Have you no heat on both sides, driver and passenger ? Remember they have individual ducts and controls. On my W140 I get heat only out of one side..Its more of a duo-valve than a monovalve, but I think I have to replace the whole thing, I dont think you can replace just the bad half. Hold your hand over both side of defroster ducts and check to see if both sides are bad. These monovalves are expensive, around $453 if I remember. If both sides are cold maybe it is something else.