'78 300D - Heat won't blow
 

i bought this car not too long ago for cheap and it came with a few problems. one of them is that the heat and a/c don't work. they don't even blow air at all. i'm not going to deal with a/c right now, but i'd like to get the heat going for the winter and perhaps just some vents blowing air from outside. anyone know where to start? here are a few things of note:

-nothing happens when heat or a/c turned on
-defrost does work
-there is no "servo" - what exactly does a servo do?

any advice appreciated. it's a 1978 300d non-turbo.

Sounds like a vacuum problem. I think default mode for a vacuum failure in the climate control is full defrost. Get a mighty vac, do some forum searches, and have fun.

no servo? up front just behind the passengers headlight should be a 4" square box with a bunch of wires and hoses coming out of it... what do you have in this place?
the servo controls the a/c and the heat. and some other voodoo as well...

Ken, does your a/c control panel have round knobs to turn or a vertical row of pushbuttons? Does the blower work in defrost mode?

If there is no servo that is the source of the problem, unless your car is a Euro with manual heat controls.

it has a vertical row of pushbuttons and the defrost does blow. i read somewhere that you could somehow bypass the servo to just have the heat work. i just found it strange that the defrost does blow, but the others don't.

Was the car sitting??
You may have a nice mouse nest in the blow motor housing.

Also you may also have troubles with the blower motor regulator and temperature regulator.

Also you might want to remove the dash to see if anything has been tampered with. Working on a 83 300d now that someone cut wires and spliced that there own to go around the mercedes system. Needless to say its a headache and the other person work around didnt work.

i think this car did sit for awhile. and i did find evidence of a mouse nest under a corner of the hood pad that was coming off, but haven't seen anything else. where exactly is the blower motor?

passenger side below the glove box. remove that cover

So last fall I got a used servo that looked to be in good shape. I installed it and have had working heat all winter. The problem is that no matter what I set the temp dial to, the air is extremely hot. However, the first day that I installed the servo, the temp dial did work. I got cool air, hot air, the a/c even worked. Since then, it has just been hot. Any idea what the problem might be?seems strange that it did work that one day.