automatic transmission

[carman57]

Just wondering if any other owners with auto trans. experience an abrupt and noisy final downshift.  Almost sounds like a "clunk" in the rear.  Is this characteristic of the car?  A mechanic recently suggested that the idle may be too high, but haven't yet had that changed.
The car only as 57k, so I'm hoping it isn't ready for a rebuild, and other than this downshift problem, works beautifully.
Any comments on this would be appreciated.

[ja17]

Hello carman57,

The most common and first item to check is the small slotted stop screw with lock nut on the intake venturi.  This is factory set and is mistakenly used as an idle adjustment screw frequently.

This stop screw must be set so that the intake linkage arm and venturi valve in the intake closes all the way! If not the switch controlling automatic transmission modulator pressure will not function properly. This switch lowers modulator pressure during downshift, thus smoothing downshifts. This switch is attached to the opposite side of the venturi.

This will make an amazing difference when set up correctly. The set screw must be backed off enough to allow the venturi valve to close all the way and just enough to keep it form binding in the intake. If you have the vacuum dash pot on your intake it must be moved out of the way so it does not interfere before setting the stop screw.

Adjustments of the other linkage and tune-up items can follow afterwards if the idle is still too high. Keep us up to date.

Download Attachment: venturi dashpot and stop screw illustrated.JPG
64.74 KB

Joe Alexander
Blacklick, Ohio

[dldubois]

I'll tell you that my car has been in my family for 25 years.  I just purchased from my father and had the same issue.  I brought it to my mechanic I've used for years and he adjusted the idle on two separate occasions but could not eliminate.   I finally ended up taking to the MB dealership and was able to get eliminated.  I know they had to make a linkage adjustment on mine which is accessed from the floor in the passenger footwell.   It purrs now though.

Dana DuBois

1969 280sl auto
blue (906)
Corinth, TX

[TheEngineer]

And check for operation of the switch using an ohm meter. When things are properly adjusted, you will not be able to feel downshifts even if you try very hard. They are sooo smooth, they are undetectable! My transmission only shifts with a detectable roughness on the upshift from 2 - 3 and never when it is cold. Only sometimes after prolonged driving and only when I drive very gentle. If I accelerate substantially at all, the upshift is always smooth. And my transmission is original with at over 100,000 miles on it. Never been touched, except oil & filter change. These transmissions do not wear in normal use because there is almost no slip during shifting. Modern transmissions shift smoother because they are permitted to slip during engagement. The trade-off is wear. But most people keep a car only 3 years before trading it in, so wearing out the transmission is not an issue.