I drive an '84 123 300TD. I have just went through 3 weeks of chaos in transmission problems. I was told that I need to rebuild the trans. I elected to replace with a rebuilt. The first rebuilt wouldn't work. They replaced it. That one seemed to work but I was told the vacuum was not working right and they didn't know what to do with it. (This was just your everyday transmission place.) They did know of a reputable Mercedes mechanic an hour or so from my place. (I had given up on the guy I had been using. I think I knew more than he did.)

By the time I got to the mechanic the climate control was alternating heat and air-conditioning all by itself. Some of the windows quit working, When I went to turn off the engine, it kept running and I had to use the shut off under the hood. At that point everyone seem to agree that I had vacuum problems.

He discovered the vacuum unit in the rear driver's side door had to be replace. Then he discovered that the kick down switch didn't work and he replaced it. While he was test driving it he noticed, as I had already noticed, the right front wheel was making noise and the bearing need to be replaced. This entire process took a couple of days and during that time he had loaned me his shop car, an '82 300 TD. (He even replace a couple of climate control buttons I had lost.)

This is all history and background. I am pleased with his work. His charges were less than $550.

The care runs fine, has much more power, runs smoother. Apparently I have had a bad transmission the entire 2 years I've had the car and the other fellow didn't recognize the problem.

Now is the rub. On the way home from work today the climate control abruptly switches off the AC and turns on high heat. Nothing I do makes any difference. Some time later, I notice I once again have AC. Later it switches back to heat again.

Last winter I replaced the monovalve and I have replace the hose to the thermostat sensor in the dash.

Any suggestions would be appreciated.

__________________
DEH
"Beauty" '84 300 TD
Navy/Navy
169,000 mi

There is probably a simple solution to the a/c problem. I recently had the same exact thing.
The climate control system turns the heat off electrically at the mono valve. If the valve looses power, it opens to the max heat position. Even though the a/c still runs, the a/c looses the battle with the heater. My problem was a less-that-perfect connection at the mono valve. So that might be your problem.
However, if your a/c is actually turning off (ie: compressor disengages) then it must be an ACC issue. But I would suggest cleaning and reseating the plug on the mono valve first.

Although I am completely exhausted after reading the entire history of your car in order to make it to the one paragraph about your problem, you should start by removing the four screws on top of the monovalve and raising the cover. See if it is flooded above the diaphragm. If so, it will need a rebuild kit. Just because you rebuilt it las Winter, does not mean that it couldn't be bad again.

Good luck,