What car are we talking about ?
On my 1990 W124 TD turbo the Hall sensor serves only to transmit the speed of the car ( read on top of the speedocable right behind the instrument cluster ) to the Tempomat cruise control. So if your Tempomat is not working correctly...you know where to start !
Depending of the age of the car, the outboard temperature reading is also coupled to the Hall sensor so it will not indicate a wrong temp when you are not driving .
If you don't have one ore both of these items there will be no Hall sensor because ther is no need of it !