I have a 1983 300D with a turbo, it has 190,000 I am the second owner and have owned the car for two years. The Problem; I have no he while driving, the ac works fine so that's leads me to believe that my control module is working. I put in a new mono valve, the aux water pump is working. If the car is at idle the heat blows warm in the car. when I start driving the heat goes from warm to cool. Has anyone have any suggestions of what I might do next.

 

Mine is doing the same thing, and have the mono valve unplugged.

 

Got mine fixed, had to switch the mono valve from the old valve body, now everything works good. The new used one I bought had a good coil, but bad mono valve, Now it works fine.

 

OK, I have been away for awhile, My heat will work if I turn it on while the car is sitting still, it will blow heat continuously as I am driving. If I turn it off and then turn it back on, nothing but cool air comes out. If I stop the car, and turn it back on it will blow hot air again. I have changed the thermostat and he mono valve. Are there any other suggestions?

 

What about the electric water pump in the heater hose, is it stuck and restricting flow? just a thought.

 

That's a good thought but it pumps and the heats works good as long as I don't turn the heat off while I am driving. If I turn it off while iam driving and turn it back on I only get cold air. As long as I don't turn the heat off it will blow hot air for as long as I am driving. Thanks for the idea

 

mine would blow heat when at idle, but when on road would start blowing cold. I think my Mono valve was about stuck closed, letting enough hot water to do heat, but not enough to keep the heater fan from cooing it down real fast. After changing the Mono valve all is well. Now on to putting the rubber bumper strip back on bumper. Know this is going be fun!!