GF54.30-D-3025H Display Speed, Function

GF54.30-D-3025H Display Speed, Function


The speedometer (A1p8) of the instrument cluster (A1) indicates the vehicle speed. The speedometer is driven by a stepper motor.

Measuring the vehicle speed
To calculate the vehicle speed, the wheel speeds from the following sensors are evaluated by the instrument cluster:
- Left front rpm sensor (L6/1)
- Right front rpm sensor (L6/2)
- Left rear rpm sensor (L6/3)
- Right rear rpm sensor (L6/4)

The instrument cluster receives the wheel speeds in rpm from the ESP control unit (N30/4) via the Controller Area Network bus class C (engine compartment) (CAN-C).

Evaluating the wheel speed signals
Since the wheels do not always rotate at the same speed, all 4 wheel speeds must be evaluated. The actual process is relatively complex, which is why only a general description is provided here:
- The individual wheel speeds are compared and submitted to a plausibility check. If there are no significant differences, the vehicle speed is determined using the rotational speed of the left front wheel.
- If the individual wheel speed signals differ considerably, the speed signals may be faulty or the vehicle may be cornering.
- If appropriate comparison of the wheel speeds (between the two sides of the vehicle) by the instrument cluster indicates that the vehicle is cornering, the speed is determined from the mean value of the front wheel speeds.
- If the comparisons indicate one or more incorrect wheel speeds, the speed is calculated from the highest wheel speed. This also applies if an error message is received instead of a rotational wheel speed. In this case, an appropriate error message is stored in the memory.

If error messages are received for all wheel speeds, the speed 0 km/h or 0 mph is shown (the indicator points to the left-hand side of the scale).
This evaluation of the wheel speed signals also applies for evaluating the distance traveled since the distance is calculated from the vehicle speed.