Displaying speed, function - GF54.30-P-3025S
Model 215, 220
Networking of components
The electronic speedometer (A1p8) is used to display the vehicle speed.
Determining the vehicle speed:
This display is furnished as the distance covered in one hour in either miles or kilometers.
To calculate the vehicle speed, the processor in the instrument cluster (A1) evaluates the wheel speed signals from the four wheel speed sensors:
- Left front wheel speed sensor (L6/1)
- Right front wheel speed sensor (L6/2)
- Left rear wheel speed sensor (L6/3)
- Right rear wheel speed sensor (L6/4)
The instrument cluster (A1) receives the wheel speeds in "rpm" from the traction systems control unit (N47) via the engine compartment CAN.
- The system also includes tire size in its speed calculations. The tire size code is entered using the STAR DIAGNOSIS TOOL.
The determined vehicle speed is displayed on the electronic speedometer (A1p8).
The speedometer is driven by a stepper motor.
Evaluating the wheel speed signals:
Since the wheels do not always rotate at the same speed, all four 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 no significant deviations exist between the individual wheel speed signals, the vehicle speed is determined using the rotational speed of the left front wheel.
If the individual wheel speed signals vary substantially, the speed signals may be faulty or the vehicle may be cornering:
- If the computer detects, by means of suitable comparisons (comparing the two sides of the vehicle), that the vehicle is cornering, the vehicle speed is calculated from the mean value of the front wheel speeds.
- On the other hand, if the comparisons tend to suggest one or more faulty wheel speed signals, the vehicle 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 bit is stored in the memory.
If error messages are received for all wheels speeds, the speed is displayed as "0" (indicator points to the left 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.
Depending on the national version, the electronic speedometer (A1p8) is calibrated either in kilometers per hour, or in miles per hour with a secondary scale for kilometers per hour.
| Instrument cluster (IC), location / task | GF54.30-P-3039S |