Automatic Trans Underpressure Regulator

[JimVillers]

With the transmission still out, I am curious on what the under-pressure regulator does.  It is connected to manifold vacuum.

In newer cars, the vacuum system smooths the  shifting.  Gary Tyre has complained about very rough shifting and I just want to make sure if there is something to "fix" or check whilethe transmission is out that we do it.  Any other thoughts about rough shifting?

[69280sl]

Jim

I drove 2 auto trans 280sl's before I bought mine. The shifts were very hard. I was told that was "normal for these cars". Mine also shifted very hard when I bought it. After having the trans overhauled by an older trans specialist, shifts are smooth as silk and barely different from a newer vintage car.

I don't know specifically what was done, but it can be done/

Gus

[J. Huber]

Hi Jim. You may already know this but hard shifting can also be a result of the fuel linkage being off.

[bpossel]

Jim,

You can adjust the modulator pressure to improve shift quality.
Check out this section of the manual...
http://www.sl113.org/wiki/Automatic/Start#VacuumModulator
Bob

[glenn]

Jim, There has been a lot of 'chatter' about the problem, but no real accurate, definitive description of how it's s'posed to work.  double action solenoid, vacuum, tranny, wiring/piping diagram, rpm settings, etc. ..  Got two 230SL trannies sitting on the floor ready for ??  One is out of a 250SE 4 door and the other is out of a 230SL.

[ja17]

Hello Jim,

While the transmssion is out, make sure that the rubber diaphram in the modulator is good. Check to make sure the three position solenoid and its linkages operate freely.

Hard shifting is usually the result of the engine linkages being off and causing the switch on the venturi to be incorrectly adjusted. The other probable cause is the linkages on the transmission are stuck from rust.

Now if someone has wrongly adjusted the modulator pressure to solve any of the above problems, then you will need to go through the modulator pressure adjustment procedure to get everything back on track.

Lastly, if engine ignition timing is drastically off, engine vacuum will be drastically altered which in turn will change the shifting characteristics of the transmission.

[JimVillers]

Thanks for all of the advice.  I'll check the modulator diaphragm.  I have checked the solenoid and it moves in both directions, solidly on the up shift and a little more gently on the down shift side bur it moves in both directions.   It would not surprise me to have the linkages out and the idle switch not to be activated.  When Gary brought the car down here, it ran so poorly and so rough that fixing the engine was "Job One".  I just do not want to "miss and easy" while everything is out.