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ABS modulation

ABS modulation



A. Speed

B. Braking force

C. Road speed

D. Wheel speed (of rotation)

E. Inlet valve open, outlet valve closed

F. Inlet valve closed, outlet valve closed

G. Outlet valve open, return pump actuated


The ABS control module continuously measures wheel speed. When any wheel exceeds the limit of optimum slip, the control module activates the ABS function and regulates the brake pressure applied to the wheel or wheels in 3 phases. The front wheels are regulated individually but the brake pressure applied to the rear wheels is the same on both, controlled by the wheel having the lowest friction (greatest slip).

Phase 1

Inlet valve closes. This prevents a rise in the brake pressure to the caliper and provides a flow of brake fluid upstream of the inlet valve for use in phase 3.

Phase 2

The outlet valve opens, at the same time opening the passage from the caliper to the hydraulic accumulator, which quickly receives the pressure from the caliper. At the instant that the outlet valve opens, the control module starts the return pump which pumps the fluid back to the master cylinder. The wheel will now rotate faster.

Phase 3

Simultaneously, the outlet valve closes and the return pump stops running. Thereafter, the inlet valve opens. This results in a drop in the wheel speed.

Phases 1 to 3

These phases are repeated until either the brake is released or sufficient adhesion (friction) returns between the tyre and the road surface. In the event of a circuit break or short circuit occurring, the valves will return to the rest position, and conventional braking without ABS modulation will be obtained.