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Wheel-speed signal output

Wheel-speed signal output

Several other systems in the car need to be aware of the wheel speed. Therefore, the TC/ABS control module, which is not connected to the bus, has a control module output for each wheel speed signal (RL and FL are connected). A transistor in the TC/ABS control module grounds each output 29 times per wheel revolution. The output is supplied with power by the receiving system.

The main instrument unit is connected to TC/ABS control module pin 23, the output for the left-hand rear wheel's speed signal. The main instrument unit converts the frequency-modulated signal to km/h, a calculation that includes a preprogrammed correction for tyre size. The wheel-speed data is then sent out on the bus.

Trionic is connected to pin 25 on the TC/ABS control module and receives speed information from the front left wheel.



A voltage reading taken across any of these pins should show the values switching between 0V and B+ when the respective wheel is rotated slowly. The voltage will be 0 or B+ when the wheel is stationary. Speed information is sent even if the TC/ABS control module is out of order and the ABS, brake fluid and central warnings lamps or the TCS OFF lamp is on. The information is sent as long as the RL/FL wheel sensors are working.

For more information on wheel-speed data from the bus, see “Electrical system, Bus and diagnostics communication ” and “Electrical system, Main instrument unit ”.

Diagnostics

If the wheel-speed signal output is faulty, a DTC will be set in the receiving system.