The tranny has been shifting rougher and rougher over past several months on my 300SD, and lately I hear louder and more frequent clunking from the differential case when I come to a stop.

Generally, the tranny shifts smoothly when it is stressed during harder acceleration, but on slower, more gradual acceleration, the shifts would be rougher (particularly first and last gears, and last gear would even clunk when not stressed). It also appears that having the AC on makes the differential case clunk louder when I come to a stop.

I am suspecting that it's more the vacuum gear-shifting mechanism and modulation than a bad flex coupling or something inside the differential case, but I don't really have any experience with these issues to say with any certainty. Do any of these symptoms point to a bad modulator valve or something related to that?

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

First and foremost check your fluid level. How long since your last filter change?

Secondly, its time to check out your vacuum system. Maybe you have a leak thats slowly getting worse.

Do you have a MityVac?

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'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

I check tranny fluid periodically - the level's good, and the fluid color is what it's supposed to be. I haven't done the fluid or filter change since I acquired the car, but I am planning to have that done soon.

I do keep an eye on my vacuum hoses and fittings, but no - I don't have a MityVac. I assume by MityVac everyone means the hand-held vacuum pump?

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

Yep.......you'll need it to further diagnose the system.

The modulator is the least of your concerns at this point.

BTW, do you still have all the connections to the black box on the top of the valve cover.........all EGR related?

No, I don't see any black box on top of the valve cover - only linkage-related stuff and the rubber breather tube. The previous owner also plugged the EGR valve connections.

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

Well, it was probably removed before your ownership.

Pick up a Mityvac and we'll sort it out.

I surely don't possess the knowledge of many of the forum members but I have experienced similar symptoms. I replaced my transmission vacuum modulator (turned into a big job) and that was not the cure. It was leaky, but not the primary problem. Under the tutelage of several of these forum members, I finally obtained smooth shifting by making adjustments to the vacuum control valve. Used a mighty vac -- great investment --- and now have a smooth shifting transmission. In the process of doing this I think I experienced all the conditions from too much vacuum to too little vacuum. Again, the mighty vac is essential.

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John Schroader
bio burnin' 83 300D, '83 300 SD, '79 240D
"I've never met a man who was good at making excuses who was good at anything else" Ben Franklin
"You cannot permanently help a man by doing for him what he could and should do for himself" Abraham Lincoln

Under the hood, it's the green one running to the firewall.

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RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

99 W210 E300 Turbo Diesel, chipped, DPF/Converter Delete. Still needs EGR Delete, 232K

90 Dodge D250 5.9 Cummins/5 speed. 400K

Gone and still missed...1982 w123 300D, 1991 w124 300D

...... this too. thanks, BC.

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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

Thnks for this Brian, I have the same problem and this step by step troubleshootig guide your creating is going to be very helpful.

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I'm sick of .sig files

Maybe this can be turned into a step by step diagnosis with pics for the DIY Articles section?

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)