Analogue Instruments: Speedometer

The speedometer is an electrically driven, analogue instrument for displaying the vehicle speed. The pointer needle is oil damped to prevent impacts on the vehicle wheels, causing deflection of the pointer needle.

The speedometer is driven by a square wave signal derived from the ABS ECU. The wheel speeds are measured by sensors reading rotational speed from toothed targets on the hubs. These speeds are passed to the ABS ECU in the form of pulsed signals. The ABS ECU is programmed that 48 pulses is equal to one revolution of the wheels and this equates in distance to approximately 2 meters. The actual signal that drives the speedometer is a buffered signal sent from the left hand rear speed sensor.

This information is passed from the ABS ECU, on a hardwired connection, to the instrument pack and is received by one of the microprocessors. The microprocessor applies a formula to the signal and calculates a numerical, 16 bit value for the signal. This signal is then used by the speedometer drive to display the current speed to the driver. A second digital signal output is also broadcast on the CAN, I and K buses for use by other systems.

To prevent incorrect readings due to spinning wheels, the microprocessor introduces a time delay on the output signal if the received speed signal increases too quickly. The delay ensures that only the true speed is displayed and transmitted on the bus systems.

Two versions of the speedometer display are used. One shows the speed in kilometers per hour and the other shows the speed in miles per hour as the main display and kilometers per hour as a secondary display.