[Luce]

Hang on there. A hall sensor is quite different in the way it senses the magnetic field. It does not use a coil of wire, but rather a transistor that is sensitive to magnet flux. What is in question here is the drive circuit inside the hall sensor. If it's being used to drive the LED inside the opto isolator, then there is no need for any pull up resistor. But if it's driving the VR circuit, there is no mechanism to pull the signal high, so a resistor is used. Also, you may not need a pull up resistor, but may need a pull down resistor. It depends on the polarity of the hall circuit.

And no, of course you never see a pull up resistor on a production car. If it's needed, it's located inside the ECU.