First let me say I've had this problem since I've owned the car (will be a year this spring), I have looked into it/troubleshot it multiple times, done extensive reading on here.

1984 300SD 126

Here are my symptoms:
1-I only get heat on defrost mode.
2-Other buttons blow air, but no heat.
3-A/C has not worked since I have owned it (have not looked into this issue at all, there is a R134a sticker from 2001, but not sure if they replaced anything or just took out R12 and added R134a without replacing compressor, etc.)
4-Defrost vents always stay open.
5-A symptom that is no longer occuring that used to was the defrost would kick on regardless of any buttons depressed or not when the RPMs would get around 3000 or higher (acceleration or freeway speeds). This doen'st occur anymore. I think it stopped occuring when I unplugged the Aux water pump, but don't remember for sure.
6-I believe (although am not positive) I am getting air coming through the system at all times. When I close the center vents (for fresh air I believe) with the sliding tabs it seems the air is diverted to the defrost vents.

Here is what I have done so far.
1-Checked into the monovalve. Ordered the rebuild and installed it, no change. Since it is winter and my heat is marginal, I just keep it unplugged for full heat.
2-Took out the CCU and tried to resolder it. I fried a trace. I believe something jumped around the 9 pin and it shorted out and then did not work at all. Ordered a rebuilt one.
3-Installed rebuilt CCU. At the same time, checked Aux Water Pump and found drawing too many amps. Unplugged it for awhile, but recently took it out and put an elbow in there.
4-Footwell vents were not opening so I wedged a beer tab in the flap to keep it open permantly. (Although now that I think about it, they shouldn't have been opening on defrost, maybe they were opening on other functions, not sure)
5-Took a Mity Vac to the switchover valves in the dash. Pulled vacuum and see if it held. The first time I tried it the top one and third from the top did not hold, all others did. Went back and tried them all a second time and this time it seemed they all held vacuum. I ran out of time and stopped. So I don't know if I simply wasn't getting a good seal the first time or what. I'll check into this again tonight. I think I was doing it correctly, but started second guessing myself when the results changed.


I get heat when the car warms up and running on defrost, but I am certain it could be hotter and I really want to get it functioning like it should as I plan on getting the air up and running this summer but want to make sure the controls are functioning as they should before I get into putting new components into the a/c system.

Lately I have been reading up on the dashpods. I will need to retest them. I plan on doing that tonight or tomorrow night to see if they hold vacuum. If they don't hold I will order new pods and rebuild?? (suggestions here appreciated)

So what do you think? I know I wrote a lot but I wanted to be sure I covered as many symptoms and covered as much of what I've done as possible. Is there anything else I should look into?

Thanks in advance everyone!
-Ryan