My 300SDL was shifting hard so I checked the line from the (blue) vacuum amplifier to the transmission and found that the vacuum modulator (on the transmission) had a leak.

Figuring that that was my problem, I ordered and installed a new vacuum modulator on the transmission. This one does not leak but now that it is installed the car shifts much harder than before.

I checked the vacuum supply tube on the input of the vacuum amplifier and it has 22" of vacuum. The line from the vacuum amplifier to the vacuum modulator on the transmission has only 8" of vacuum instead of the 16" to 19 that I have read it is supposed to have. My guess is that the transmission shifts harder now because the new modulator is much stiffer than the old one which probably moved more in spite of the leak.

So my problem is that I am not getting enough vacuum from the vacuum amplifier to the modulator for smooth shifts. My question is... how do I troubleshoot it? I can't find any leaks so I either have a bad vacuum amplifier or a bad vacuum control valve or some sort of electrical problem.

Has anybody been through this?

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Tom Hughes
St. Louis
84 300SD
92 300D
86 300SDL