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A closer look at ABS

All anti-lock brake systems are designed to control tire skid and maintain vehicle stability and steering control during panic stopping. By continually monitoring the relative speeds of the wheel assemblies, the processor is able to respond to a skid situation by momentarily reducing the pressure to the brake assembly on the affected wheel(s). By rapidly pulsing the affected brake circuits, the braking load is reduced and allows traction to be regained, thus preventing lock up. Once the need for anti-lock passes, the system returns to normal brake operation.

Basic components consist of wheel speed sensors, electronic control module, hydraulic modulator assembly (which contains electrically operated solenoid valves), a pump motor, accumulator and an ABS light.

    • Wheel speed sensors consist of a magnetic pick-up and a toothed sensor ring. As the wheel assembly rotates, the magnetic pick-up develops an AC (alternating current) signal by making and breaking a magnetic field. The signal is converted into a DC (direct current) signal by the processor, which it uses as a speed indicator.
    • The ABS electronic control module is a microprocessor that uses key inputs from the wheel sensors and the brake switch. If the control module detects a difference in the deceleration rate between two or more wheels, it cycles the solenoid valves to regulate hydraulic pressure to the affected brake circuit(s) until the need for ABS control passes and the system returns to normal brake control.
    • The hydraulic modulator contains the solenoid valves for each brake circuit. The solenoids in the hydraulic modulator are used to open and close the passages to any given brake circuit to prevent lock up during hard or severe braking.
    • The pump motor is used on some applications to provide power assist for normal braking as well as brake pressure for ABS braking. Fluid pumped under pressure is stored in the accumulator and used to provide power assist.

Most ABS failures are caused by contamination, misdiagnosis by the installer, or use of the wrong fluid. The top two complaints about ABS from consumers are:

    • #1 - pedal pulsation during ABS mode, which is normal.
    • #2 - the vehicle does not stop any better than without ABS, this is due to incorrect operation by the driver pumping the brake pedal.

The proper way to use ABS when a panic stop is necessary is to simply press the brake pedal hard until braking is no longer necessary. Pumping the brake pedal will never allow the ABS to control the system.

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