Why would a car overheat when the defrost is on?

The defrost selection cycles the AC pump, regardless of what temperature you are choosing to defrost at. This cycles the freon, and cools the evaporator (inside dash), and heats up the condenser (in front of radiator) I would check for 1. the fan working in front of the motor... 2. road debris, bugs, cotton from weeds plugging the space between the radiator and the condenser (common)