Fan Speed Sensor (FSS)
WARNING: This page is about a different car, the 2013 Ford F-550 Super Duty, 2013 Ford F-450 Super Duty, 2013 Ford F-350 Super Duty, and 2013 Ford F-250 Super Duty. However, it is still accessible from the selected car via links, so may be relevant.
The FSS is a Hall effect sensor integral to the cooling fan clutch. The PCM monitors the sensor input and controls the cooling fan speed based upon the engine coolant temperature, the transmission fluid temperature, and the intake air temperature requirements. When an increase in cooling fan speed for vehicle cooling is requested, the PCM monitors the FSS signal and outputs the required pulse width modulation (PWM) signal to a fluid port valve within the cooling fan.