transmission shifts late....w126 - PeachParts Mercedes-Benz Forum

mbtjc · #1 03-24-2004, 11:07 AM

I did a search on this topic and didn't get much. My transmission is acting odd. It starts in first gear and makes a whining noise when you first start (like when you are backing up), and then waits till about 2500-3000RPM to shift into 2nd under light load. Same with the 2-3 upshift. Then, it will not shift into 4th unless I am doing about 50-60mph on the highway. Even then, I have to stab the gas pedal very quickly for it to shift into 4th. So when I am in town, it stays in 3rd. When in 4th on the hwy however, it's fine, it goes between 3rd and 4th just fine. No slipping from what I can tell.

Fluid and filter changed maybe 10,000 miles ago, and fluid level on the mark. Car has 180,000 miles on the clock

Thanks for any help.

haasman · #2 03-24-2004, 12:19 PM

If it were my car I would check the vacuum line going to the vacuum modulator, the rubber hose that connects and also check to make sure the modulator on the side of the transmission is in good shape.

Haasman

mbtjc · #3 03-24-2004, 03:24 PM

Haasman, a friend and I put the car up on a lift and the line was attached, we pulled it, and it was getting 20 inches of vacuum at idle. Of course, this doesn't check the modulator itself, but it is getting vacuum. I guess it could be a bad modulator, but is there some test to do to confirm this? I don't want to end up just throwing parts at it. Thanks!

mbtjc · #4 03-24-2004, 06:55 PM

One thing I forgot to mention. The problem used to be intermittent. Maybe a week or so it went away for a day and then it was back. Or before, sometimes if I brought it up to 60 and shifted back and forth a few times between 3rd and 4th it would go away. But now it looks like it's here to stay.

C32AMG · #5 03-24-2004, 08:34 PM

On 722.3, 4 and 5 transmissions, the vacuum modulator is the load input to the transmission, it raises and lowers working pressure (line pressure) Only; controlling shift feel, it does not control shift timing that is the function of the Bowden (TV) cable; early or late shifts. That is why if you disconnect the vacuum line to the modulator the transmission will only shift harder not later.

The whining noise could be a plugged filer, metal to metal wear, planetary whine or torque converter whine; turbine fins use a mechanics stethoscope and try to isolate the whine.

Lates shifts can be an out of adjustment or sticking Bowden cable. Also check for a stuck closed kick down switch under the gas pedal. Or the valve body is bad. Assuming the Trans is good.

Theurig · #6 03-24-2004, 08:38 PM

when I had some work done on my car. (intake manifold was R & R'd) my transmission was shifting late just like you in your case, 2500-3000 for 1-2 shift, anyway went to my mechanic the next day, and he apologized, some cable that attaches to the trans was too tight hence the late shift. ( He has a real good reputation, but began to kind of do stuff in a hurry, because of higher volume.) anyway trans shifts perfect now. Do a search on late shift, I had read a similar problem somewhere some time back.

mbtjc · #7 03-24-2004, 10:12 PM

Thanks guys for the replys. If it is in fact the Bowden cable (or whatever it is for that matter) why would it be intermittent? And how does this cable go out of adjustment? Also, is checking/adjusting the Bowden cable at all a DIY job or should I leave this to the pros? I'd say I'm an intermediate DIYer, I did my t-chain. Thanks again!

haasman · #8 03-24-2004, 10:21 PM

This site might help you adjust the cable and yes you can do it yourself. It sometimes takes a bit of adjusting to get it right, but well worth it.

Also, from this site is another member's comments:

"Transmission bowden cable adjustment

These transmissions do have the habit of skipping through the intermediate gears darn quickly. My '87 300TD does this to a degree every now and again.

I'd start by adjusting the bowden cable. It sounds as if it has stretched a bit. The cable should be just tight with the throttle closed - no slack, but no real tension either. The bowden cable controls how long the transmission holds each gear. Opening the throttle more pulls on the bowden cable, telling the transmission to hold each gear longer. Too much slack in the cable and you get early upshifts - the transmission doesn't think you have your foot on the gas so much.

Driving style can also cause these stacked shifts. If toe way into the throttle to get started from rest then back off as the turbo spools up you are encouraging the transmission to shift up. Less throttle = less demand for acceleration = time to shift into a higher gear.

The vacuum modulator adjusts shift quality - quick and firm vs. soft and sloppy. It doesn't affect shift timing. Assuming fluid levels are correct, it sounds as if a bit more firmness might be in order."

Haasman

mbtjc · #9 03-25-2004, 09:45 AM

Awesome! Since the weather looks decent today and I've got some time, I may try that today. I can't tell from the pics on that website and it doesn't say, so before I spend 30 minutes hunting, where is this cable exactly located?

Also, why is my tranny starting in first gear?

I'm still wondering what causes this cable to go out of adjustment why this was intermittent.......

Well anyway....I'll see what I can do. Thanks again!

mbtjc · #10 03-25-2004, 02:51 PM

Ok found the cable fine, easier than I thought. From where it was at, it already had some slack in it. So it most def. was not too tight. So I thought what the hell, I loosened it a little (maybe took the slide out 1/4 inch or so. Took it out for a drive. Yes, it shifts better now from the 1-2 under light load, but that's it. It still starts in 1st, it still doesn't want to shift into 4th unless I get up to hwy speed. When I slow down to a stop, you can hear and feel it go to first. What would be causing that? Also, if I am on the hwy, and go into 4th, if then I slow down to 40mph or so, it stays in 4th, and shifts between 3rd and 4th just fine. It just won't upshift into 4th when coming from a slower speed. Also, now with the cable moved, it shifts too soon (surprise surprise) under hard acceleration. So I don't think the cable did anything except mask a problem under certain conditions (light load). So I don't think it was the cable adjustment, it was already kinda slack. I think I'll move it back.

As far as the whine....it only happens when first taking off in 1st, just like the noise like when backing up in reverse (if you go kinda fast in R you can hear it). So I am guessing that noise is planetary or torque converter noise because of the lower gear ratio in the tranny.

I guess if it's not the cable...it may be time for a shop to look at it. Any ideas.......??

mbtjc · #11 04-15-2004, 04:09 PM

Everything is still as described before. Some more things: When in 4th and at non-hwy speeds, depressing the gas to speed up, it should just shift down to 3rd so you can gradually speed up, only downshifting to 2nd if you hit the kickdown switch. Now, if you depress it w/o hitting the kickdown, it abruptly downshifts to 2nd. It only downshifts to 3rd if you barely press on it or at hwy speeds, but then hitting the kickdown switch does nothing. So driving is a bit erratic in the city and passing power is gone on the hwy.

Took it into an indy (who by the way put green coolant in with my MB coolant, so needless to say I won't be going back there)....well he "wasn't sure" what was wrong with the tranny. He suggested "trying" a tranny service, but at his $130 asking price and not knowing if it was actually going to to anything, I passed. So I just did it today myself. Start with the cheapest thing. New MB filter, new fluid. Drove it.....no change. None. I am thinking about using Auto-Rx, but with the chances of it doing anything being very uncertain, the $25 bottle sounds mighty expensive. The indy suggested sending off the valve body and having it rebuilt at a tune of $600. Not knowing that that is actually it, kind of a lot of money to swallow.

The actual shifts seem fine, no slipping. The first shift may be a bit slow, but again no slipping. The pan had just traces of fine metal shavings in it, barely noticible. No chunks or piles of anything. Also, the bowden cable has slack in it, so if anything that should cause it to shift early, not later.

Needless to say I am getting very aggravated. What was once a car that was a dream to drive is now the opposite. The engine runs like a gem, a tranny that runs the same would be nice.

Anyone?

WANT '71 280SEL · #12 04-15-2004, 08:49 PM

Did yours have the red bellow thing around it? Mine does... Can I just pry that bellow thing out of the way to somehow adjust the cable?
Thanks
David

P.S.
I am not trying to hijack your thread or anything, just curious.

Q · #13 04-15-2004, 09:03 PM

Do not take this as a recommendation to check, just a curiosity question. Do kick-down switches ever fail closed? Like it is always depressed? Would that cause the problem he describes?

joe p · #14 04-15-2004, 09:11 PM

Quote:

Originally posted by Q
Do not take this as a recommendation to check, just a curiosity question. Do kick-down switches ever fail closed? Like it is always depressed? Would that cause the problem he describes?

I was reading through this and thought the same thing.

Unplug either the kickdown switch or the kickdown servo on the tranny. (r/h side of the tail housing, single black rubber cover wire).

Joe