Weren't 1977 MB's 123's and not 115's? My 1977 123 300d had a transmission that was controlled by a series of rods that went back to the right side of the transmission from the throttle linkage down thru the intake manifold area. Not sure if the 78's had that same rod linkage or not but my impression is that there has to be some mechanical connection similar to the Bowden cable which controls the shift points. Vacuum controlled the harshness/softness of shifts on later 123's but the shift points were controlled by the mechanical bowden cable.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13