1981 300SD Climate Control quick diagnosis - PeachParts Mercedes-Benz Forum

DesertStar · #1 11-10-2006, 06:24 AM

Hello all, I havent posted in quite awhile as I have been preoccupied restoring my Gwagen.

My symptom is no(very little) heat to the feet while in heat mode (arrow down button) and temp dial in red and most of heat going to windshield and sides.

Last year I replaced all vaccuum servos even though one was bad and monovalve as well as the tube to the temperature sensor. All connections are correct and no hoses leaking, crimped etc. All flaps are in working order as I checked after servo install. When in "heater only mode" most of the air comes through the defrost with dial in red with a leak share coming to the floor....same scenario in "EC heater mode" When I depress the "bi-level" button (arrows up and down) I get heat to the feet as well as the door panels and leak share for defrost. This tells me my pods are somewhat in order as all flaps are opening and closing...just seems not corresponding to the prescribed buttons. The W126 repair manual shows when depressing "heater" button that most heat should go to floor with leak sahre to windshield...mine seems to be reveresed.
I can just live with this as it does the job, but I am a bit anal about things working as they are suppose to and quite frankly there are times I just want heat on my feet.

Does this sound like a control unit issue or is this common to some of you with same model ? I did notice that my control unit was a rebuilt unit dated year 2000 which tells me previous owner had replaced. I would hate to purchase another unit only to have it do the same thing.

Any comments are appreciated.
Thanks, Mike

mbdoc · #2 11-10-2006, 09:19 AM

Testing the vacuum switch-over valves behind the pushbutton control unit should the reveal problem.

Brian Carlton · #3 11-10-2006, 09:42 AM

If the vacuum pods are fully functional, and you're sure you have no leaks...........and you test the switchover valves to be sure that they are signaling the pod.........and you still have no heat out of the floor registers..........then it's the CCU.

The floor registers don't have a "leak" position. Either they are open or they are closed. Any "leaks" are caused by leakage past the door........it's not by design.

The defrost register does have a "leak" position and will leak when in economy mode. Be sure the defrost pod is not defective and will hold vacuum. If the defrost door won't close under vacuum, then, by default, all the air will head out to the windshield.

DesertStar · #4 11-10-2006, 06:17 PM

Thanks guys I will dig deeper. The defrost flap does open and close while in economy or heat mode and I turn the temp wheel to blue (flap shuts) and to red( flap opens). I will take a sunday pull all out again and test again as per W126 repair manual...just to be sure before I dive to a CCU.
Mike