I know heating is a common problem in these cars so hopefully someone will be able to help me out before winter comes! My heat doesnt work. The fan blows when on defrost, but not heated air. On all other setting no fan, but I can hear the valves switching to panel or floor. Please help me.

Do a search for monovalve.

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1984 300Sd 210k

Former cars:
1984 300D 445k (!!) (Strider) Original (and not rebuilt) engine and transmission. Currently running on V80 ( 80% vegetable oil, 20% petroleum products). Actually not, taking a WVO break.
1993 300d 2.5 275k. Current 120/day commuter
1981 300SD 188k (Hans) Killed by a deer

The monovalve is an easy fix.

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Thank You!
Fred
2009 ML350
2004 SL600
2004 SL500
1996 SL600
2002 SLK32
2005 CLK320 cabrio
2003 ML350
1997 C280 Sport

This could be a fan problem, with worn brushes that only allow it to run a full speed. It could be a bad Control Unit, not signaling the fan to run except in Defrost.

The switching you hear is probably the solenoids that apply vacuum to the various air flaps.