What are symptoms of a worn tranny modulator valve?
 

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?

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?

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?

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.

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.

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.

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

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

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?

May I add?
 

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.

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.

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.