91 300D, seem to shift fairly smooth in the morning when it is cooler but a bit rougher when temps are up to 95 to 100 degrees outside.

The temp seems to be the only variable as I don't drive it any differently on the trip to and from work. Is this indicative of something going wrong with the vacuum or is it normal?

Also, I am a newbie so please be gentle. When you refer to flaring in the transmission, what exactly is that?

A pod in your AC is kicking on at warmer temperatures and it has a vacuum leak causing the harder shifting.

__________________
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

As Kerry has indicated the transmission relies upon vacuum from the vacuum pump to function properly.

Diagnosis usually requires the vacuum diagram for your car and a vacuum pump/gauge. People usually refer to a brand unit called Mity-Vac but then many of us end up buying the cheaper unit from Harbor Freight.

I have the '90 version of your car so I know it fairly well. You have a number of items on the passenger-side of the engine compartment that may be a source of a vacuum leak as well as the usual stuff on the driver's side.

If you search through the various threads you'll find quite a few that discuss vacuum and transmission. You'll be kept busy but the process is not too hard.

I'd suggest you consider gsxr's EGR modification but that may be too much too fast.

If you find the vac is good, the trans does have a component in it that changes for temperature changes in the fluid. I do think the vac is your problem though. Turn off the climate control & see if you still have the problem.

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

my 1982 300SD shifts hard when it's cold and smooths out when it warms up. just got the car and havent changed the fluid / filter yet. I will post more info on this later on this week.

This is typically a result of the lower horsepower of a cold diesel. Since there is less power, the accelerator has to pushed further down to get the same amount of oomph out of the engine. The vacuum to the transmision is controlled by the motion of the pedal. More pedal=less vacuum=harder shift.

__________________
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

Welcome to the forum!
a shift flare is the engine RPM jumping up during a shift. some call it slipping between gears. same thing.

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Well, I tested the hypothesis today. Turned off my air (albeit for a short amount of time) and the car was back to shifting smoothly. So now it's off to find where I am losing vacuum.

Kerry - something does not sound right. For smooth upshifts there is supposed to be no vacuum; that's why you get an immediate vacuum bleed down from the proportioning valve at WOT. For smooth downshifts, it is the opposite - need vacuum for smooth downshifts. Is this not correct? So in post #6 I believe it should say more pedal = less vacuum = smoother upshift. Of course I am relating to an 81 300SD so maybe something changed by 1992??? Am I off base?

So the poster should be aware of when he needs vac and when he does not.

How the climate control works into this, I have no idea. Could someone explain?

You should disconnect the vacuum line to your modulator and take a test drive. The flaw in your theory should be immediately apparent.

Already did that. Am I remembering backwards? Need to go back and look at my tranny thread.

Tango - I looked back at my posts and I believe I am correct. In the SD the proportioning valve bleeds off the vac at WOT so you get a smooth upshift. I drove around with the line to the tranny pulled off and got smooth upshifts and clunk downshifts. Then I plugged the mityvac to the same line, drove around and pumped up the vac. The downshift worked perfectly under 15" of vacuum from the mityvac. (I kept driving around with vacuum released while it was upshifting, then after it reached fourth gear, I would pump the mityvac to 15" and come to a full stop. Worked each time.)

So I think my post above is correct. What are you thinking?

I am thinking that, if you were correct, vacuum to the transmission would need to drop to zero as soon as the accelerator pedal was depressed. That doesn't happen; the vacuum level is reduced in a fairly linear manner as the pedal moves through its range toward WOT.

Tango - You may be right about the way it is designed to release the vacuum. I can only report on my new used tranny; in mine the vcv just about immediately released the vac when I hit the accelerator pedal. I watched it on the mityvac while sitting in the garage; it went to the zero box in the mityvac immediately. And at WOT it has zero vacuum. The tranny needs zero vac to upshift and it needs positive vac to downshift.

For sure, most upshifts do not occur at atmospheric pressure.