300TD shifting problems

[rrrs]

Hi, I got a really strange, temperature related shifting problem on my '87 300TD Wagon. It started few months back and the car would not shift into 4th gear, but only once it has wormed up; for the first half hour or so it all works perfect and then, once wormed up (not overheated) it stops shifting to 4th and it even once downshifted into 3rd while just cruising on the highway!
Then, again, next day in the morning when car is cold it all works fine.
I did change fluid and filter and it seemed to have helped; it all worked for about 2 weeks but then it got back.
I did disconnect kick-down switch, but this did not help either.
I did visual inspection of the vacuum lines but could not find any obviously broken.
Now, just recently, the car stopped to turn of with the key; I need to open hood and manually stop the pump.
Could someone please help on how to diagnose what is really going on; are there any steps to follow?
Was looking around but could not really find related thread.
I went to couple of local transmission shops, but non wanted to touch my car; they said that they could spend tons of time and not find a problem "because it is Marcedes and on top of that it is diesel" !

[CarpeDiem51392]

Please be considerate and proof read before you post.

Car not shutting off with the key means you have a vacuum leak, try eliminating everything except vacuum to the tranny.

Nothing external to the transmission should stop it from going into 4th, how many miles?

[rrrs]

Sorry for quick post!

There are only 100K miles on the car; I bought it few years ago form the original owner and all (accept AC) worked great till recent transmission problems.

I did do all maintenance including transmission fluid and filter change.

What is really controlling when the car shifts on this model; is there any electronic control box that could fail and cause this problems?

As for the shut-of valve, is there a simple way to test if it is a vacuum leak or maybe the actual valve that needs to be replaced?

[Maxbumpo]

Two different issues (engine shut-off and transmission shifting).

Recommend you start a new thread for the shut-off valve issue. Very simple test: attach a rubber line to the shut-off valve, suck on the other end with your mouth, and you should easily be able to pull the "stop" lever from the up to the down (stop) position. If that fails, the valve needs to be replaced.

Transmission shifting: Have you checked the fluid level? Shift point is controlled by the bowden cable, shift quality (harsh to soft) is controlled by vacuum modulator. Downshifting is controlled by kick-down switch under the accelerator pedal via the KLIMA relay. Research all of these here on this forum and other interweb locations, ask here if you don't understand what you read, and start testing components to eliminate potential problems. I think this is something that can be fixed without removing or replacing the transmission.

[pmckechnie]

Also check the shifter bushings. Mainly the ones on each end of the rod from the shifter to the transmission, under the car.

PaulM

[rrrs]

Thanks for suggestions, will go ahead and try to find threads on how to check for bowden cable and vacuum modulator.

I do keep checking fluid level, and, although I find it really a bit confusing to read the dipstick it seems fine (with car worm and running in Park position my level is between the marks on the lower side of the stick; other side of the stick seem to get totally covered in fluid).

When I first bought the car few years ago I got a local shop do a full service including transmission fluid ad filter change. It seems that they did not get the pan to seal really good as after 6 months or so the car started to shift strange, especially going slow uphill between 1st and 2nd gear.
After checking, the fluid was very low (below the dipstick); I then filed it up and transmission started shifting fine again.
After that I kept watching and filling fluid until I did myself another fluid/filter change and got the pan to sill fine (there are now just a few drops that leak a day, as was not able to seal it completely).

Is it possible that transmission got damaged in that period when was used with very low fluid level?
It would be strange as it worked fine for 6 months after that; than it stopped shifting into fourth and sometimes it even refuses to go from second to third.

I did replace all shifting bushings when I got the car as the lever was loose, so this should not be a problem.

As suggested, I will try looking into posts on how to check Bowden cable and vacuum modulator, but would really appreciate if someone could post some related links!

[Maxbumpo]

Neither the bowden cable nor the vacuum modulator are likely to be your issue. An easy/cheap place to start is the kick-down switch, which can introduce all sorts of weird behavior.

After that, I'd try a fluid/filter change, including draining the torque converter.

After that, I'd consult some experts. Maybe the valve body is messing up?

[rrrs]

I already disconnected kick-down switch and this did not help; is this test enough to prove that Kick-down is not one causing the issue?

I will go ahead and do another complete fluid/filter change; last time I did the change it was shifting fine for about one month but then the problem got back....

Is there any sense in trying adding Lucas Transmission oil additive, or can that actually do any harm?

[rrrs]

I did another fluid change, this time I did drain the converter (did not do this first time), but left filter unchanged this time.
I got recommended at Napa to use Valvoline Max Life full synthetic fluid instead of Dextron III I used last time.
Now the car is shifting even much worse; it is not even working well when cold (before the issue would only come up once the car would get hot; about 20-30 min driving).

I really do not know what to do as a next step; local transmission shop (which has a very good reputation) does not want to work on Mercedes and there is really no other place to got to on the island.
I am at the point of selling the car with bad transmission for a very low price, but am really sad to do so as it is very low mileage (just over 100K) and engine, brakes, suspension as well as all other parts are really in excellent condition.

I would really appreciate if someone could point me to some links on how
to possible diagnose my problem.

Thanks a lot!