Originally Posted by
onehundred80
The problem probably lies in the solenoids under the hood that switch off the hot water to the heater core. There are two cores and two solenoids.
These adjust the amount of hot water flowing when they are in heat mode but shut off when in full AC mode. But if you have one of the ***** that adjust the heat set to hot then hot air and cold air will mix when in AC mode. The **** has to be in the "Blue" zone to see more cooling.
That's roughly the way it works.
Sounds exactly like what onehundered80 mentioned... rather common to MB's