I have done a search and found some info, but not quite this problem.
All components of my climate control system used to work (last week).
Now, I get nothing when I turn on just the defrost fan (all three settings) without A/C.
When I turn on defrost fan with A/C, only the third setting produces results (air and cold).
This happened suddenly, overnight. Any thoughts? Alex

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"Frau Blue Car" 1980 240D think Frau Bluker, cue the horses, formerly known as "China" from JimmyL
"Gunther" 1982 300D.....Turbo!! A/C!! Sunroof!!
"Fraulein" 1975 240D----Donated to NPR

Your CCu may have gone, some of the electrical may have gone like the temperature and blower speed regulator units.

Cant tell until you start digging in and finding out whats wrong.

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)