Displaying speed, function - GF54.30-P-3025PP
MODEL 203.0 /2 /7 up to 23.4.04
The purpose of the electronic speedometer (A1p8) is to provide a display of vehicular velocity. The electronic speedometer (A1p8) is actuated by a stepper motor.
The speed can also be displayed digitally:
- The speed can be displayed instead of the outside temperature in 3 digits in km/h or mph on the multifunction display (A1p13). Changeover is effected in the "Settings" menu.
- With the exception of vehicles for Great Britain (code (833)), the speed can also be displayed instead of the total and trip mileage on the multifunction display (A1p13). Changeover is effected in the "Settings" menu.
Measuring the vehicle speed
To calculate the vehicle speed, the instrument cluster (A1) evaluates the wheel speeds of the following sensors:
- 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 by means of signals on the class C (engine compartment) controller area network bus (CAN-C).
As the tire circumference differs only very slightly among all the approved tires, no coding of the tire circumference is necessary.
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 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 instrument cluster's (A1) (lateral) comparisons of wheel speed indicate that the vehicle is cornering, the speed is calculated from the average for both front wheels.
- 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 wheels speeds, the speed is displayed as 0 km/h or 0 mph (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.
| Instrument cluster (IC), location/task | GF54.30-P-3039PP |