Greetings folks, I've been all over the innernets and my local mechanics' trying to find the answers to my problem. It's becoming apparent that there is no ONE answer, but I'd thought I'd try here too.

Changed the tranny fluid and filter (yes drained the TC). After a bit of trying to figure out if the fluid level is correct (hard to read the stick), I took it to a shop that's got some pretty seasoned repairmen. Said even if it's overfilled that the extra will manage to escape via an overflow tube (?). But, I'm pretty sure I've got teh correct amount of ATF fluid in.

So, I get this really bad flare between third and fourth gears. It wasn't present AT ALL before the fluid/filter change. It's the first change in a year since I bought the car. I'm wondering if the previous owner put some sort of 'anti-flare' additive in it (is there even such a thing?). I've pulled vacuum on the system (except for the vac modulator on the side of the tranny, which I'll get to in a minute), and even drilled out the T fittings that some websites called for, nothing worked. I even turned the modulator 'key' clockwise a full turn and that did nothing. Don't worry, I've got the Bowden cable adjusted correctly, even though that doesn't have much of a part in the problem.

So, suspecting a bad vacuum modulator, I've unplugged the vac line going to it. I wanted it to shift as hard as it possibly can (that is what it's supposed to do without ANY vacuum line attached right?). Guess what...it still flares between 3rd & 4th gears! I'm having a hard time believing that I have a bad tranny since it worked before the fluid change. I was sure not to get any junk into the tranny fluid pan when I changed the filter (I've changed my other MB the exact same way and it works fine).


Please, someone...insight?!!!!1

I'm at the end of my rope..lawl

might be some dirt got in valve control body or tranny worn out and old fluid was thick enough to keep it from slipping. You could try pulling the valve body and cleaning but watch location of small parts. Ball bearing and springs have to be put back in right order and right location. MB is not very helpful providing diagrams of their trannys

Try turning in the modulator key a half turn in at a time to increase line pressure.

When slip is gone, check the vac' at the BLACK trans' line with a tee piece with engine idling.
Needs to be about 12 inches at idle.
If higher, remove plastic dome cover from side of valve on i/p.
With a small wrench on the shaft nut, set vac' to 12 to 14 inches.

If you can't get vac' down to 12 inches, check vac' at main vac' line from pump....with the correct orifice ( 0.5mm.) you should see about 15 inches to the trans' vac' system.....if more, the orifice has been drilled out or changed and too much vac' signal is getting to valve.

( FYI... the valve is a vacuum bleed, to change the signal to the trans'.)

Open throttle and watch for vac' to decay to zero at wot.
At half throttle there should be about 5 inches of vac'.

Road test.....if upshift too hash, increase vac' at idle to 15inches and try again....
If it's flaring.......reduce vac' to 10 inches at idle and test again.

Small amounts of vac' change will make shift quality change a lot.....





.

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Thanks a LOT for both of y'all's replies. The whole thing regarding messing about with the modulator to get the vac. right is good and all, but I have completely removed all vacuum from the line going to the tranny. Shouldn't this result in the hardest shift possible? Yet it's still slipping.

Also, if it were a band brake issue, which one would it be?

Thanks!

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alimony

I had the same problem many years ago when I did the first fluid change on a rebuilt transmission. The transmission was rebuilt by a shop that only does Mercedes and BMW transmissions so there was no question about the quality of the rebuild.

Anyway, I changed the fluid and filter. I used a "generic" filter, not a name brand. I had flare in the 3/4 shift. Suspecting the filter, I went back a repeated the fluid/filter change with a Mann filter. Problem resolved until I did the next fluid/filter change 25,000 miles later. This time the 3/4 flare was there for good. I did note that the transmission shifted better as the transmission aged (debris in the fluid tightening up the shift?).

My problem was not a severe as yours sounds as I would experience harsh shifts when I disconnected the vacuum line to the modulator.

When I finally got tired of the sloppy shifts 14 years later, I got another rebuilt transmission from the same rebuilder. He reported that he had seen situations of bits of glue used in the manufacture of the generic filters that came loose and lodged within the valve body. He always used Mann filters.

The 3/4 shift is particularly bothersome in that it requires the precise timing of two unrelated events: 1) release of the B2 band and 2) the application of the K2 clutch pack. If the pressure to the clutch pack is not fast enough or of sufficient quantity, the shift will flare.

Good luck with this one.

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Tom Evans
69 280SE 91K (wish I still had it)
73 220D 98K
78 300D 134K
78 300D 185K
81 300SD 301K (still have)
82 300SD 321K (still have)
84 300D 268K
84 6.2 Diesel GMC Suburban 225K (a whole different story!)

Awesome information, thank you. I used one of those Meyle filter/gasket set. Interesting, since I did the change on my other benz with a DIFFERENT filter and didn't run into any problems. I really hate to, but I guess if i get a warm day then I'll redo it with a different filter. Sucks, b/c i thought the Meyle was pretty good.......hmmm....have to get on to ol' Russell @ dieselgiant about that;P

I hope another change will get the bits out of the filter, or at least correct the problem. grrr....It would probably be a royal PIA to rebuild a tranny huh?
I saw some TSB regarding drilling out of the holes in the valve body, possibly to let more fluid in faster to the pistons or clutch pack or something like that........I'm learning as I go here.



What is a fair price that I should expect to pay for another tranny? Used, of course...

Thanks!!!

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alimony

i was going to suggest the K1 spring kit but i remembered that its for the 2-3 shift flare...wonder if you tried it maybe it would do something? i mean it couldnt hurt and its only $12 or so. I also wonder if there is a K# spring for the 3-4 shift?

just a thought

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Yes, that's one of the things I'd like to know. There's so much information on this, but for some reason I can't find the answer I'm looking for. I'll keep bumping this darn thread myself if I have to, b/c I know one of you knows what my problem could be. I don't want to have to buy a new tranny unless I have to-after all options are exhausted....

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alimony

" Mojool ",
Try to stay away from internal tranny fixes for your problem... that is until you have exhausted the several possibilities that the vacuum control system presents.

So I wanted to add this sidebar thought that was brought on when I read about:
(a) your drilling out the main vacuum supply "T" and
(b) " dkveuro's " POST above that made me think
... made me think of this:

IF you are not able to adjust the vacuum low enough " dkveuor's " included in his POST, then your drilling out the " T " might have contributed to such an inability to adjust the control syste using the Vacuum Control Valve as he instructed... the component that sits on the back of your IP.

With this in mind, browse the following THREAD I started that is a lengthy discussion [220 responses] but includes something I think often figures into properly setting up the vacuum control system on these auto tranny cars of this vintage... namely that the use of restricted orifices that are installed and included in the "T" that branches off the main vacuum supply line supplying the tranny shifting controls.

Take a look at this THREAD that starts at:
http://www.peachparts.com/shopforum/showthread.php?t=158216
and try the "Search this Thread" option that is on the bar. I see that there are 32 of the 220 POSTS that includes the word "orifices". Check these out.

I'm not saying that these orifices are the solution to all problems but IF you are unable to adjust your control system to a low enough vacuum using the Vacuum Control Valve, then it could very well be because you drilled out the "T" coming off of your main vacuum supply line... the one running over to the brake booster. I also believe that such drilling can offset the engineered balance MBZ designed into the vacuum system of these cars... a "balance" that was intended to reserve enough vacuum for the brakes even if you had major vacuum failure in any number of vacuum components.

IMHO, this is one of the reasons that MBZ switched to a separate electric vacuum pump for the door lock systems in their 300SD [W126] models.

IF I'm correct, a possible easy solution for you might be to add back in any restriction you drilled out... this by putting an in-line orifice on the tranny vacuum control line just after where it branches off the main vacuum pump-to-brake booster line.

Sorry to run-on so much!
Regards,

Thanks for your input! But, as of right now, I've taken the vac line TOTALLY off of the modulator on the side of the tranny (yes i put a golf tee in it). So now I'm getting NO vacuum and still sloppy flaring (taking the vac line totally off should result in the hardest shift possible, from what I've been able to gather).


Thanks again though!!

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alimony

... I canNOT say from experience for certain how these automatic trannies are suppose to shift when you completely disconnect the Vacuum Modulator... but let's see if we can think this through. I know that if everything is working correctly, as the vacuum vehicle accelerates and the Vacuum Control Valve bleeds off the initial strong vacuum [ 15-20 ], I know that the Vacuum Modulator should see a lower and lower level of vacuum… until it finally reaches ~zero. Then my way of thinking is that IF we disconnect the Modulator, I would expect it to shift upward more quickly ["sloppy flaring"] than if it were receiving the proper high-dradually decreasing-to-zero vacuum. So I don’t think I go along with the notion that disconnecting the Modulator should cause you to have hard/ abrupt upward shifts !
I could be wrong and in the interest of knowing what's correct on this point, I welcome yours and anyone's rebuttal explanation of the error in my trying to think this one through on logic alone ! I once disconnected the modulator as a diagnostic tool, but for the life of me I canNOT remember the details and thus my shot at this one using logic!
Regards,

from what my newbie tranny mind can understand is that the Bowden cable (the one on top of the valve cover that connects to the throttle linkage) determines the time (how quickly or slowly or the 'when') of the shift, and the vac. modulator determines the firmness of the shift.

i've seen in several places that taking off the vac line results in a hard shift.

also, when you turn the little 'T' key on the side of the modulator, it is supposed to increase the vac leak (no, really...it regulates it by actually leaking), so that you have less vacuum, resulting in a harder/firmer shift.

on my other 300D, when doing the exact same fluid/filter change, i noticed there was NO vac line hooked up at all...and she runs pretty well.

__________________
alimony

Regarding (A) above - ...about these vintage MBZ(s) that have the Bowden cables etc for I have had NO direct experience with the earlier and these later models lik yours with both Bowden cables and vacuum control components. Our two 1980 model MBZ(s) have vacuum tranny control only [to my knowledge] and a simpler vacuum control system at that!
So have you monitored [of late] the vacuum by "T"ing off in the engine compartment at the vacuum line that takes off and goes down to the tranny... monitored what vacuum reading it starts at and IF/how well it slowly transitions down to/toward ~zero vacuum. IF I were in your shoes, I would hook things back together and give it a try. Surely you have a vacuum gauge. Take a reading at idle from the "T" you drilled out, and then connect the gauge to the line to the modulator and position it so you can clearly see it while driving. I usually wedge the pump/gauge under my windshield wiper blade assembly.
It's worth a try... and IF you are NOT starting a reading of ~10 to 15 [ at just above idle ] and then slowly and reasonably smoothly dropping the vacuum reading eventually to ~zero as the car speeds up, well then we have something concrete to talk about.
Regarding (B) above - Let me say this - turning the adjuster "T" on the modulator has nothing to do with the rate of bleeding off vacuum from the tranny vacuum control system... for bleeding down the vacuum is the function of the Vacuum Control Valve [VCV at ~$200] which resides on the back side of the Injection Pump (IP). Maybe you have reversed the names of these two in your mind? I have seen a number of different names of various vacuum components so we must be dilligent when we use these names. Most of the time I try to stay with the names on the diagrams available at: http://www.peterschmid.com/ for I think these are from official MBZ sources.
Regarding (C) above - It's a somewhat common story/tale on this FORUM... where people report misadjusted automatic MBZ trannyies seemingly shifting OK... but I suspect that many of these cases are from people who have never driven a MBZ this is setup and shifting properly.

The Guru of knowledge of these vintage trannies [designed in the 1930-40(s) by GM I have heard]... the Guru [ Steve Brotherton (?) ] once said that he believes that many of these trannies have been needlessly ripped out and replaced with rebuilds at considerable expense because so few people really understand these external shift control systems... particularly the vacuum components!

Where is Brian Carlton of this FORUM when he is needed?!
Regards,

The bowden cable adjusts the shifts according to throttle position. That's why at WOT the RPM gets pretty high before the tranny shifts tp the next higher gear. Modulator determines the firmness of the shift.

You are correct, no vacuum should result in really firm shifts.

Keep chasing the vacuum issue, and also check to see if your modulator hold vacuum as well.

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RRGrassi


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... thanks for chiming in.
Mojool - What I'm trying to do right now is verify whether or not your vacuum modulator is receiving the correct vacuum signature... or at least something in the ball park. Once that has been confirmed, I would want to know just were your vacuum modulator is adjusted. You can do this by carefully adjusting it all the way to the left and then right to the limits and note how many turns it takes in each direction and end up with it adjusted ~mid way. I am not very familiar with the modulator on your newer tranny but by starting mid-way, I feel you will have it in the best postion to start adjusting for shift firmness.

But first verify what vacuum signature you are sending your modulator!!!
Regards,