Anti-Lock Brake System With Traction Control System - BOSCH 5.3: Operation

During normal driving and braking operations, ABS acts like a conventional braking system. Wheel speed sensors transmit an AC voltage signal about wheel acceleration, deceleration and slip value to EBCM. The EBCM controls braking by activating and deactivating BPMV.

The modulator valve consists of rapidly switching solenoid valves, accumulator chamber and a return pump. The valves decrease brake fluid pressure to each wheel to prevent wheel lock-up.

The modulator valve can supply only as much fluid pressure as applied through the master cylinder. The modulator valve alone cannot apply the brakes. A slight pulsation should be felt through brake pedal when ABS is active.

The BRAKE warning indicator and ABS indicator will illuminate when ignition switch is turned to RUN position and should go out after a 3 second self-test. If any indicator remains illuminated after vehicle is started, a system malfunction is indicated. See DIAGNOSIS & TESTING .

The BRAKE warning indicator will activate if parking brake is applied or brake fluid is low. The ABS indicator is controlled by EBCM and comes on if problem is detected with ABS. When indicator is illuminated, ABS is disabled and braking is accomplished by conventional brake system. If both warning indicators are illuminated, check conventional brake system for hydraulic system failure.

The TRAC OFF indicator illuminates when EBCM inhibits the traction control system due to a malfunction in the TCS. LOW TRAC indicator is illuminated when BCM detects that the traction control switch has been pressed, disabling the traction control or when EBCM detects a traction control event. The Instrument Panel Cluster (IPC) receives a class 2 signal from the EBCM requesting illumination. The IPC also sends a class 2 message to the BCM to activate an audible warning signal.