See how to test magneto resistive wheel speed sensors using a PicoScope Lab scope. This video shows the connection, scope set up and signal on a wheel speed sensor, other magneto resistive sensors will have similar voltage peak to peak fluctuations.
Magnetoresistive sensors are based on the magnetoresistive effect. The magnetoresistive effect is the change of resistance of a current carrying ferromagnetic material due to a magnetic field. Magnetoresistive sensors can also be called magnetically controllable resistors.
When current is passed through the ferromagnetic material the internal magnetisation vector (M) of the ferromagnetic material is parallel to the current flow. When an external magnetic field is applied in applied opposite to the direction of the current flow as shown in the figure the internal magnetisation vector changes its position (M1) by an angle depending on the strength of the magnetic field. The resistance depends on the angle formed by the internal magnetisation vector (M) of the ferromagnetic material and the direction of the current (I) flow. Resistance is largest if the current flow and the internal magnetisation vector are parallel. The resistance in ferromagnetic material is smallest if the angle is 90° between the current flow and the internal magnetisation vector.