Displaying speed, function - GF54.30-P-3025Q
Models 209.3 /4
The electronic speedometer (A1p8) and speedometer segment ring are used to display the vehicle speed.
Speedometer segment ring
This is located on the second, passive level of the central display, which contains the special active areas for displaying the segments. The display of the speedometer segment ring contains 52 segments in the ECE variants and 64 segments in the USA/UK variants; these segments are visible around the inner edge of the speedometer scale.
Measuring 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 following wheel speed signals:
- 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 (A1) receives the wheel speeds in RPM via the engine compartment CAN from the ESP and BAS control unit (N47-5).
Speed limit (winter tires)
as of 1.6.03
If a speed limit is activated using the 2nd level of the instrument cluster (A1) the speedometer segment ring is hidden.
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 checked for plausibility. 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 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.
Digital speedometer
The vehicle speed is shown in digital form on the display. The default value for the display unit (km/h or mph) is stored as a country-specific unit in nonvolatile memory and can be changed via "Settings" in the Configurations/Settings menu item. The digital speed display is matched to the speedometer display by a performance map. This ensures that the value of the analog needle position and the reading shown on the display are synchronized.
No speed display appears if error messages are received for all four wheel speeds. As the distance readings are also based on monitored speed, the odometer will cease to operate.
The scale of the electronic speedometer (A1p8) displays either kilometers per hour or miles per hour depending on the national vehicle version.
| Instrument cluster (IC), location / task | GF54.30-P-3039Q |