Why 2001 honda civic still shuts down when temp. gets high?

my 2001 honda civic jus shuts down when the temp is high.y icantsay but this started to happen after i changed the timin belt. i didnt trace it to dat but when i drove slow for like 20 minutes the temp rises.i did try cleanin the radiator using washing detergent. the car drove fine but when i drained out the detergent and used rain water the temp starter to work high and at dat temp the car feels slugish any advice on how to make fan switch work at lower temp. wat sensor is located infront of the ac condensor. it is like a head lamp bulb and black thanks