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  #1  
Old 03-04-2007, 07:20 PM
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Clunky Downshift on 300d

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?

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Last edited by ramjensen; 03-04-2007 at 07:30 PM.
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Old 03-04-2007, 07:49 PM
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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..
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  #3  
Old 03-04-2007, 08:19 PM
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Testing the vcv

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.
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Old 03-04-2007, 08:41 PM
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Subscribing to thread...

Same car here - identical problem except the R or D jerk
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Last edited by C Sean Watts; 03-04-2007 at 08:46 PM. Reason: adding
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Old 03-04-2007, 08:47 PM
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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
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  #6  
Old 03-04-2007, 09:19 PM
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My leak is too leaky

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.
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Last edited by ramjensen; 03-04-2007 at 09:30 PM.
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  #7  
Old 03-04-2007, 11:33 PM
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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...
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  #8  
Old 03-04-2007, 11:45 PM
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I seem to recall that the set screw on the bottom of the amplifier will reduce vacuum when turned fully clockwise..........but........I haven't adjusted it in awhile.

In any case, try the amplifer with the set screw fully counter-clockwise. It takes a 4mm socket........which you probably already realize.

Also, try a test with just the Mityvac connected to the modulator. Pump up the Mityvac to 15" and leave it there. Take the vehicle for a test drive and see how it does. If the vacuum falls during the test, pump it back up to 15". See if the clunk disappears.
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Old 03-05-2007, 08:21 PM
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Test Drive Info

Tested the vcv today, and found that contrary to what I had previously thought, it loses vacuum at what Diesel Dan considers a proper rate, that is about 1 inHg per second.

Test drove the car today.

First, with mityvac teed into vcv output:
10 inHg at idle
8 inHg driving at constant speed
5 inHg with light pedal
0 inHg with 1/3 pedal

Second, with mityvac teed into vac amplifier's output to modulator on tranny:
12.5 inHg at idle
10.5 inHg driving at constant speed
5 inHg with light pedal
0 inHg with 1/3 pedal
My clunky downshift took place with about 7 inHg going to the modulator

Third, with mityvac hooked only to vac line going to modulator and with mityvac pumped up to 15 inHg as you suggested, Brian:
Tranny flared when shifting, and flared immediately before producing the clunky downshift yet again. Vac stayed at a constant 15 inHg the whole time. Tried to produce the clunk again and got it on the same downshift, this time with no flare coming first.
So, the clunky downshift is not the result of a low vacuum condition. Brilliant idea to test it this way, Brian. What does it mean that I still get the clunk? I assume that the downshift that is taking place is a 3-2 downshift. As I go from high speed to braking to 20 mph, the tranny shifts from 4 down to 3 I would guess, although I can't feel it. When I try to accelerate, the tranny kicks down to 2 unless I feather the pedal, and the shift control allows me to stay in 3 since I am not demanding much acceleration.
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Old 03-05-2007, 08:46 PM
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Good job in the testing. But, the results are a bit baffling. I would have expected the downshift to be smoother with more vacuum.

Now try this:

Go to the Bowden cable adjustment screw which is down below the oil filter housing.

Adjust the cable slightly to provide more slack. Turn the adjuster inward (clockwise, aft looking forward) by two turns.

Take it for a drive and see what happens.
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Old 03-06-2007, 08:44 PM
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Eureka!

Brian, I turned the screw below the oil filter housing 2 turns as you suggested. Now it takes more pedal to get a downshift (when slowing down from 60mph to 20 mph then hitting the accelerator). When I do get a downshift, it is firm but not as hard as before. It feels just about normal, if a tad too firm. Also, when maneuvering the car around my driveway, it only takes a light touch of the pedal to move the car. Before I was having to push the pedal much further to do low speed parking maneuvers. I was also having to push the pedal much further when accelerating away from a stop. I assume that the adjustment you had me do somehow changed my shift point relative to throttle position? What you referred to as the bowden cable adjustment screw is called the control thrust cable in my manual. The manual states "Detach ball socket (212). Push control thrust cable (211) forward until slight resistance is felt. In this position, attach ball socket free of tension; adjust at bowden cable, if necessary". According to the manual, the bowden cable is the cable up above this control thrust cable. Which is which?
I'm not sure what you had me do, but it helped. I would love to understand more. I plan to follow my manual's procedure for adjustment of the accelerator control linkage including this control thrust cable to get things fine tuned.
My car was operating without turbo boost when I got it. Is screwing around with this control thrust cable something that people (the previous owner) might do when the car has no boost and therefore no power? I would assume that they would do this to force an earlier downshift and make the car more driveable when it is lacking power? Thanks a million for the advice, and I look forward to hearing what I did.
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Old 03-06-2007, 09:00 PM
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Impressive

I learn a great deal from reading Brian's trouble-shooting advice. 14,000+ posts - absolutely amazing.

I am equally impressed with your skills, Ramjensen, with helping to diagnose the problem - too many owners look for a quick & easy answer.

Finally, I apologize for not paying closer attention to your model - I thought at first reply you were in a w123.

I'll go back to 'lurking' now...
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  #13  
Old 03-06-2007, 10:09 PM
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Quote:
Originally Posted by ramjensen View Post
Brian, I turned the screw below the oil filter housing 2 turns as you suggested. Now it takes more pedal to get a downshift (when slowing down from 60mph to 20 mph then hitting the accelerator). When I do get a downshift, it is firm but not as hard as before. It feels just about normal, if a tad too firm. Also, when maneuvering the car around my driveway, it only takes a light touch of the pedal to move the car. Before I was having to push the pedal much further to do low speed parking maneuvers. I was also having to push the pedal much further when accelerating away from a stop. I assume that the adjustment you had me do somehow changed my shift point relative to throttle position? What you referred to as the bowden cable adjustment screw is called the control thrust cable in my manual. The manual states "Detach ball socket (212). Push control thrust cable (211) forward until slight resistance is felt. In this position, attach ball socket free of tension; adjust at bowden cable, if necessary". According to the manual, the bowden cable is the cable up above this control thrust cable. Which is which?
I'm not sure what you had me do, but it helped. I would love to understand more. I plan to follow my manual's procedure for adjustment of the accelerator control linkage including this control thrust cable to get things fine tuned.
My car was operating without turbo boost when I got it. Is screwing around with this control thrust cable something that people (the previous owner) might do when the car has no boost and therefore no power? I would assume that they would do this to force an earlier downshift and make the car more driveable when it is lacking power? Thanks a million for the advice, and I look forward to hearing what I did.

Good job.

The adjustment that you made was to the Bowden cable. What it does is to increase or reduce throttle pressure relative to engine speed. With higher throttle pressure, the transmission will shift later and downshift earlier.

In your case, with an early downshift, the engine speed is a bit too far from the vehicle speed and you get a clunk. To avoid the clunk, we reduce throttle pressure so that it downshifts a bit later........when the engine is turning a little slower.......so the shift is not as dramatic.

I really cannot fathom how the low speed performance would be improved. We did nothing to the relative position of the rack with respect to the accelerator pedal and we did nothing to the ALDA.
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  #14  
Old 03-06-2007, 10:16 PM
Craig
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Originally Posted by Brian Carlton View Post
The adjustment that you made was to the Bowden cable...
It sounds like he adjusted the linkage to the rack, not the bowden cable. He's talking about adjusting a "ball socket" in his post.
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Old 03-06-2007, 10:19 PM
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Quote:
Originally Posted by Craig View Post
It sounds like he adjusted the linkage to the rack, not the bowden cable. He's talking about adjusting a "ball socket" in his post.
The Bowden cable has a ball socket at the very end. There is no adjustment to the rack down below the oil filter housing. The only cable in that area is the Bowden cable.

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