I crawled around under the dash this weekend and found three big vacuum leaks--one of them was at the center vent element. Replaced it (what a hassle!) and fixed other loose rubber joints. Now blower and vents all seem to work properly. But, STILL NO HEAT!

To summarize:

--new servo
--replaced amplifier
--new aux water pump
--air purged from coolant
--tight vacuum systems
--interior and ambient temp sensors check OK
--temp selector wheel checks OK
--replaced foam tube in dash for cabin temp sensor

The blower did not come on (except in DEF mode) until coolant reached ~80 C. Still no heat in cabin--drove to work this morning with frozen fingers and toes.

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1978 300SD 149K miles silver-blue "Maxine" (mine!)
1979 300SD 318K miles red "Helga" (SOLD)
1980 300SD 321K miles yellow "Linda" (younger daughter's)
1980 300SD 148K miles blue (SOLD)
1980 300SD 153K miles green(parts car, SOLD)
Yes, I have a W116 fetish :-)