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ABS modulation

ABS modulation



A. Speed

B. Braking force

C. Road speed

D. Rotational speed of wheel

E. Inlet valve open, outlet valve closed

F. Inlet valve closed, outlet valve closed

G. Outlet valve open, return pump actuated


The TC/ABS control module continuously monitors the rotational speed of the wheels. When any wheel exceeds the slip threshold, the control module activates the ABS function and modulates brake pressure to the wheel or wheels in 3 phases. The front wheels are modulated individually but rear-wheel brake pressure is the same on both wheels, being controlled by the wheel having the lowest friction (greatest degree of 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 the brakes are released or until there is sufficient friction between the tyres and the road surface.