i have the same problem on my prelude

everything works great but the stupid radiator fan doesnt come on when the car gets hot while idling at a stop for a long period of time.

if i turn off the car the fan comes on and will cool the radiator

if i turn my heater on full blast while the car is running and its hot it will come back down to normal.

i dont know what the problem is. does anyone have a clue?