Displaying Vehicle Speed

Networking:

The electronic speedometer (A1p8) serves to display the vehicle speed.

Determining vehicle speed
The speed is the distance travelled in kilometers or miles per hour.
- To calculate the vehicle speed the computer of the instrument cluster (A1) evaluates the wheel speeds of the four wheel speed sensors (L6/1, L6/2, L6/3, L6/4). The instrument cluster (A1) receives the wheel speeds in "rpm" from the traction systems control module (N47) via the CAN engine compartment.
- The tire size is still taken into account for calculating the vehicle speed. Tire size is coded with the STAR diagnosis tool.

The vehicle speed determined is displayed on the electronic speedometer (A1p8).
It is driven by a stepper motor.

Evaluation of wheel speed signals
As not all wheels always have the same speed, the four wheel speeds must be evaluated. As this is relatively involved, the procedure is only covered briefly here:

- The individual wheel speeds are compared and then checked for plausibility.
If no significant differences in the individual wheel speed signals can be recognized, the vehicle speed is determined from the speed of the left front wheel.

- If the wheel speed signals differ significantly from each other, this may mean defective wheel speed signals or cornering.
- If by means of suitable comparisons of the wheel speeds (side comparisons) the computer determines that cornering is involved, the speed is determined from the mean value of the front wheel speeds.
- If the comparisons indicate one or several defective wheel speeds instead, the vehicle speed is determined from the highest wheel speed. This also applies if a fault message is received instead of a wheel speed. In this case a corresponding fault bit is also set.


NOTE:
- If fault messages are received for all wheel speeds, the vehicle speed "0" (pointer of vehicle speed display on left end of scale) is displayed.
- As the distance travelled is determined from the vehicle speed, the evaluation of the wheel speed signals described here also applies to the distance travelled.


NOTE: The electronic speedometer (A1p8) has a scale in kilometers per hour or a scale in kilometers per hour and a scale in miles per hour depending on the national variant.