How to bypass mono-valve..
 

I have a 79 300sd The heater stopped working the fan still works no heat though. The mono-valve was replaced it looks like. Its a nice shiny aluminum one. There's a sticker on it says lifetime warranty on it. I bought the car about 6 months ago from original owner. I was hoping someone could point me in the right direction to getting the heat working..

Thanks

That is the servo unit not monovalve. The aluminum ones are all made by a company whos names I cant remember but I am sure someone will chime in. Have you done the tests per the FSM? Those will easily diagnose the problem, unlike the later models with the Monovalve.

No FSM tests, what some that I can try out?

Yes go buy the FSM from MBUSA on cd for $20 and read the section on climate control.

On my 84 I had just taken asome copper pipe and soldered an "L" shape, just so that it would fit in place of the monovalve. I used a rasp to make the ends rough so the hose wouldnt slip off and used some hose clamps to hold it in, and it worked fine. Basicaly what your doing is making the heater on HOT mode all the time, and the only way to modulate heater temp is to turn the fan down or up. You just wont get the functionality of the blue-white-red dial. Im not sure if your ac will work with this mod, my ac didnt work anyway. But it will be good to get some hot air flowing until you can afford a ripoff repair kit