My 2002 ford explorer the temp. Control knob when you change it it makes a loud clicking sound and it will no longer blow cold air only hot.

There is an actuator that controlls a blend door that is called a blend door actuator. This "door" is inside the evaporator case and it opens and closes to divert air over either the evaporator (for air conditioning) or the heater core (for heat) and can be somewhere in between for warm air. your temp knob sends an electric signal to this actuator to controll that door. the actuator has plastic gears inside of it that can become stripped of its teeth causing a "tapping" sound. These trucks comonly have this problem. The actuator is screwed on to the evaporator case and the blend door actuator is a mounted on top either behind the radio or the glove box. It;s about the size of a deck of cards