If Frank’s tests show the switch is good, it could be that the mechanism has too much drag in one direction and that heavy current draw is pulling the voltage down. I sort of remember the 300SD having an issue where the motor was loose. When it twisted in the housing, it would bind and draw high current and run sluggishly if at all. In the other direction, it ran much better but no perfect.

Michael

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Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120