no heat

new to this forum..need help to get to constant flow of in car heat throgh the vents....every once in a while the hot or warm air stops and minnesota cold air blasts out air vents....this may last for several miles or hours.....if anyone could give me direction on what maybe the problem sure would appreciate the help.. 1991 420 sel.. thanks train loads..jack

[John Plut]

No answer yet Jack...just sympathy. My '88 260e just started doing the same thing about a week ago. Even on max heat the center vents open up and blow cold air from outside. I tried pushing all the buttons and eventually the hot air will start again but I know something is going wrong. I suspect my pushbutton controller may be going bad.