Main instrument unit, detailed description: Odometer/tripmeter
The main instrument unit stores the driven distance and shows it on the odometer display. The odometer display incorporates a six-digit register that is continuously updated from 000000 to 999999 km (or miles) by the control module. The trip meter incorporates a five-digit register that starts at 0.0 and runs to 9999.9 km (or miles).
This information is generated in the active wheel sensors of the TCS or ESP system (rear left and right for petrol-engined vehicles, front left and rear right for diesels). The TCS/ESP control module converts the speed signal (48 pulses per revolution) into wheel speed. This speed signal is transmitted on the P-bus as the number of revolutions per minute and used by the engine control module, which converts the signal into distance and takes the programmed wheel dimensions into account. The engine control module (ECM) transmits the distance for the respective wheels on the P-bus. The CIM forwards the information on distance to the I-bus. The main instrument unit uses the distance and calculates an average between the distances of two wheels, subsequently providing road and trip information in the display.
The mileage is transmitted as bus information and is thus accessible to other control modules.