W124 '92 300D 2.5 downshift clunks won't go away - PeachParts Mercedes-Benz Forum

Wayfarer · #1 02-22-2010, 09:07 PM

I have been battling a downshift clunk whenever coming to a stop. It's pretty hard, bad enough that I am worried for the driveline components. Upshifts are good, a little on the hard side, if anything.

The downshift clunk is right around 16-18 mph and is especially bad when rapidly slowing; if I let off the brake and ease down through 20-15 mph, then I can stop without the clunk. Normal driving though, and it clunks. I am guessing that this is the 3-2 downshift, but I really can't tell as the other downshifts are smooth.

This started off as a 2-3 flare. Took it to my indy who put in a K1 spring kit for me. The 2-3 flare went away completely but introduced this new downshift clunk.

Here's where it gets interesting. In reading all the posts on the forum, I figured out (or so I thought) that I must not be getting full vacuum to the modulator due to either a leak or a bad/misadjusted vacuum control valve (VCV). Out came the trusty mity-vac.

Supply to the VCV is a steady 21" Hg. Output to the tranny modulator is rock steady 11" Hg at idle, decreasing to zero by the time 1/3 pedal is reached. It drops from 11" to 9" or so with even the slightest pedal travel; by 1/8 pedal travel, it's already down to 5". These readings were taken both in the driveway and with the mity-vac teed in while driving around. Best I can tell from reading posts here, that's about right. This is after I adjusted the VCV per the FSM procedure, which was easy and simple, and left the VCV at about midway within its adjustment range.

Next I rested the modulator itself. Hold vacuum perfectly with no leakdown.

Next I tried adjusting the modulator itself. Popped the black cap off and started twisting the metal T each way, counting the turns, and test driving between adjustments. I didn't seem to notice much if any difference even when I would turn the modulator 2-3 turns in either direction

. I noticed that the resistance I felt when adjusting the T increased when I went 3-4 turns clockwise; conversely, resistance decreased as I turned the T counter-clockwise until it would just spin with no resistance at all. I don't know if this is normal, as I've never tried to (or needed to) adjust the modulator more than a half-turn or so on any of the four MB diesels I've owned.

Next I tried giving the modulator direct 21" Hg vacuum, bypassing the VCV altogether. Shifts softened up, giving moderate flaring on all shifts, but the downshift clunk was totally unchanged

. Even used the mity-vac & confirmed the modulator was getting full 21" vacuum when the downshift clunking occurred.

I'm out of ideas, but am leaning toward a failed modulator. Possibilities:

1) Either my indy or I broke the innards of the modulator trying to adjust it too far, I know I turned it at least 3 turns in either direction. Maybe this destroys something internal to the modulator?
2) My indy reinstalled the modulator incorrectly, in particular the pin.
3) The modulator is blue plastic, which I don't think is the correct modulator for the car, but it did seem to work OK before I had my indy install the K1 spring kit.
4) The K1 spring kit somehow increased the downshift force to where it clunks, maybe because internal wear makes it so to get a crisp 2-3 upshift, the tranny gives a clunk on the downshift.
5) Maybe my indy didn't install the K1 kit properly?
6) PO removed the vacuum amplifier as part of the EGR delete he did (he did it right, too

, thanks Wayne2970), so maybe that has something to do with the downshift, but I doubt it since it downshifted fine before this recent transmission work.

Oh, and indy changed tranny fluid and filter at the last service and the fluid level is good.

So there we have it. The vacuum system seems to be fine, so I think the modulator has failed somehow during all the recent transmission work that has been done. Frankly, I'd rather go back to dealing with a moderate 2-3 flare than the bone-jarring downshift clunks I'm now getting.

Thanks for any advice. I have a modulator on order, but that won't arrive until next week. Anything I could try in the meantime? Anything I'm overlooking?

Wayfarer · #2 02-25-2010, 11:40 PM

A little more tweaking and a few more clues. Still having the downshift clunk with no change though.

I had adjusted the VCV per the FSM and put it right where in was supposed to go. It was easy to tell right where the VCV encountered resistance when you loosened the mounting bolds and twisted the VCV on the IP. No problem there, it ended right at midway in the adjustment range, which is what everybody says it where it is supposed to be. Still hard downshifts and somewhet, but tolerable, hard upshifts in all gears. 11" to the tranny at idle dropping almost immediately to zero with 1/4 pedal or greater.

So today, in a mood to experiment, I set out to see what would happen if I adjusted the VCV way out of spec, to see what effect it would have. So I twisted it all the way left (CCW) to the stop, hooked up the mity-vac to the tranny modulator line and went for a test drive.

I was now getting 12" at idle, a bit more, but instead of dropping to zero with pedal, it would go to about 5" with 1/4-1/3 pedal, dropping only to about 3" at full throttle. The upshifts were suddenly butter smooth! Even got just a bit of flaring on the 2-3 shift at times. A very noticable difference and much better! Unfortunately, I am still getting the same clunk on the downshift. When the clunk occurs, it is getting a steady 12" (11" when the VCV is set per the FSM).

So clearly, the lack of a vacuum amplifier changes the system and changes the optimal set point for the VCV. It shifts better when the VCV is set at the CCW limit of its adjustment. The tranny does better when it gets some vacuum at all times, in this case around 3-5" at most part-"throttle" conditions.

Also, I think the downshift clunk is a matter of the vacuum at idle , with 11"-12" just not enough to prevent clunking. Can someone tell me what the proper spec is for idle vacuum to the modulator for the OM602? I'm guessing it must be around 15"-17" and for some reason my VCV just can't give it more than 12" even when adjusted all the way.

Which leads to my last conclusion: that the vacuum amplifier (the "flying saucer"), which is long gone from my car, probably brings this idle vacuum up to 15" and gives the modulator enough vacuum to provide smooth downshifts. Has anybody gotten their 602 W124 to shift properly after removing the vacuum amplifier? If so, how did you do it?

Although I haven't yet solved my downshift clunk, all this tinkering sure has taught me about my car's vacuum system. It's pretty cool really. I'd sure rather have a car with vacuum controlled everything than computer controlled everything

.

One more thought: I'm not even totally sure my car is supposed to have a vacuum amplifier, but I assume it did and was removed by the PO who also did the pressure wastegate conversion. Everything I've read says that all diesels from 85-96 had it, so that would include my '92, although without a vacuum diagram, I cannot confirm.

Wayfarer · #3 03-12-2010, 10:34 AM

Any suggestions? I've tweaked and tweaked the VCV and vacuum modulator to no avail. I am getting sufficient vacuum to the modulator, which holds pressure, so I don't know what it could be. I am about to give up and take it to my mechanic (defeat.....

).

Also, can someone confirm whether my car is supposed to have a vacuum amplifier (aka "blue saucer") or not? I don't have one and it doesn't appear from the vacuum diagram that there is supposed to be one.

Oh, and one last question: I bought a new tranny modulator I am going to have my indy install. It's the correct Mercedes part, the "new and improved" version with the white plastic cap and no black rubber nipple like the old style. Now the question: is this new style modulator supposed to leak vacuum? Because mine does. Excahnged it for a second one and it still leaks. Apparently this is part of the new Mercedes design, i.e. some sort of controlled vacuum leak. Can someone confirm. Thanks.

wantamb · #4 04-09-2010, 09:58 AM

My 1981 300TD has two small clunks when accelerate from standstill to 50km/hour. And my 300dt has delay when selected rear. I am also trying to check vacuum first, then replace spring kit.

Wayfarer · #5 04-09-2010, 11:49 AM

After trying everything: two fluid and filter changes, K1 spring kit, replacing the modulator and all vacuum lines, numerous adjustments to the VCV and modulator, I finally gave up and took it to two different indy's for their appraisal. They both said my transmission is on its way out after they too could not get the clunk to go away.

It's interesting that the downshift clunk can be a sign of impending transmission failure. With all the other MB 722 tranny failures I've had over the years (at least 4 that I can remeber over the last 20 years

), they always slipped (aka "flared") uncontrollably to signal their upcoming demise. This is the first one that doesn't slip or flare but rather clunks when coming up to a stop, that apparently is about ready to fail.

I am going to try one last desperation measure: Trans-X. They say never to use it unless and until everything else has failed to fix the problem and you're about to rebuild it anyway. Looks like I am at that point

.

bobodaclown · #6 04-09-2010, 12:24 PM

I'm out of my league here but are all the other components ok. Flex Disk and such.

Olivier · #7 04-09-2010, 12:26 PM

Did you had a look at the front rear sub-frame bushes?

Wayfarer · #8 04-09-2010, 12:35 PM

Flex disks and rear bushings are ok. I know the symptoms of both of those, and my clunk is definitely internal to the transmission. Thanks for the thoughts though.

Wayfarer · #9 06-26-2010, 12:41 AM

Bump. Still need help with this one. Update: the coasting-to-a-stop downshift clunk is still not fixed but here's another clue: I replaced the vacuum modulator with the new MB replacement part (126 270 91 79) and tweaked and tweaked a half turn at a time for several turns in each direction to see what would happen.

Here's what I found. I can get the downshift clunk to go away completely if I crank the modulator adjustment 3-4 turns clockwise, i.e. harden the shifts. However when it's adjusted this way, the upshifts are brutally harsh. If I crank the modulator adjustment counter clockwise a few turns, I can get the upshifts buttery smooth, crisp and fast and with no slamming at all, perfect. But then the downshift clunk is back and is is harsh as ever. In between I can find a spot where the upshift and downshift clunks are both there and tolerable, but far from ideal.

What would this indicate? It seems the transmission is capable of shifting smoothly both up and down, just not at the same adjustment point in the vacuum modulator. And yes, I've re-verified that the modulator is getting the requisite 11-12" with my foot off the pedal, and the modulator valve holds vacuum just fine. Could this mean that somehow the modulator piston (part no. 123 277 10 31) inside the tranny is not working properly or was installed improperly when the last tech installed the K1 spring? Could a missing vacuum orifice cause this problem?

Remember, this downshift clunk problem did not appear at all until my indy installed the K1 spring kit (to resolve a moderate 2-3 flare) and did a transmission service along with modulator adjustment. Is there anything that he could have done to create this problem?

I don't think it's an internal transmission wear issue since it appeared suddenly at the same time I had transmission work done. That's what gives me hope that there is a fix.

I am getting desperate to fix this once and for all. This car is my daily driver around town and the clunking is driving me crazy, and probably prematurely wearing the rest of the driveline. Help!

P.S. I'm now pretty sure that my car does not have a vacuum amplifier, aka Blue Flying Saucer. It looks like that was deleted starting with the 1992 model. At least that mystery is solved.

sasquatchgeoff · #10 06-26-2010, 01:02 AM

I am with you in your belief that it has to do with the valve in the tranny itself. Mine has an occasional downshift clunk and my transmission is only at about 50K. After shaking down all the driveline and rear suspension components and not finding any suspect looseness, I also reasoned that it has to do with my worn modulator valve. I have not checked for vacuum leaks, but I know that it is the original modulator valve. I was very interested to read of your adjustments and subsequent transmission behavior.

Apparently, when I slow down taking more time, there is no clunk. Only when I slow down normally or relatively quickly do I get the clunk and not every time at that. Its about 70% non-clunk and 30% clunk.

I figure if I get another 75K out of this rebuilt transmission it will have paid for itself (its a warranty unit) so I will watch and (and listen) carefully. Thanks for posting your experience.

Wayfarer · #11 06-26-2010, 01:27 AM

Quote:

Originally Posted by sasquatchgeoff

when I slow down taking more time, there is no clunk. Only when I slow down normally or relatively quickly do I get the clunk and not every time at that. Its about 70% non-clunk and 30% clunk.

Same with my tranny, except that I get the clunk 100% if I slow down normally. I can avoid the clunk almost everytime if I slow down very gradually, especially from 20 mph to 15 mph. If I slow to a stop going uphill, however, it's hard to avoid a clunk no matter what I do. I have to believe that this phenomenon is not indicative of impending doom, but is fixable. Thanks for your reply.

biopete · #12 06-26-2010, 02:11 AM

Quote:

Although I haven't yet solved my downshift clunk, all this tinkering sure has taught me about my car's vacuum system. It's pretty cool really. I'd sure rather have a car with vacuum controlled everything than computer controlled everything

.

http://forums.tdiclub.com/showthread.php?t=205732

You mustnhave owned a pd Tdi from VW. Check out above thread. What a nightmare. Dealerbreplaced head trying to solve problem on car with 60000 miles about.

My bro has 05 passat wagon tdi pd with same problems as above thread abouts He replaced maf as codes were maf related and unplugging maf makes it run good Thats a big clue its the maf often No help. Last nig we check wasregate, vacuum hoses , puull things apart and look At n75 valve, replace a vac hose that almost had hep but held vacuum.

Drive it and it works. No more black smoke taking off. But it threw a code. He Take it to Indy who knows tdis apparently, he says. "your egr was unplugged. But that's not problem. I tested it. I want to replace maf and boost pipes to intercooler. 559.00"

He left and unplugged egr . Ran great. So we stumbled upon problem leaving cable unplugged. Dealer and mechanics couldnt find it All tests for egrchecked out. That's the wonderful world of VW electronics. The manual is not detaied in troubleshooting system.
Read a little of that Tdi club post when your having vac problems. Make ya feel better

Also dealer replaced turbo 6 mos ago. 2000.00 He's thinking wasn't turbo after all.

Wish I could help with your prob. Maybe he used kit from wrong car on k1 spring. I'd do it over. Also maybe wrong ATF? Dexcron iii is what I use.

Also

biopete · #13 09-25-2010, 10:36 AM

Any luck on fixing your problem wayfarer?

Wayfarer · #14 09-25-2010, 10:59 AM

Nope. I have tried two colors of modulator black & blue. I think blue is the right color but it behaved the same as black.

Adjusting the modulator, I can make the shifts REALLY hard, up and down, then as I back off the adjustment I can soften it only to the point where the shifts are reasonable, but still a little too hard. When I continue to back it off to soften it more, it has no effect. It only softens shifting so far then nothing, no matter how much further I go, it doesn't make any additional difference.

Since the problem happened suddenly when I had my indy remove the valve body to replace seals (at his recommendation), I have to believe it is not a wear issue but rather something internal. The internal modulator pressure just doesn't seem right, it's not a vacuum modulator issue. Perhaps something was not hooked back up properly, an o'ring didn't seat properly or something like that. I am not a transmission expert so I really dont know what it could be.

For now I am driving it very gingerly, slowing VERY slowly through 17 mph (the point at which the clunk happens upon slowing) to minimize the clunk, bt I know I am causing premature driveline wear by driving this way. I've taken it back to my indy a couple of times to look at the problem, another good indy here in town, and the dealership, and they all tell me the same thing: internal wear requiring a new tranny. But I have to believe that it is not wear-related since it happened when I had the tranny work done. Before that, I had no problem other than the 2-3 flare which I brought it in for in the first place. At least the 2-3 flare is gone

.

biopete · #15 09-25-2010, 11:38 AM

Did you try bowden cable adjustment? Turning in clockwise will make it downshift later. Try 2 turns. From another thread
Youve probably read

. Brian Carlton advises

Quote:
Originally Posted by ramjensen
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.

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