I also posted this under the w126 section. Sorry for the double posting. I saw this area after I posted it.

Before posting, I did a search and did not find the same problem.

With my 1990 300se, you can set the heat on "econo" and it seems to work fine, including when you change the temp setting up and down. Here's the problem.

When you change from the "econo" to say "defrost" it starts to blow cold air, if you go back to "econo" it blows cold air. If you turn off the car and turn it back on, it works correctly again. If you turn the car off, set it on defrost, turn the car back on, it blows hot air. like it should.

It appears by turning the car off and back on it resets something, any ideas??

Paul Waddell
1990 300se
Front Royal, VA

 

You most likey have a vacuum leak for the a/c control's.

 

Not sure if its vaccum leak... I mean how would it not affect the ac when car is started after setting it to defrost or something...

 

All the a/c box flaps are controled by engine vacuum.

It also could be you'r a/c pushbuttons, but I dont really know all that much about the older cars.

 

when in defrost mode you go to full heat default, if you are not getting heat you probably have a bad mono valve or push button panel,

 

I would look into the pushbuttons first. Bad mono valves usually have the complaint of no heat no matter what.

 

When you go to defrost, you can here the flaps change, air goes to high and shoots out the top on the windshield. The same thing happens when ever I change it from enco to both the up/down arrows, one down arrow, etc.... In fact, if I go to "high fan" on enco I don't get as much heat as I do on low fan. It just seems the heat goes away. And the only way I've found to fix it, is by turning the car off and back on. I guess I'm resetting something. I didn't know if an AC charge would help?? I know the AC is low on freon. I know the rebuild kit for the mono-valve does not cost a bunch of money. It maybe worth starting there?? I see the were I can get the manuals out of Perf. Products out of Calf. for about $39.00. Has anyone had luck with these??


All of your help comes with many THANKS!!! This is my first MB, I call it my "Linda Blair". I'm just waiting for it's head to spin around. Don't get me wrong, I like the car and the way it rides, the looks etc...., I'm just use to the car from the USA and Japan. I'm trying to understand way MB used all of these different things for something like heat.

My aunt and uncle owned the car since it was new. When she died, I got the car, it had sit for almost two years. I guess, I'm just working out all the bugs.

Paul Waddell
Front Royal, Va
1990 300se

 

You can disassemble the mono valve easily, just keep the parts in order. When they fail its usually the rubber seals inside, you can tell if its bad if you can see any rips/tears in the seal.