Date registered: May 2009
Vehicle: S430 93K; E420 243K; 240D 242K
Location: Near Sublimity, Orygun
Mentioned: 1 Post(s)
Quoted: 65 Post(s)
What Kajtek said, failing that, based on what info you gave:
It sounds like you may have a 280S, W116 and it is at least 35 y.o., so the reason is a failed automatic climate control and or a stuck bypass valve.
If that is the case, do a search on the W116 forum and find out how to bypass or repair or replace the ACC/Servo . Don