Recently got at 79 W116 300sd. At light and moderate throttle shifts are too soft with flaring on 3-4 and sometimes on 2-3. Shifts crisp at full throttle.
Vacuum to modulator is 13 inches at idle reducing to 0 at wide open throttle.
Just for the hell of it, I disconnected the transmission modulator vacuum home for the drive home tonight. All shifts under all conditions were crisp. 1-2 was a little harsh but not as harsh as the 1-2 shift on my 77 300d with a transmission without vacuum controls.
Is there any downside to simply leaving the transmission modulator vacuum line off permanently? The car has 111k and I know the original owner had the modulator replaced at some time in in the past after complaining about transmission problems.
The only downside is that the 1-2 shift is slightly harsh. It's not harsh enough that I would worry about it if I didn't know I had disconnected the vacuum line.

__________________
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

I don't think it will hurt anything, but it;s not going to fix the real transmission issues that are causing the flaring.

What are the real transmission issues causing the flaring? I thought flaring was caused by high vacuum at the modulator.

__________________
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

If it's not slamming on the 2-1 downshift, I don't see why it would hurt anything at all fairly longterm.

__________________
80 300SD (129k mi) 82 240D stick (193k mi)77 240D auto - stick to be (153k mi) 85 380SL (145k mi) 89 BMW 535i 82 Diesel Rabbit Pickup (374k mi) 91 Jetta IDI Diesel (155k mi) 81 VW Rabbit Convertible Diesel 70 Triumph Spitfire Mk III (63kmi)66 Triumph TR4a IRS (90k mi)67 Ford F-100 (??)

I believe I'm correct in thinking that the 79 SD does not actually make the 1-2 downshift until you come to a full stop. At least that's the way the car is behaving and I'm pretty sure I read that is the way it was designed.

__________________
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

It's my understanding that the modulator will only soften the shifts, but should not allow flaring (increase in rpm between gears). You should be able to make the shifts as soft as you want without flaring. My transmission would also flair with the modulator set "too soft," until I replaced the transmission.

Your flex discs certainly won't last as long.

__________________
Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John

They'll still last longer than the flexdiscs on my 77 because that has much tighter shifts all the time in all gears.

__________________
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

Did you adjust the modulator itself on that flaring transmission or the vacuum to the modulator?
I'm pretty sure the modulator setting needs to be determined with some kind of pressure gauge, is that correct? It's possible that since my modulator was replaced, the pressure wasn't set correctly.

__________________
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

The W116 300SD has an electromechanical device that does the 2-1 downshift when you come to a stop

__________________
80 300SD (129k mi) 82 240D stick (193k mi)77 240D auto - stick to be (153k mi) 85 380SL (145k mi) 89 BMW 535i 82 Diesel Rabbit Pickup (374k mi) 91 Jetta IDI Diesel (155k mi) 81 VW Rabbit Convertible Diesel 70 Triumph Spitfire Mk III (63kmi)66 Triumph TR4a IRS (90k mi)67 Ford F-100 (??)

If it doesn't bang on 3-2 and 2-1 downshifts, there is no downside.

On my old tranny, I just had the modulator turned way down so it shifted "too hard" to try to minimize the flaring. I also got in the habit of letting of the throttle between gears, because it would still flare if I kept my foot in it. Basically, the tranny was shot and I probably got away with it for another 20K miles before I just had it replaced. I do not believe you can get a good tranny to flare at any modulator adjustment, but running with it "too soft" will increase the wear.

kerry - if you want to try setting the modulator -

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


Is it true that the diff will much faster from hard shifts?? I have read this elsewhere on the site...which of course doesnt mean its true, but I though its worth mentioning.

dd

__________________
-------------------------------
'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

if your really really desperate and dont want to fix the tranny just yet, throw some sawdust into the pan before you puti t back together. (not really thck and grainy stuff, but fine powdered stuff.) it will help the flaring. but thats more of a last ditch effort than anything.

__________________
Have you ever noticed that anybody driving slower than you is an idiot, and anyone going faster than you is a maniac?

As long as they would add one additional commandment for you to keep thy religion to thyself.
George Carlin (Wonder where he is now..)

1981 240d (engine donor 1983 240d) recently rebuilt engine hurray! - No more.. fought a tree and the tree won.

pearl black 1983 240d 4speed (Converted!@$$%) atleast the tranny was rebuilt.

I don't think the tranny needs any work. It shifts fine with no vacuum. I assume from reading the modulator setting wiki that this probably means the modulator pressure is too low. But as long as it's shifting fine, I don't see a pressing need to change the modulator pressure.
The 79 SD is probably the one vacuum controlled transmission that can best deal with low vacuum to the modulator. Since the 2-1 shift, which typically is the hardest one with no vacuum, is controlled electromechanically and doesn't occur until the car is stopped,

__________________
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