I went through the climate control testing procedures and found that my heat exchanger temp. sensor was not changing at all with respect to the increased temp of the heater core. Put a volt/ohmmeter on the sensor, drove the car and it read a consistent, very high resistance. I have ordered a new part and will install on Monday to see if that cures the problem. I hope it does - it's a relatively cheap part and easy install! Certainly MY preference in a fix to a problem. I'll let you know how it goes...
1987 300SDL 265,000 mi.
1982 300SD 325,000 mi. (and holding)
1956 Packard Clipper 150,000 mi.