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Old 06-09-2003, 01:33 PM
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Join Date: Jun 2001
Posts: 150
Transmission questions

I have a 1991 300D with the usual (from what I have read) shifting problems. Harsh shifts at slow speeds and somewhat harsh at higher acceleration. I understand that the vacuum and cable to the throttle linkage control this but I would like to know the exact function and how these work together to make the tranny shift properly.
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Old 06-09-2003, 01:37 PM
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Hi bud
Im very familiar and sick of these MB automatic transmissions
they just take alot of juice from these cars

So ill explain to u how it basically works
The bowden cable that you see linked to your throttle
linkage is used for the transmission to determine at which
points to shift E.G. ( shift from 2nd to 3rd at 80 km/h )
Now what controls the smoothness of your shifts is the modulator
which is vacuum controlled.
What you can do to help control the smoothness of the shift is to turn the ajusting T handle on the modulator ( usually this handle is under a rubber black cap) turn it just 1/8 of a turn anti clock wise or clock wise ( i dont remember) and try the car. Make sure u dont make shifts too soft as u will make the transmission slip and thus wear ur bands and clutches.

Good luck
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Old 06-09-2003, 02:35 PM
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Join Date: Jun 2001
Posts: 150
It looks like the vacuum line from the transmission also is effected by the injection pump ( unless I followed the wrong line)

I guess all the I can do is check for leaks and make adjustments.

If these cars weren't so darn nice they wouldn't be worth the hassles. Guess there's no such thing a free lunch.
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Old 06-09-2003, 02:43 PM
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Join Date: Jan 2002
Location: Colorado Springs, CO
Posts: 1,006
There are a number of possibliites, some are harder to check than others. To set the modulator by the book requires hooking up a fluid pressure guage but many are quick to make adjustments.

On the injection pump is the vacuum control valve (VCV). Tthe black line is fresh air vent (like the transducers). There is another line, with a green filter on it. This should read 385 mbar +/- 25 mbar (~10-15 inHg) (engine off) and zero out at full throttle (engine off). When you let off full throttle it should go back to 385 mbar +/- 25 mbar. If that is not working, then that assembly needs to be replaced ($70-80).
Brian Toscano
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Old 06-10-2003, 08:38 AM
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Join Date: Jun 2001
Posts: 150
I bought a mityvac and started checking things. The vacuum modulator worked fine. In checking it out I found the problem. I noticed that the hose came off very easily.....found the leak !!!!!

Thanks for the help. The car shifts like silk now
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