The viscous fan is not electrically operated (older models had an electrically operated magnetic clutch fan which was activated by one of the coolant sensor switches) and so only comes into action when the hot air coming from the radiator is above +- 95 deg C.

The Auxilary fan is the electric one in front of the radiator and goes on at hi speed (the roaring sound you get) if the coolant sensor is over 105 deg C. If it runs continuously then definitely the sensor you have identified is not operational and therefore the wire short circuited out somewhere.

The auxilary fan will only operate on low speed if the air conditioner is on (and only when the aircon circuit high pressure switch is activated due to thermal overload).

Both circuits are relay operated. On most of these engines all the sensors are usually situated in the front part od the engine.