AC problem
 

Heating on my 94 E320 works fine. Maintains the temp set on the dial.

In the cooling mode, if the temp dial is on the max cool, works fine. As soon as some other temperature is set on the dial, it starts blowing hot air through the center vents.

Aux water pump, mono valve?

If it were my car I would check the Aux water pump and also make sure the overhead temp sensor system is clean and working.

Haasman

Of haasmans suggestions I would suspect the temp sensor, or more specifically the sample blower.

To test it, cut out a piece of paper 1CM square, turn on the key and hold the piece of paper over the small grille near the inside rearview mirror. If the paper does not stay there when you take your finger off of it, the sample blower is not drawing air across the sensor.

You will need to consult the service manual to find out exactly where the sample blower is located on your particular car. The location varies. Some are near the console and others are to the right of and slightly below the glove box.

Good luck,

That little fan motor, aka aspirator motor, is behind the passenger side air vent.

It sucks in air and blows it up the A pillar, through the headliner, and out through the vent that is right next to your dome light / sunroof control button.

The temp sensor for your a/c is up there, which registers the temp for your wheel, and also blower speed.

My mechanic replaced the one on my former W124. I watched him do it, and it looked like he inserted two thin plastic sticks into each side of the air vent, where they go into tabs. He pulled the tabs toward the middle, and the plastic vent pulled right out.

He could pull the aspirator motor right out so that we could see it and hear it run. Mine was noisy.

He simply unplugged it and installed a new one. Took all of 20 minutes.