Antilock System Operation

Fig. 1 Bosch ABS/ASR Brake System.:

DESCRIPTION
The anti-lock brake system (ABS) with integrated Acceleration Slip Regulation (ASR) system or Traction Control System (TCS), Figs. 1 and 2, maintains vehicle maneuverability under severe braking conditions on most road surfaces. The ABS/ASR system monitors each wheel speed during braking. The Electronic Brake & Traction Control Module (EBTCM) uses this information to produce signals to prevent the braking wheels from locking.

The modulator valve regulates brake fluid pressure based on road conditions as interpreted by the EBTCM, regardless of master cylinder output pressure. The modulator valve can maintain, reduce and increase brake fluid pressure to the calipers under ABS conditions, but it cannot increase pressure above that applied from the master cylinder. However, during ASR or TCS operation, the modulator can apply pressure to the rear calipers if the requires that a wheel be slowed or regulated.

The ASR and TCS traction systems use engine spark retard, throttle close-down and rear brake intervention to provide better acceleration and vehicle stability. These systems can function up to maximum vehicle speed. They are intended to enhance the directional control, traction and maneuverability. The TCS and ASR systems use input information from wheel speed and vehicle speed to allow traction priority to low speeds and directional control at high speeds, vehicle acceleration and throttle position, during low vehicle acceleration the controls are more sensitive and brake application input.

When the ignition switch is turned to the On position, an 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 the anti-lock brake 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 system may not be used every day, there is an additional test which actually runs the modulator valve. Whenever the ignition switch is turned to the 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, the 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.