Display speed, function - GF54.30-P-3025MCU

MODEL 451.3/4 

Shown on code (K07) Speedometer (miles) 

Fig 1: Identifying Instrument Cluster And Electronic Speedometer

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Vehicle speed display 

The electronic speedometer is used to display vehicle speed. The speedometer is driven by a stepper motor.

Measuring the vehicle speed 

The instrument cluster receives from the ESP control unit (N47-5) the wheel speeds required for the computation of vehicle speed via the Controller Area Network (data bus/CAN bus) (CAN). The tire size is taken into account when calculating the speed. Coding takes place using STAR DIAGNOSIS.

Assessment of wheel speed signals:

• The wheel speed signals are compared against each other and thus checked in terms of plausibility. If there are no significant deviations, 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, by comparing the wheel speeds on both sides of the vehicle, the instrument cluster determines that the car is cornering, the vehicle speed is calculated from the mean value of the front wheel speeds.
• If the comparisons indicate one or more faulty wheel speed signals, the vehicle speed is then calculated on the basis of highest wheel speed. This also applies if an error message is received instead of a wheel speed signal.

The travel distance is determined as for the vehicle speed.