97 1.5 rad fan @#&@

1997 protege a/t, 64000 miles, rad fan don,t want to start up . when i start the car, soon the temp gage goes about 45% and stays there. i stuck a thermometer in rad and got it up to 200 degrees. fan did not start. i turned on the a/c and both fans started immediately. is there a sensor causing all this to happen and where is it located. thanks for any help. jim.