Today, I encountered a transmission problem on my 300SDL. When I shift to D, it engages and upshifts normally up to 3rd gear. When it's about time to shift to 4th gear, it suddenly disengages and the rpm just suddenly shoots up as if it's in neutral position. I have to put the shifter to S position for it to engage but that is only up to 3rd gear. There is no problem downshifting and reverse is working fine.

I already checked the the fluid level and i added 1 quart but still the same problem exists. Transmission filter and fluid was changed a a year and a half ago.

What could be the problem?

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You've described a "flare".

On the OM603, the chances are good that the VCV is not properly adjusted. For some reason, it will flare badly if it's not set by the factory procedure.

Do a search on Dave's posts (Gsxr). He's described it several times.

Thanks Brian.

I just read threads about flaring but "flaring" seems to be a temporary slide/rev between shifts. My transmission totally disengages when it shifts to 4th gear.

When I put it to D it shifts fine up to 3rd and just revs to high rpm when it shifts to 4th. 4th gear does not engage and all you hear suddenly is the high engine rev as if it's in neutral. I have to shift to S to keep up to speed. When I again shift to D at 50 mph it just revs to high rpm as if I'm in neutral. It seems like just I lost my 4th gear.

I just talked to my mechanic on the phone and says it could be a clutch disk problem. I'm hoping it's just a vacuum problem.

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It sounds like you have troubles with the hydraulics of your transmission or worn out band. This problem will happen either when the friction material on clutch bands are wearing out or when you losing or dont have enough hydraulic pressure to engage and clamp the bands.

You are either looking at rebuild or having someone rebuild the transmission. Those transmissions are very hard to work on and most places wont touch them.

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)

If it will not shift to fourth when you lift off the pedal, then you're correct..............the problem is not vacuum related and you don't have the traditional "flare".

However, just to be sure............disconnect all vacuum to the transmission and take it for a drive. If the problem remains, you have deeper issues with the trans, unfortunately.

I brought it to my mechanic this morning and i asked him to detach all vacuum connections to the transmision so we can trouble shoot. Shifted to all gears and everything is fine. He then traced the problem with a vacuum leak somewhere along the turbo system and fixed it.

Everything is working well again. Thanks to all those who replied.

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Sounds like the clutch plates are worn or getting worn.
How many miles on the car? I had to rebuild my trans at 190K

The problem with diagnosing the issue as a vacuum leak is the fact that disconnecting the vacuum from the modulator gives you the same exact situation. If the leak prevented vacuum from reaching the modulator, then disconnecting the vacuum line to the modulator cannot solve the problem.

While it's great that the problem is solved............it was not caused by a vacuum leak.