home
Previous cars:
96 Caravan
87 Camry
84 Cressida
82 Vanagon
80 Fiesta
78 Nova
Ford Cortina
Opel Kadet
68 Kombi
Contessa

The '84 should have a small round dome on the side of the VCV. Remove the dome with a blade and there will be an adjusting screw and, possibly a locknut.

Loosen the locknut and the adjustment screw must be turned in very small increments (10 degrees or so) to increase or decrease the vacuum at idle.......depending on what you want to accomplish.