My son says he is getting air conditioning out of front vents but heat comes out of the defrost vents. I hope I am telling correct. Any suggestions?

 

Sometimes, for whatever reason, when the charge in the A/C system gets low, it can cause it to be cold on one side and warm on the other. Make sure there is enough R134 in the A/C system.

 

That is very similar to a problem I am having with my '89 190D. I get practically no cool/cold ac from the front central vents but the defroster is pouring heat in to the car as if is -30. My side vents(left and right) I keep shut off because I was told they don't provide AC anyway. I am swealtering in the heat and don't know what do about fixing it. I have read some other threads about vacuum valves, flaps and switches etc. But is there a basic cause/solution to this problem?