Wheel Rpm Sensors (Active) - Fiat 1998 Marea Service Manual

Braking system
Anti-lock brakes
33.

WHEEL RPM SENSORS (ACTIVE)

The operation of the "active sensors" is based on the variation of the internal electrical resistance de-
pending on the intensity and the direction of the lines of force of an external magnetic field producing a
square wave signal whose frequency varies according to the rotation speed of the wheel, but whose am-
plitude is constant.
The external magnetic field is produced by a so-called "multi-polar ring" made up of an elastomer with a
certain quantity of magnetic particles which, through a particular magnetization technique, are directed
to create different magents with alternating North - South polarities around the circumference.
The multi-polar ring can be fitted on the wheel bearing seal or on the hub (rear wheels).
The sensor is supplied by the ABS electronic control unit and produces a square wave signal with an al-
most constant amplitude and a frequency proportional to the wheel rotation speed.
The use of this new type of sensor offers the following advantages compared with traditional magnetic
induction sensors:
- less sensitivity to the distance between the sensor and the magnetic ring;
- improved immunity to electro-magnetic fields;
- the capacity of the sensor to measure the speed of the wheel up to zero (rather than 2.5 kph for the
passive sensor);
- the reduction in weight and size in the vehicle.
1. Active sensor casing
2. Multi-polar ring
3. Gap
4. Exampleof wheel rpm sensor output signal
6
Marea - Marea Weekend
Print no. 506.763/10