After you have checked to see that you have 16 to 18 inches coming out of the vacuum pump and that all of your vacuum lines and connectors are OK you may want to focus on the Vacuum control valve. You should have over 15 inches of vacuum at idle diminishing on down to about 8 as you move the rack. I say that bcause I had hard shift problems with my 300SDL and it turned out to be a faulty vacuum control valve.

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