Clunky Downshift on 300d

[ramjensen]

My '87 300d downshifts with a clunk under a certain condition. It happens when I get off the highway and slow from 50 mph or so to 20 to 25 mph briefly to cruise through the tollbooth with my electronic pass. After I coast through the tollbooth, the car downshifts with a clunk even when I only give it moderate pedal (about halfway down or less). I have watched the rpms and am usually turning about 2000 when I hit the pedal. If I feather the pedal until I get to about 2500, I don't get the clunk. Here's what I've tried so far:
1. Adjusted Vacuum Control Valve (VCV) according to manual.
2. Cranked adjustment on Vacuum Amplifier fully clockwise (for max. vacuum)
Test drove and still got the clunk, so:
3. Adjusted VCV fully clockwise, still got the clunk
4. Adjusted VCV fully counterclockwise, still got the clunk
5. Teed into VCV and checked at idle, 10 inHg
6. Teed into Vacuum Amp output to tranny and test drove, 13 inHg at idle (I believe that Steve Brotherton's article http://www.continentalimports.com/ser_ic20242.html specifies 15-17 should be going to the modulator)
7. Vac tested modulator with mityvac and it held vac for about 20 mins perfectly.

I have not touched the modulator on the transmission. My other symptom is that the car jerks back when put into R and forward when put into D. I have heard this is normal, but I have my doubts. To this point I have not adjusted my throttle linkage, my bowden cable, or the little two piece slide with an adjustment which I read about on mbz.org http://www.mercedesdismantlers.com/722.3and722.416_transmission_adjustments.html. Is the clunky downshift normal? Is the jerky shift out of neutral normal? Should I first adjust the throttle linkage even though the car seems to have good power and the shift points seem fine?

[running-snail]

Nice analysis - while we're waiting for Mr. Carlton to step in... you mention you've vac tested the modulator and it held. Did you also test the VCV (engine off) to see if it is holding vacuum, or how quickly it drops?

It would be interesting to have a guage in the car "t-ed" to the modulator line to see what amount of vacuum you're getting when you begin to press on the accelerator. Based upon previous reading, I associate clunking to mean too little vacuum..

[ramjensen]

I read in the manual that upon shutting the engine down after checking the vcv output at idle, the vac gauge should drop to zero. Mine does drop to zero within a second or two. This makes sense to me. Isn't the vcv supposed to be a constant, controlled vacuum leak which increases as more throttle is applied thereby stiffening the shift under heavy pedal? If this is the case it should not hold vacuum, correct? Educate me please if I don't get what you are saying. I will take your suggestion to tee into the line which goes from the vac amplifier to the modulator on the tranny and observe my vac levels, particularly when I get the clunk. Thanks.

[C Sean Watts]

Same car here - identical problem except the R or D jerk

[running-snail]

I'm still learning the ropes here - hopefully a posting link will show up. This is a definitive posting regarding VCV & modulator settings - w/ great pics. If I post this correctly, this should take you to Dieseldan's posting regarding his VCV testing.

The answer is yes, there is a leak occuring with the VCV - it's just a question of how fast. Particularly if you test with the engine stopped and no throttle, there should not be a very fast leak, IIRC

http://www.peachparts.com/shopforum/showthread.php?p=1348968
Click Me!

[ramjensen]

Running Snail, thanks for the link and the advice. Sounds like my vcv is bleeding off vacuum too quickly, like I said I think like 1 or 2 seconds with the engine stopped and no throttle. I will check again tomorrow and report. If Diesel Dan was correct when he stated the vcv should bleed off 1 inHg per second, I think mine is going much faster. One thing, though: my car is an '87 and my vcv looks quite different than what Diesel Dan pictured. My car was worked on by someone who had no clue about diesels and they replaced the injection pump. When they did so they did some stupid things like not replacing the injector line retaining clips, and smashing the alda against the intake and cracking it. Sooo I wouldn't be surprised if they messed up the adjustment of my vcv before putting it on the new IP or if the thing was bad and they put it back on because they had no clue or didn't care. I will report back and thanks for the help.
Sean, stay tuned maybe we can help each other.

[holthoff]

Some of those clunks could be coming from worn rear axles. If they are not tight, the axle can rotate freely a moment as you first engage drive or reverse; when it catches, you get a clunk and a lurch. I'm not sure it would cause a downshift clunk, though...