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My 1985 380 sl has developed a problem with the heater...it won't blow warm air in any mode, a/c or economy setting.  Thinking it was a poorly functioning thermostat this was replaced to no avail.  It is interesting in that if you start the car, turn up the heat all the way and let it idle, it will eventually start blowing warm air but as soon as the car is driven, only cold air come out of any of the registers.  I would appreciate any and all suggestions.  Thanks,

 

Joe 

I just purchased a 1982 380 SL with the exact same problem.  Blows warm air for a while then starts to blow cold air.  If you stop for a short period even as short of a duration as to pick up a toll ticket, the heat returns.  Drive for a few miles and it goes cold again.  Exactly the same as Joe's 1985