I searched the forum but didn't see any topic on this.

My transmission tends to rev up several hundred RPM before shifting intothe next gear. This typically happens when I first start up and goes away (most of the time) after 15 minutes of cruising. I have a '83 Mercedes 500SE that does the same thing (the mileage is about the same as the Porsche at 108K).

Anyone else have the same symptom? Is this an indication that something may be going wrong with the AT, or is it just a MB transmission "thing"?

Any assistance would be helpful.

 

Hmm...this is very similar to what my car did last year. Mine would "slip"on the shift into 3rd gear...engine would rev for a bit before the car and transmission caught up to one another. This quickly degenerated into no-go in 3rd at all. Luckily, I caught things early, had the transmission rebuilt by a local shop familiar with MB units. I had a broken clutch band in 3rd gear...plus a number of other problems. Gave me the opportunity to do the torque tube bearings (which were very noisy) at the same time!

You can pick up on the thread here: https://rennlist.com/forums/showthre...threadid=38252

Looking back after the fact, I recall the car would shift slower when cold and would often "rev"a bit between the 2-3 shift until it warmed up. I just thought it was normal until it started happening all the time. My essentially new transmission now shifts as smooth as silk when driven camly, but becomes a no-nonsense unit when power is applied...just, I suspect, as it was intended to do.

I got lots of great advise from the forum, so I would suggest you start with the basics and work your way up:
1. Check the fluid level in the transmission...make sure it is neither too low or too high (check the manual for detailson how to check this).
2. If you have not done so recently, you should pull the transmission base pan and check the filter and condition of the fluid. Good time to change both.
3. Check the working pressure of the transmission...if it is not correct (it can be adjusted), then it will not work correctly. Working pressure effects the firmness of the shifts (hard or soft).
4. Check the tension on the shift cables. These should be the last thing checked...working pressure has to be set first. Cable tension effects the shift point (early or late).

If all this checks out, there are a number of things that could effect the slipping or reving between gears. Probably time to consult with a local MB transmission guru for a more rigorous diagnosis.

Regards,
SteveCo in St. John's

 

Automatic transmissions do not tolerate slippage , they are designed to be in or out of a "gear" but the brake bands planetary drums are quickly damaged if allowed to slip "between " gears . They operate off hydraulic pressure from an internal oil pump but something must be failing for them to slip !

 

Thanks, Steve and Jim... anyone know how much a transmission rebuild go for?

 

Does the transmission slip between all the up shifts or only between 2nd and 3rd?


Possibilities;

Oil level low. Check transmission oil hot car level in park.fluild level should be between the 0 max and 0min on the fill tank

Worn valve body

Low oil pressure.

 

Steve Cattaneo is the guru on 928 transmissions...you are in great hands with him...I'm sure he can help you figure out what is wrong and how best to fix it.

I'm sure 928 Intl can fix you up with a rebuilt transmission. Shipping costs made this impractical for me...plus my transmission was an excellent candidate for a local rebuild.

Regards,
SteveCo

 

It's good to know that I have experts here to help. Steve C, at leisurely acceleration, the transmission tends to rev-up on all gears. During medium acceleration, typically the rev-up is between 3 - 4. In hard acceleration, no rev-up, just good ol' hard shifts that keep me glued to the back of my seat! During the last oil change I did notice a little leak on the transmission... nothing that left a pool of fluids on the garage door. I will check the fluid level first.

 

pdfoot,


If you have a slip in all the gears. You have a oil pressure problem, the right way to check working pressure is with an oil gauge
Connect a vacuum gauge to the disconnect vacuum line at the modulator, with the engine idling in park, normal vacuum readings would be 14 in to 22 in HG, if not you have a problem with the vacuum line going to the engine. Quickly open and close the throttle, the vacuum should drop to below 5 in, then rebound to your highest vacuum reading, 14” to 22”if it does not you have a restriction in the vacuum line


You should change the modulator, the diaphragm leaks with age. M. B dealer, tell them the color of the modulator... Make the TV cable on the linkage tighter.

 

That leaking you're getting is a lot of times the front pump and its just an o-ring (from what I remember). All the MB trannies of this type do it. I'd put my money on the valve body as the problem of the slippage, and if you're going to have it repaired, have the 1st gear start adjustment done too.