Displaying Vehicle Speed

Networking:

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

Determining vehicle speed
The speed is the distance driven in kilometers or miles per hour
- To calculate the speed the instrument duster (A1) computer evaluates the wheel speed 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 also taken into account for calculating the speed.

The speed determined is displayed on the electronic speedometer (A1p8).
The drive is effected by a stepper motor.

Evaluation of wheel speed signals
As not all the wheels have the same speed, the four wheel speeds must be evaluated. As this is relatively time-consuming, the process is only outlined here:
- the individual wheel speeds are compared and checked for plausibility.
If no significant differences in individual wheel speed signals can be recognized, the vehicle speed is determined from the speed of the left front wheel.
- If wheel speed signals differ significantly, there may be incorrect wheel speed signals or the vehicle may be cornering.
- If the computer establishes by means of suitable comparisons (side comparisons) of the wheel speeds, that the vehicle is cornering, the speed is determined from the mean value of the front wheel speeds.
- If the comparisons point to one or several incorrect wheel speed signals, the 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, a vehicle speed of "0" (speed display pointer on left end of scale) is displayed.
- As the distance is determined from the speed, the evaluation of wheel speed signals described here also applies to distance.


NOTE: Depending on the national variant, 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.