I've read the archives and, unless I missed one, I can't find reference to this ACC problem.

The AC compressor activates and operates at any temperature setting of the temperture wheel if any pushbutton on the ACC unit is engaged, except for the "O" button or "EC" button; for these last two buttons, the AC compressor does not operate.

The AC compressor does not cycle on and off with the three other button settings but runs constantly. The temperature wheel setting does not control temperature and hot air blasts out from the side vents and footwell vents - no air from center vents under these circumstances. Evidently the hot air overwhelms the chilled air from the AC. BUT, if the temperature wheel is turned to the full cold detent position, the side vents and footwell vents immediately close, and cold air blasts from the center vents (and rear vent).

I changed the monovalve insert; no change. If temperature wheel is turned to full hot detent, AC compressor continues to operate but hot, hot air blasts from side vents and footwell vents.

I can't follow the logic of what is happening. Evidently there is vacuum and the flaps operate as they should but something is signaling the ACC system to function as if the temperture in the cabin is very cold. Any ideas before I order a new ACC control?

I would think it's possible that the interior temp sensor is bad.

Gilly

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To add to what Gilly said. I would be wondering if the aspirator motor is working. This little motor draws air across the inside temp sensor which would give the same affect as a bad sensor if it is not working. I quick simple test is to take a small piece of paper and see if, with the key on, it will stay stuck to the grill on the sensor. A dirty sensor will react slow to temp changes too. The sensor is located next to the sunroof switch. The motor is located to under the right outside dash vent.

fasthair

Thanks to Gillybenztech for the suggestion that cabin temperture sensor could be bad. I had measured the resisitance at different Temps and felt that the values were close enough. However, because the cooling system thermostat locked up and the engine started to overheat as I was passing within 100 yards of a Benz dealer, I used the opportunity to pay retail and buy a new cabin temperature sensor as well. Talk about serendipity. The new sensor tripped the ACC system to operate as Mercedes intended...or close enough. The ACC problem appears to be solved. And to think I was about to order a rebuilt ACC pushbutton unit out of desperation. thanks for the help.