I got the AC working in my car about a month ago by replacing the monovalve with one I pulled from a junkyard. At the time I was only getting hot air, and after the repair I was getting every temperature that I selected on the wheel and air was blowing through all the vents with the exception of the center vents from time to time. After reading a lot about this HVAC system, I realized that I'd be lucky if this system would be perfect for more than a month.
About two weeks ago I wasn't getting heat on the economy setting, but the blower was blowing fresh, cold air. I pressed defrost and got heat, then went back to economy and got heat again. This scenario happened a couple of times until today when I went to hit defrost I didn't get any heat. After I let the car sit for a few hours, I went out again and had heat in the economy setting. I stopped to fill up with diesel, and when I started the car I heard what sounded like rubber stretching in the center console. All I got was fresh cold air on the way home. I tried the defrost trick, and only got heat on defrost, but not economy.
Is there a flap that's not opening to allow the hot air in? If this sounds like a CCU issue, is there a way to test the ones that are in the junkyard with a multimeter? Thanks

__________________
'81 300SD

I'd bet money on a vacuum pod... but I have no idea which one it would be...

After going without heat this evening, I disconnected the monovalve and got heat in the economy setting. Does this mean that my temperature wheel is bad? Thanks

__________________
'81 300SD

The temperature "wheel" is not the sole component involved in temp regulation.

Don't bet much; you are going to lose it all.

there are DOZENS of solder joints in the CCU board that can cause the monovalve to stay cold.
if you have some rosin core solder, and a decent soldering iron, it's not difficult to touch them up. the wheel is much harder to repair, but it's not as likely to be the problem.
also, when you open it up, and if you find a badly burned board, check your aux pump. it can lock up and actually burn up your car.

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Wow! that's the first thing I've read about 126's catching on fire due to a bad aux pump. If I drive around for the next week with the monovalve disconnected and consistently get heat, does that mean the aux pump is doing it's job of pumping coolant through the heater core? Is the aux pump located below the monovalve assembly? Thanks

__________________
'81 300SD