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ABS modulation

ABS modulation



A. Speed

B. Brake force

C. Vehicle speed

D. Wheel speed

E. Inlet valve open, outlet valve closed

F. Inlet valve closed, outlet valve closed

G. Outlet valve open, return pump activated


The ESP control module continually monitors wheel speed. If the value for a wheel exceeds the slip limit, the control module activates the ABS function and modulates the brake pressure to the wheel(s) in 3 phases. The modulation of the front brakes is individual, while the rear brake pressure is distributed evenly and determined by the wheel with least grip (greatest slip).

Phase 1

The intake valve closes. This prevents an increase in brake pressure to the brake unit and allows brake fluid to accumulate in front of the intake valve for use in phase 3.

Phase 2

The outlet valve opens and also the passage from the brake unit to the pressure accumulator that can quickly take away the pressure from the brake unit. At the same moment that the outlet valve opens the control module starts the return pump, which pumps the pressure back to the master cylinder. The wheel rotational speed will increase.

Phase 3

The outlet valve closes, the return pump stops and then the intake valve opens. This causes the wheel rotation speed to decrease.

Phase 1 to 3

Repeats until braking ceases or sufficient friction between the tyre and road surface is achieved.