ABS Hydraulic Controls
During normal braking, the ABS lies dormant. The electronic control module monitors wheel speed for wheel skid or lockup. If an impending skid is detected, the EBCM activates solenoid valves in the hydraulic modulator to prevent the tire from locking up and skidding.
Some modulators contain a single three-way valve, and others contain two valves per brake unit. The hydraulic modulator above contains two valves for each brake caliper. A normally open inlet valve that allows fluid to flow from the master cylinder through the valve and to the brake units, and a normally closed outlet valve that blocks fluid from flowing back to the accumulator, reservoir, or pump. During normal braking, the inlet valve remains open, and the outlet valve remains closed. Brake fluid flow to the brake calipers is unaffected.
During a panic braking situation, the ABS operates these valves to prevent a wheel or wheels from skidding. If a wheel is about to skid, the inlet or isolation solenoid valve is activated to block off flow from the master cylinder and maintain pressure in the brake caliper. If the tire is still going to skid, even after isolation mode, the outlet valve is activated, and fluid pressure from the brake unit is exhausted to the accumulator. To quickly restore pressure, pump pressure is supplied to the brake caliper through the inlet valve as needed. This process happens very rapidly and can be felt as a pulsation in the brake pedal.