When the ABS is activated, a rapid pulsation is felt on the brake pedal. This pulsation is due to rapid changes in the position of individual solenoid valves that act to prevent the wheels from locking up. This pulsation occurs only when the ABS is activated and stops as soon as normal braking is restored or when the vehicle comes to a stop. The presence of extraneous sounds and pulsations when the ABS is activated is not a malfunction.
Vehicles equipped with ABS can be stopped by simply depressing the brake pedal and the operation of the brake pedal during normal braking is no different from non-ABS equipped systems. Maintaining a constant force on the brake pedal provides a shorter stopping distance while maintaining vehicle directional stability. A typical ABS system cycle is as follows:
Pressure holding
The ECM closes the isolation valve and holds the bleed valve closed to isolate a locked wheel when slip is detected. At the same time, the pressure remains constant, neither increasing nor decreasing.
Pressure drop
If the wheel continues to slip while holding pressure, there is a decrease in pressure in the brake line. The electronic brake control unit reduces the pressure in the brake circuits of individual wheels when wheel lock is detected. The isolation valve is closed and the bleed valve opens. Excess fluid enters the battery until the pump returns it to the expansion tank or brake master cylinder.
Increase in pressure
After the wheel slip stops, the pressure in the brake line increases. The electronic brake control unit increases the pressure in the brake circuits of individual wheels to reduce the speed of their rotation. The isolation valve opens and the bleed valve closes. The increase in pressure occurs due to the fluid coming from the brake master cylinder.
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