Speedometer with odometer and trip meter, Trionic T7: Speedometer
The main instrument unit is fitted with an electronic speedometer. The ABS control module receives information on vehicle speed from the rear left-hand wheel speed sensor. This information is processed and conveyed continuously to terminal 10 (K12) of the main instrument unit's control module.
The information is in the form of a square pulse which varies periodically between B+ and 0 V. The main instrument unit supplies the output with current and the ABS control module grounds it, This takes place 29 times per wheel revolution.
With the wheels stationary, the voltage may be either B+ or 0 V.
The main instrument unit uses the frequency to calculate the vehicle speed.
The speedometer is supplied with current and controlled by the main instrument unit's control module which converts the frequency modulated signal into km/h or mph. A correction, depending on the programmed tire size, is performed and the speed information is sent out on the bus.