ABS General System Description

Fig. 1 Bosch Anti-lock Brake (ABS II) System:

The Bosch anti-lock brake II system (ABS II), Fig. 1, maintains vehicle steerablity, directional stability, and maximum deceleration under severe braking conditions. The ABS II performs this function by monitoring rotational speed and controlling line pressure of each vehicle wheel during a braking maneuver, to prevent the braked wheels from locking.
When ignition switch is turned to On position, amber ANTI-LOCK warning lamp in the instrument panel illuminates. When engine is started, it goes out. If ABS warning light does not go out or comes on permanently while driving, it indicates a fault in ABS system.
When starting vehicle, the control module performs a functional check of electrical circuitry. The test cycle checks components of the monitoring circuit as well as the logic section. Test sample signals are sent to the control module, which checks whether or not the correct output signals are available.
Since the ABS system may not be used every day, there is an additional test which actually runs the modulator valve. Whenever ignition switch is turned to On position and vehicle reaches 4 mph, the test starts. Operation of the modular valve can be heard and felt, if the driver's foot is on the brake pedal. This test is performed once, each time the vehicle is started.
This system continuously monitors the following components: modulator valve, control module, lateral acceleration switch, wheel speed sensors, wiring harness and relays.
The control module constantly monitors its own supply voltage. If supply voltage drops below a predetermined value, ABS will be switched Off and the amber ANTI-LOCK lamp will come on. When supplied voltage returns to or exceeds the predetermined value, ANTI-LOCK lamp will go off. If a fault occurs in ABS, the ANTI-LOCK warning lamp comes on and ABS will be switched off. The system will remain off until vehicle is restarted at which time the functional check is repeated.