Displaying speed, function - GF54.30-P-3025R

MODEL 230

Networking

Fig 1: Displaying Speed - Signal Communication

Courtesy of MERCEDES-BENZ USA

The electronic speedometer (A1p8) displays the speed of the vehicle.

Measuring the vehicle speed

The vehicle speed is the distance covered in kilometers or miles per hour.

To calculate the vehicle speed, the RPM signals from the
- 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)
  are evaluated by the processor in the instrument cluster.
The instrument cluster (A1) receives the wheel speeds in RPM over the engine compartment CAN from the ESP, SPS and BAS control module (N47-5).
The tire size is taken into account when calculating the vehicle speed. The tire size is encoded using the STAR Diagnosis tool.

The calculated vehicle speed is displayed by the electronic speedometer (A1p8).

The speedometer is driven by a stepper motor.

Evaluation of 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 at the same time. If there are no significant variations between the wheel speed signals, the vehicle speed is calculated from 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 wheel speed. In this case, an appropriate error bit is stored in the memory.

No speed display appears if error messages are received for all four wheel speeds.

As the mileage readings are calculated from the vehicle speed, the odometer will cease to operate.

Depending upon national version, the electronic speedometer (A1p8) has a scale in either kilometers per hour or in kilometers per hour and miles per hour.

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