Good Morning, I'm a new poster here,but I have read the archives and have not really found the same issue i am having with the ac/heater ...I bought this car a couple of years ago,it has 123 mi on her and I bought her from a good friend who is also a MB mechanic ....The issue currenly is as we drive that we get door heat and floor heat when we don't want it and the AC works thru the center vent ,the side vents and the defrost vents blow a little warm air also ....What we have done so far ....Replaced the monovalve (the original was fine)voltage at the mono valve is 12+ volts ,or .7 volts,we swaped out the control unit with another ,and we have swaped out the heater core sensor ....all out of a known working w126....still the same issues ....My MB mechanic ,is still trouble shooting the issue,but I would like to help speed this process along so I can drive the car ,and he can work on another W126 for me ....that I want to buy.
He suspects vacuum issues,but his memory is a bit foggy after all these years ,I know in time he'll resolve this ,but it never hurts to ask...any reponses would ne most appreactied...TIA JIM

Jim there are two area's that I would look at. Check the vacuum pods with a Miti Vac - the diaphrams go bad. Also the switch-over valve located in the same area can cause this symptom.

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Well my good friend and MB mechanic ,got up Saturday morning and decided to pull the head liner down on my 420sel,the mice had built a nest around the front courtesy light (just in front of the sun roof) and chewed thru the wires to one of the sensors ,not the vacuum tube ??? Anyhow he soldered the wires and the climate control is working again,we also found a nest uned the brake booster and under the battery compartment as mentioned in this forum,we cleaned them out ,and now with the climate contro system funtioning I have the sweet aroma of Mice pee in the car....again not to be out done ,I squirted in some "doggie" carpet cleaner that nuetralizes ammonia ....it worked like a champ.More mouse traps in the gargae this year .....and in the Benz....Thanks for your help,until next time.Happy fathers day ,Jim