Transmission dying?

[braverichard]

Yesterday while getting on a highway ramp in my '84 300SD, my engine reved and the transmission just refused to shift into 4th gear. I pulled over from the highway, shifted back to Park and then drove off and everything worked well.

Today while driving in town, after I took off from a stop light, transmission began to shift really late. It would rev at about 2800 rpm at 25mph and then would shift into the next gear at around 4500 rpm. Then the next time it would shift would be late as well. I drove home at 40mph with the tach showing about 2700 rpm which is too high. That meant it was in the wrong gear, though I don't know which gear it would have been in and which it should have been in. When I got into my neighborhood with a 25mph speed limit, it reved at about 2800rpm.

Anyway, I drove it home, checked the fluid level while engine was running, hot and on level ground. It was overfilled a little bit, however that can't be the cause because no one has touched that transmission for over 9,000 miles. The fluid didn't smell burnt either.

I've read the archives and I couldn't really see posts that gave me sharp answers to my situation. Is there excess vacuum? If so how did it develop, as no one has touched anything on that car for a long time? Reverse still works perfectly and transmission has just 165,228 miles on it. The car is on ramps right now. Where do I start?

[mbz240d1981]

Over long periods of time having the transmission overfilled can do damage. It can cause an excess of pressure internally and blow out internal gaskets and such and you will never know until you notice a problem with how it shifts or sounds.

Having said that, there are a multitude of other issues it could be. I would immediately try replacing the vacuum modulator on the transmission. Also, I would change the filter and fluid and make sure the level is correct. Check to make sure that the linkage on the valve cover is secure and that the kick down cable is not broken.

The downside of owning these wonderful old cars is that the Mercedes dealer will rarely have a technician still working from the 70's and 80's thus causing you more pain and suffering for paying for work that gets you no where nearer to solving your problem but only emptying your wallet.

I have had over 100 Mercedes from this era and they can do the strangest things. Please let me know what you find when you try some of these ideas.

[braverichard]

1) Linkages on valve cover seem fine, just like they did before all this started
2) Bowden cable is intact - no issue there, again just as it was prior to all this.
3) I have drained some fluid from transmission. I'll check the levels once I get the car running and then I'll go for a test drive right after doing so to see how it drives.
4) Vacuum line still runs from engine bay down to transmission modulator. Now why exactly do you advise me to change the vacuum modulator?

[braverichard]

Seems like I have an intermittent problem on my hands!

I drained the excess fluid and took the car on a test drive. I'm not trying to get my hopes up too high here, but the fluid drained was very clear and red, didn't smell funny and showed no signs of internal transmission problems. During the initial 1.1 mile drive, all shifts were late and hard. However, I reached 70mph and it shifted through all gears and into 4th eventually. Then I reversed and on my return journey, the car shifted normally like no problem existed all of a sudden. All shifts were not too soft but just a little hard as they used to be before these problems came up but again, transmission shifted through all gears. However, after stopping at a traffic light, I took off gently and then "stood" on the accelerator to kickdown into 1st gear. After that, the problem came back - late, hard shifts. In fact, the car was stuck in 1st gear for a while but shifted after hitting a very high 4500 rpm. When I got home, I checked the fluid level while the car was running, hot and on level ground and it was just perfect - full, but not over the highest mark.

So what's next?

[Brian Carlton]

Originally Posted by braverichard

So what's next?

Don, go and disconnect the kickdown switch located under the accelerator pedal. I'll bet it's sticking and delaying the shifts.

Drive it and see if the problem goes away.

[blueranger]

there is a vacuum shift controler sitting on top of the fuel pump
disconnect it and your tranny will probably work...

it controls the quality of your shift by decreasing vacuum from high to zero... when it hits zero you should be in high gear... if this thing breaks then your shift messes up..... there is a post here of a 400k mile car and the guy says they change it 2 or 3 times.... so this is a normal faliure....

[braverichard]

Brian, I did exactly as you said. Over about 1 mile of city driving and 5 miles on the highway the car was back to its usual self - the shifts were just like they normally were, the usual "clunk" I hear when I slow down to a stop (downshift into 1st) is back and everything is just like nothing happened. The kickdown switch is currently in my glovebox. I'll just have to purchase a replacement.

So what exactly happened here? How can the kickdown switch delay shifts that don't involve it - for instance, 2-3 and 3-4 shifts which don't involve usage of the kickdown switch? Is it just the fact that it is connected to the system that enables it to do so? Enlighten me please.

But for now, a massive THANK YOU to all those who have contributed here! I truly, truly, appreciate it.

[Brian Carlton]

I can't say exactly how the kickdown switch is wired to the trans, but, it has happened more than a few times that that switch sticks closed and delays 1-2 and 2-3 and sometimes there is no 3-4 because the speed is too fast (something like 70 mph).

If it went away, you found the problem.

Just replace the switch and rest easy.

Help is what we do here.

[braverichard]

And thank you Mr. Carlton for the help.

Looks like we've won this one. I've since logged 55 miles in city driving and about 180 miles in highway driving and the transmission seems to be back to its normal self. I still haven't replaced the kickdown switch, though I can obtain a new one from my local MB parts guy for $9.

I'm definitely glad that I won't be the owner of the 126 diesel with the most short lived transmission.

[patshier]

great thread. Went so far as to pick up the car from the shop before they could get to it - in a fit of over-confidence.

I had dropped the car off yesterday on the way to work, unannounced, when it quit shifting into top gear, attended with late, uber-crisp changes in the lower realms. It is getting close to the time I generally park it for the winter, anyway, and didn't care that they were not sure they could get to it soon.

So I read the thread, pick up the car after work, and unplug the kick-down switch... no joy.

So after the business trip, I will be chasing down the vacuum system. Can you be more specific on the location and possibly color of the vacuum valves I should be after? There is a green one right after the "T" up toward the top of the engine, there are also a couple of white ones - one right at the top of the fuel pump, another close by with at least three connections. One writier suggested simply disconnecting the vac to the xmission modulator as a way to confirm the xmission still operates - true?

Also, this was a sudden occurrance: driving fine, stop to drop off the lovely bride at her office, hit the on ramp and symptoms as described. Is this typical failure mode?

BTW: fluid in the middle of the marks, red, no burnt smell or taste. 140,000 on engine and transmission. Great car, though the steering is more like an approximation, and i DO miss my older 200D with the fins! THanks!

PS: I do have the vacuum tester-guage set up I bought for chasing down the door lock leaks.

[Brian Carlton]

Originally Posted by patshier

I had dropped the car off yesterday on the way to work, unannounced, when it quit shifting into top gear, attended with late, uber-crisp changes in the lower realms. It is getting close to the time I generally park it for the winter, anyway, and didn't care that they were not sure they could get to it soon.

Welcome to the forum.

In this case, there are two issues.

The first is the fact that it shifts late and won't shift into high gear. This points to the kickdown switch sticking, however, the shifts would have to be very late.

The second thing to check is the Bowden cable to the transmission. Normally, it doesn't suddenly go out of adjustment and cause late shifts, however, the more the cable is moved, when the throttle is opened, the later the shift becomes. So, if it's adjustment is off, and I can't immediately determine why it would do this suddenly, the symptoms could be as described. I'm presuming that this vehicle has a Bowden cable, however, I'm not that familiar with the 240D.

The second issue is the hard shifting. This could be caused by a vacuum leak. However, if the hard shifting is also related to the late shifting, curing a vacuum leak won't fix this issue. I have a suspicion that the two events are related and the problem is not a vacuum leak.

However, you can check for a vacuum leak by simply monitoring the vacuum to the transmission when you drive it. Just run a "T" into the line and run the gauge into the cabin. If the vacuum is not very high (about 20") at low or no pedal, then you must find the leak and correct it.

BTW, when was the last time the fluid and filter were changed in the tranny?
It's probably not the problem, but, it can't be ruled out.

[blueranger]

on top of the fuel pump is a vacuum control for the tranny.
it controls the quality of the shift... it goes from high preasure down
to zero and by zero you should be in high gear... this thing gets old and
weak... if it is not getting all the way to low preasure then that is your
shift problem... you can hook a preasure guage up to it or you can just
disconnect it....

i disconnected mine and have been driving without it... you just shift through all your gears quickly instead of them being drawan out.

[braverichard]

"Quality of the shift"

Does this refer to the firmness / softness of the shift?

[patshier]

Thanks for the replies. Tried some preliminary checks before I could get back to this site and discovered that the vacuum is over 22" at the "T". Tried disconnecting the vacuum line that disapears behind the engine down toward the tranny - no effect.

With the info supplied, I will press on. Just moved, can't put my hands on the service manual CD with the schematic for the vac. lines, and the garage is a wreck, so it might be a few days before I report back. Since the monsoons have started in beautiful SE AK, I'm not inclined to try it out in the driveway!

[Brian Carlton]

Originally Posted by patshier

Thanks for the replies. Tried some preliminary checks before I could get back to this site and discovered that the vacuum is over 22" at the "T". Tried disconnecting the vacuum line that disapears behind the engine down toward the tranny - no effect.

Which "T"??

The check needs to be made in the line that disappears down behind the engine toward the tranny. Find the upper end, where it connects and T into it. Then take the vehicle for a drive and watch the gauge while you drive it.

Report back.