Only three times in four months has this symptom occurred, but I'm wondering how serious it is.......The climate control in my 90 500SL will select full fan and full heat and won't relent, almost regardless of the temperature setting. I normally leave things in AUTOMATIC mode at about 70 degrees. I have to open the center "Cold" vent just to make things bearable. This has occurred in outside temperatures ranging from 25-40 degrees. Things seem to get back to normal operating mode the next time I use the car, which is often the next day.....Any ideas of the extent and expense of this problem???

Some of the malfunctioning problems of the climate control can be reset by the dealer!!
I had mine reset recently after my climate control (C250D)switched from Celsius to Fahrenheit (and back) several times and stopped opening any ventilation ducts besides the windshield ones. I couldn't regulate the airflow at all and the only thing I had to do was resetting the system.

Otherwise it can be a bad temp.sensor inside the car which detects a lower temperature as is and therefore activates the full power of the heating system.

greetingz,

I'm afraid the problem with your 500SL is a little more serious than a "reset". I had the same problem and eventually had to have the aux water pump, a/c switch assembly and heater solenoid valve replaced. MB climate control systems often require a few dollars to fix, especially the older systems. "Mercedesshop" postings are full of similar problems, and similar solutions.

JoeB