The problem: The trani shifts smooth as glass when cold, up and down.
But after fifteen minutes,as the trani heats the down shift starts to go bad, thump when coming to a stop.
The vacuum system is tight. runs at 19Hg and will hold all night. The heater is all rebuilt(new pods through out). The modulator holds vacuum.
Is it possible the modulator hangs up when hot. Is it possible to lube the modulator piston? Would that help?
On these cars is perfection even possible?
Thanks for your comments and past help. Without this board I could not have the pleasure of driving this car.

__________________
1984 300SD turbo 126
"My true love"

God made me an atheist and who am I to question His wisdom

You are supposed to have more than 19Hg of Vacuum. Somewhere around 21-22Hg is normal for a well used but good Vac Pump.

Connect your Vac Gauge and see what happens when you Brake. It may be that the Vacuum is plunging when you brake.

When I accidently disconnected the Vac Line at the Modulator I the Trans upshifted OK but I got HARSH down shifts.
So I think it is possible if when you brake and the Vacuum drops drops too much it could do the same harsh down shifts.

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84 300D, 82 Volvo 244Gl Diesel

T a vacuum gauge into the system and monitor the vacuum levels when it downshifts hard. MityVac will work with a hose run out thru the cowl and Mity Vac balanced in front of the windshield.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Kindly T a vacuum gauge into the line that goes down to the modulator at the transmission and put the gauge in front of the driver during a road test.

Any other location for measuring vacuum is irrelevant.

Reply back with results at various speeds, gears, and pedal positions.

The rain finally stopped, teed my vacuum gauge into the line to the modulator
and taped the gauge to the window.
Cold idle=17Hg same when hot.
Takes five brake applications to dump the vacuum.
No vacuum change when normal braking.
Vacuum holds twenty minutes when parked.
Full throttle=10Hg, no throttle=17HG.
Smooth down shift when cold.
After twenty minutes of town driving the down shifts start to
get harsh. After a few minutes more the down shifts get real thumpy.
I grit my teeth in anticipation when coming to a stop.
If 17Hg works when cold what is going on when hot.
Thanks for involving yourselves in my problem.

__________________
1984 300SD turbo 126
"My true love"

God made me an atheist and who am I to question His wisdom

17" vacuum should result in perfectly smooth shifts and possibly some flaring.

The fact that it does not and starts to bang into gear leads to the conclusion that the modulator is non-functional.

FWIW, the SD has been reduced to 8" at idle...........less with some pedal. It does not bang into any gear nor does it have any hard downshifts.

Thanks Brian.
You say your SD is at 8Hg. I guess if the Vacuum -pressure is in balance with the Modular pressure all will be OK. Finding that balance is the trick.
Can the Modulator be serviced or just replaced.
Also I am considering scrapping the Vacuum pump and installing a
12 volt rotary vane pump from a Volvo. Do you have any thoughts on this.

__________________
1984 300SD turbo 126
"My true love"

God made me an atheist and who am I to question His wisdom

Check and see if the Cap is still on the Transmission Modulator and if it is still there see if the Cap is cracked.

What Lubricates the Rotary Vane Vac Pump on the Volvo?

Outside of the Transmission issue you might unscrew the Vacuum Pump Check Valve and clean out the gunk inside and see if the parts are still intact.
I cleaned mine out with WD-40 and it restored My Normal Vacuum.

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84 300D, 82 Volvo 244Gl Diesel

If cracked or not intact, there wouldn't be 17" vacuum.

I did not measure the Vacuum but when I accidently disconnected the Modulator Vac Line My Transmission up shifted normally and I had harsh down shifts.
But, My Power Brakes worked normally.

I did not drive the Car like that long enough for the Transmission to get Hot so I can't comment on how it would have shifted hot.

There is that restricted Orifice/Fitting between the Main Vacuum Line and the Vacuum going into the Transmission.

In any event it is easy to check the Modulator Cap.

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84 300D, 82 Volvo 244Gl Diesel

Got under the SD today. The cap is still nice and tight as is the line into the modulator.
Is the modulator a piston? Could a small shot of WD4 help?
What is involved in replacing the Modulator?
Thanks

__________________
1984 300SD turbo 126
"My true love"

God made me an atheist and who am I to question His wisdom

No one has mentioned it so I will. How is the fluid in it. Normal level, good red color. Not burnt smelling?

could your cable be out of adjustment?

__________________
85 Merc 300D - Unwinding 31 years of wear
86 VW TD
Mahindra Diesel
Iseki Diesel

In 2007 I didn't own a diesel.