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On a 722.6 5 speed gearbox the speed sensor issue would usually mean the conductor plate mounted above the valve body, which can be replaced seperatly
On a 722.9 7 speed gearbox it usually means the entire electrohydraulic valve body. Benz won't even let their dealers work on the 7G valve body. It has to be replaced complete from Germany. Benz knows they had speed sensor trouble on early 722.9 transmissions. I would try approaching MBUK for a part payment at minimum.
as for parts try
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