i have a 1985 300CD

i have had the car for almost two years, when i first bought it the heat worked fine - the a/c did not, but as i'm in Canada, not too bothered. As time progressed, when i pushed in any of the switches to turn on the heat/fan, it would not always turn on; however, if i randomly hit one button after another the heat would suddenly come blowing out.

Now, i can hit the buttons, slide the dial etc. and it will occasionally come on, but most often not. Likewise, if i leave the buttons on, occasionally while driving the blower will suddenly turn on and heat blows out.

when the blower turns on there is always heat. so i don't know whether it has something to do with the vacuum system (not even sure if the heat is controlled by the vacuum system). Actually, i haven't a clue why the system would sporadically work.

based on your excellent descripotion, I am certain I know what it is.
The control unit (switch module) is notorious for going out due to poor design, the solder cracks on the end where the cables plug in. Its a DIY job if you have any soldering skills and can remove it. There are other items that tend to fail in the climate control system also but it doesn't sound like vacuum is involved in your case.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

thanks a lot, i'll definitely try that tomorrow.

cheers