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.

Sorry, I need a clarification here... What rack and how do I open it? Are you talking about something on the white plastic valve?

The rack controls the injectors. Just move the "throttle" linkage from idle toward maximum power and the rack will move to provide more fuel to the engine. We don't call it "throttle" because a diesel engine is not throttled.

I am thrilled you started this thread--I have the EXACT same issues. Low load clunking.

Which MityVac did you get?

Question for others--What mode of MityVac is suited to do this job? Would this one do it or do you have to have the 0400 model? Is there a recommended place to get one? Thanks in advance!

http://www.amazon.com/Mityvac-7000-Automotive-Tune-Up-Bleeding/dp/B000BPFB3A/ref=pd_bbs_sr_1/104-9001271-1222314?ie=UTF8&s=automotive&qid=1188520535&sr=1-1

The plastic one seemed kind of flimsy when I was shopping for one. I ended up buying the metal one.

Brian - When the Mityvac is fully pumped up, I get about 12.0 mm Hg to start. Then,

in 20 sec - it is down to 11.0 mm Hg
in 1 min - down to 10.0
in 1 min 45 sec - down to 9.0
in 2 min 35 sec - down to 8.0
in 4 min - down to 7.0

I did do this test nonetheless, and it behaves exactly like you described it should - losing vacuum in-sync with the rack movement.

The modulator has a slight leak.........nothing serious. The VCV is modulating the vacuum signal properly.

So, if the transmission is still exhibiting the characteristic clunking and very firm shifts.........we need to see if an adjustment to the VCV or the modulator is in order.

Disconnect the vacuum line to the transmission and connect the Mityvac. Run the hose into the cabin. Pump up the Mityvac to 15" and keep it there (give it a squeeze every so often as it falls).

Now drive the vehicle and see if the hard shifts disappear. Make sure the Mityvac stays between 12" and 15" during this test.

Brian - you mean bypass the T-junction on top of the green damper and pump the tranny line directly with Mityvac? But then I don't understand how the shifting will be controlled with the gas pedal, if we're bypassing the modulator... Could you please explain.

I believe he means connect as it is in this pic (but run the mityvac into the car so you can see it while driving):
http://img252.imageshack.us/img252/2...uum3ay7.th.jpg

You are using mity vac instead of the car's vacuum system to feed the trann'y modulator. This is only for testing. If the tranny shift nice and smooth through all gears, then the problem lies elswhere.

That's what I assumed and did - I connected the Mityvac into the T-junction, in place of the line from the vacuum pump. I ran the line into the cab.

Wow, I had to pump the Mityvac hard, pretty much all the time to keep up with the modulator during accelerations and such. The shifts did feel a good bit smoother, although not seamless - probably because my hand couldn't keep up with the demand for vacuum all the time. I didn't hear clunking, but I didn't drive around a whole lot like this either - my hand was about to kill me.

Also a thing to keep in mind - my shifts are generally much smoother on hot engine/tranny than during the first drive in the morning.

That's not the setup that I want. I want the Mityvac to go directly to the modulator valve........bypassing the T junction.........so that the VCV doesn't leak vacuum while you drive.

The test failed because the VCV leaks far more vacuum than you can possibly pump with your hand. The modulator doesn't.

So, repeat the test with the proper connections..........I want to see if the modulator will respond with higher vacuum levels.

In the pic, the mityvac is not connected to the T junction, it's connected to the line which goes into the T junction.

I'm following .....
 

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

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

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

Scott

Ok, this morning, I connected the Mityvac directly into the line running to the tranny, bypassing the T-junction. I pumped it to about 14 mm Hg vacuum, and it has remained there - no leaking over the short drive, so I didn't even need to pump the Mityvac at all.

The up-shifts were somewhat smoother although not totally smooth, but the downshifts were definitely less so - they seemed to lurch a bit. The shift points were fine, though - responding to the gas pedal in the same manner I am used to.

Good tests.

You've confirmed that we cannot be successful with any vacuum adjustments to smooth the shifts. The only viable alternative is to attempt an adjustment to the modulator itself. I'm hesitant to do this as the modulator is typically set by using a gauge and setting it to a specific pressure. However, you're out of options.

The modulator adjustment is found at the point where the vacuum line enters the transmission. The rubber cap is removed and you'll be able to pull out a T handle for the adjustment. Rotate the handle exactly three turns counter-clockwise..........fold up the handle..........connect up the vacuum line............and take it for a drive. See if the shifts are noticeably softer. If the do soften up..........we can tune it with vacuum.

Here's an article on modulator adjustment:

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

Let us know what happens.

Scott

Would it be of any use to try the varied levels of vacuum supplied to the modulator? I used ~ 14 mm Hg, but I could also try to make separate runs with 12.5, 13, and 13.5 to see if there's any sensitivity in shift quality to that level...

I think I don't want to adjust the modulator blindly - I'll have to rig up the gauge contraption described in the Wiki and get some numbers out of it.

I had my car at the Benz dealer this week, and they told me the tranny is leaking around the B1 band. Could that have any bearing on the internal tranny pressure and shift quality?

No, I don't think you need to make further tests. If it won't smooth the shifts at a steady 15" vacuum level, then vacuum adjustments won't cure the problem.

If you can properly rig up the gauge, that would certainly be the better approach. Hopefully, the pressure is higher than desired, however, if not, I'd still take a shot at reducing it and testing to see how it performs. You can always return it to the starting point if you are careful in noting the adjustment.

Don't have enough knowledge of the internals to state whether the leak can cause these issues.

Brian, theres one thing I am not clear on. I actually had set up a remote vac gage at one time on my 84D teeing off the line going to the modulator and drove the car while reading the gage. As I went up in speed the vac dropped to zero vac. It was my understanding that it is suppose to be that way. Did I get it wrong?

My symptoms are similar to whats described in this thread. I had adjusted the modulator just like you had suggested here with slight improvement in downshift clunking - I'm guessing from 3-2.

If you T off the line going to the modulator with the VCV in the system, the VCV will properly bleed more vacuum as you apply more pedal. At full pedal, the VCV will bleed enough vacuum so that the resulting vacuum at the modulator will be close to zero.

If you send vacuum straight to the modulator from the Mityvac, there will be no bleeding and the vacuum should basically remain at the same level that you originally pump up on the Mityvac. The VCV is not in the system at this point.

The first point of attack for hard shifts and clunking downshifts is vacuum. If you can increase the vacuum and soften the shifts, that's the way to go. But, sometimes, it's not going to be successful, as I'm inclined to believe in the present situation as posed by the OP.

What would you suggest the next step should be? Also, say I simulate a steady vac to the mod with the Mytivac resulting in corrected shifting, how do you introduce vac to the mod from the system after the Mytivac is removed?

The '84 should have a small round dome on the side of the VCV. Remove the dome with a blade and there will be an adjusting screw and, possibly a locknut.

Loosen the locknut and the adjustment screw must be turned in very small increments (10 degrees or so) to increase or decrease the vacuum at idle.......depending on what you want to accomplish.