l have a 1989 MB 420 SEL that will only blow hot air. the controls will not make any difference. Can anyone tell me what to do?? l am desperate for answers. someone suggested a vacuum valve located on the firewall. thank you in advance......
AV Toronto

 

Does it blow hot air when the AC is on? Then it could be your freon is gone. Go to an auto part store and get some dye and the light that usually come in a leak test kit, to check for leaks. If the evaporator was or is leaking, they rust up inside and that might need to be replaced. Or as someone suggested, it could be a simple as a vacuum leak. Do you hear the flap doors opening and closing when you switch the AC on or when you switch from AC to heat?

 

is your compressor running ..if so it might be your mono heating valve ..by the battery ..they had replacement elements 4 screws..take it out see if the rubber inside is ripped ...i bet it is ...