300SD heater problem
 

Car = 1985 300SD. I have read about this exact problem on this forum but have not found the solution. When parked, the heat works fine. When driving, the air turns cool. Putting on the defrost blows the same cool air. I think it is not the mono-valve which is supposed to be bypassed in defrost mode. It is repeatable. Suggestions?

classic monovalve symptoms. super easy to replace.