Principles of Operation: Abs

The ABS module manages anti-lock braking to maintain vehicle control during deceleration.

When the ignition switch is turned to the RUN position, the ABS module does a preliminary electrical check of the wheel speed sensors and their circuits. At approximately 20 km/h (12 mph), the hydraulic pump motor is turned on for approximately 1/2 second. During this time a buzzing or humming noise may be heard and a vibration may be felt in the brake pedal. This is a normal condition. If a malfunction is detected, the system is disabled and the ABS warning indicator illuminates. However, the base power-assist braking system functions normally.

The ABS module monitors and compares the rotational speed of each wheel. Wheel speeds are measured by the wheel speed sensor, which electrically senses each tooth of the sensor ring as it passes through the magnetic field of the sensor. This generates a square-wave signal that is sent to the ABS module. The frequency of the square-wave signal will increase or decrease as wheel speed increases or decreases. The ABS module compares all 4 wheel speed sensor signals to each other. The vehicle speed signal it receives over the high-speed controller area network (HS-CAN) bus and the brake pedal input to determine if a wheel lock event is occurring. When the ABS module detects an impending wheel lock, the ABS module commands the hydraulic pump motor on and commands the hydraulic control unit (HCU) to open and close the appropriate solenoid valves to modulate the brake pressure to the individual brake caliper(s). Once the affected wheel(s) return to the desired speed, the ABS module commands the HCU to return the solenoid valves to their normal position.