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

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

Well, it was probably removed before your ownership.

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

If the AC makes the tranny symptom worse, I would guess you have a leaking AC vent vacuum pods.

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'85 300D Turbo - CA Version

I am suspecting the same thing as well. Where can I find the vacuum line that supplies vacuum to the ACC?

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

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

Alright, I finally got Mityvac and a little bit of time... So now I'd like to sort out why my drivetrain is so clunky.

Where do I begin?

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

I have somewhat the same problem, however mine shifts the best when A/C is on. If it is turned off it can be very rough. I am going to spend time on it this long weekend. Any ideas.

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Junqueyardjim
Christianity, if false, is of no importance, and if true, of infinite importance. The only thing it cannot be is moderately important. C.S. Lewis



1983 Mercedes W123 240D 4 Speed 285,000 on the road with a 617 turbo, beautiful butter yellow, license plate # 83 240D INDIANA

2003 Jaguar Type X, AWD. beautiful, good mileage,
Mom's car, but I won't let her drive it!

First thing to check is the supply vacuum to the VCV. You'll see a T above the VCV. One leg comes from the vacuum pump and the opposite leg goes to the transmission. The bull is connected to the VCV via the green vacuum damper.

Disconnect the supply line that goes into the T and measure the vacuum from the source (the vacuum pump) at idle.

Then, reconnect the line that goes into the T and disconnect the line to the transmission (from the T). Connect the Mityvac to the open T and read the vacuum at idle.

Report back.

Once it equilibrates, around 20.3 mm Hg

Around 11.9 - 12.0 mm Hg.

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

Good job.

Now, with the engine off:

Reconnect the Mityvac in place of the source vacuum (remove the line from the vacuum pump and connect up the Mityvac).

Pump up the Mityvac fully. You should get the same 11.9-12.0 mm Hg and it should hold there. If it does not hold..........then report back.

If it does hold, then slowly open the rack and observe the gauge on the Mityvac. It should slowly fall toward zero.........getting to zero just about the point when the rack is fully open.

Report back with results.