Wheel-speed signal output: Notes
Several other systems in the car need to receive wheel-speed data. Because the ABS control module is not connected to a bus, it has a wheel-speed output for each wheel (only the rear left and right front are connected at present). A transistor in the ABS control module grounds the respective outputs 29 times per wheel revolution. The voltage across the output is monitored by the systems receiving the data.
The main instrument unit (MIU) receives the wheel-speed signal via a lead connected to the rear left wheel. The reason that a rear-wheel sensor has been chosen is that the distance travelled (recorded millage) can be measured more accurately on a non-driving wheel. The MIU converts the frequency-modulated signal to km/h, a computation that includes a preprogrammed correction for tire size.
Because the MIU is connected to a bus, all the other control modules connected to the bus are able to share the RL wheel-speed and millimeter (odometer) data.
The Trionic system is connected to the ABS control module via pin 26, and it receives wheel-speed data from the front right wheel.
For more information on wheel-speed data from the bus, see "BUS AND DIAGNOSTICS COMMUNICATION " and "MAIN INSTRUMENT PANEL, PETROL ".
If voltmeter measurements are made at any of the relevant terminal pins, the reading should alternate between 0 V and B+ voltage when the respective wheel is rotated slowly.
Provided that the RL and FR wheel sensors are functioning, the wheel-speed signal is sent even if the ABS system is inoperative and the ABS warning light is on.