Cars with ABS

WARNING: This page is about a different car, the 2001 Saab 9-3. However, it is still accessible from the selected car via links, so may be relevant.

Vehicle speed is sensed by the ABS through an inductive sensor at each wheel. The sensor supplies the control module with alternating current, the amplitude and frequency of which increase with increasing wheel speed. The frequency is proportional to wheel speed, 29 Hz at a speed of one wheel revolution per second. The ABS control module uses the 4 wheel speeds to prevent wheel lock-up on braking.

Several other systems in the car require information concerning the vehicle speed. Cars with ABS control modules not connected to the bus have a control module output for the speed of each wheel. A transistor grounds the respective output 29 times per wheel revolution.

The main instrument unit is connected through a lead to the output for the rear left wheel speed. The rear wheel is used as a non-driving wheel provides the most precise measurement of the road distance. The frequency modulated signal is converted to km/h in the main instrument unit, where a correction is also made depending on the programmed type of tire.