Speed Recording (Sensing)

NOTE: Numbers in parentheses ( ) indicate a component or tool in the associated illustration.

WHEEL SPEED SENSORS
Operation
On vehicles with Acceleration Slip Regulation (ASR) the Parameter Steering (PML) control unit (N49/1) calculates a vehicle speed signal from the two speed signals.

By calculating the mean value, the PML control unit can detect a failure between the left and right speed sensor.

The ASR control unit (N30/1) is fined in place of the ABS control unit (N30).








Design And Function
Speed sensors comprise a magnetic core and a coil. They sense the wheel speed on toothed rotors. The speed sensors and rotor are at a set distance from each other.

If the rotor turns then the Magnetic field changes, whereby an alternating voltage is induced in the coil. Us frequency depends on the rotor speed.










INDUCTIVE ROAD SPEED SENSOR SIGNAL, TRANSMISSION
Operation
A second speed signal is recorded by the inductive road speed sensor (L2)on the transmission and is transmitted directly to the PML control unit (N49/1).

From the speed signals of the speed sensor and the inductive road speed sensor, transmission (L2), the vehicle speed is calculated and the steering force assistance then controlled. The signal from the rear axle is to be viewed as an operating signal and the signal from the transmission as a check signal.

Two speed signals are necessary for safety reasons in order to differentiate between a vehicle being at a standstill and any failure of a speed signal.

When the vehicle is stationary, no signal comes from either sensor, maximum steering force assistance is provided.

In the event of the failure of a sensor, a signal is transmitted from the other sensor and the system is shut off (normal function of power steering).








Design And Function
The inductive road speed sensor, transmission (L2) is located on the rear transmission cover. It provides the signals for the speed check directly to the PML control unit.

The inductive road speed sensor comprises a permanent magnet and a coil which is wound around the permanent magnet. A pulse disk with 4 segments is located on the transmission output shaft.

If a segment is located adjacent to the inductive sensor, the magnetic field over the slight gap is closed. If the segment moves away from the inductive sensor, the magnetic field collapses and an induction voltage is thus induced in the coil.

The next segment closes the magnetic field again and an induction voltage is once again induced in the coil. This induction voltage has a sinusoidal shape. It is converted into a square-wave voltage in the PML control unit (49/1) and processed as a speed signal check signal in the computer.