There is a door which - when actuated by the temperature control **** on the head unit - switches or blends the air flow. For heat, more air flow will come through the heater core, and for cooling, more air flow will come through the A/C evaporator core. There is an electrical device called the "blend door actuator" which actually positions this door according to where you have the heat/cool **** set. The blend door actuator could be at fault (probably most likely), or the blend door could be obstructed from movement, or the head unit's heat/cool control switch (or associated wiring) could be at fault.
I've attached a couple of illustrations that I hope will help.
Images (click to enlarge)
Wednesday, October 5th, 2011 AT 9:57 PM