BMW Repair: 1998 740il has heat at defrost vent with a/c on, stepper motors, stepper motor

Question
Last 7 of VIN DM22075.  When I select the AUTO mode on my climate control with the A/C on, I get heat out of the defrost vent.  If I just select the dash vents, A/C works great but I constantly have to adjust fan speed.  I also get cold air when foot vent is selected.  If I also select the upper vents(defrost) I get hot air.  Not sure how I can be getting cold A/C from dash and hot air at the defrost at the same time?  Could this be blend door stuck?  Or could it be a stuck stepper motor(dealer said that is probably what it is)?  Or is it the heater valve under the hood not shutting off hot water to heater core when A/C is selected.

Answer
Pete, you are correct, it could be anyone of the three. Most of the internal doors are operated by stepper motors. Clamp off the hoses to the heater valve and see what happens to the vent temp at defrost. With full A/C the Auxiliary water pump and heater valve should disallow hot coolant from entering the heater core. Ask the tech to look at the heater core temperature sensor reading. when Max A/C - cold is selected. You can also try tapping on the body of the heater valve/pump and see if the solenoid is stuck. I also can't figure out why you have to adjust fan speed? If you have Auto A/C maxed to cold temp setting the defrost vents should have no air at all coming from them aswell as the footwell. You may have multiple issues. I can't believe your heater valve/pump has lasted this long, they usually leak at around 6-7years old.
-Evan