ABS HYDRAULIC CIRCUITS AND VALVE OPERATION

Through the following operation descriptions and diagrams. The function of the various hydraulic control valves in the ABS system will be described. The fluid control valves mentioned below, control the flow of pressurized brake fluid to the wheel brakes during the different modes of Anti-Lock Braking.

NORMAL BRAKING

ISOLATION VALVES

Open to primary and secondary master cylinder brake fluid supply (Fig. 11).

DECAY AND BUILD VALVES

Closed, not allowing for the build-up or release of brake fluid supply (Fig. 11).

The brake pedal is applied. The travel of the brake pedal closes primary, secondary and booster servo circuits from fluid supply at the fluid reservoir. Brake fluid from the primary and secondary circuits flows through the open isolation valves and applies the wheel brakes. Fluid from the booster servo circuit

does not flow to the wheel brakes. The fluid flow is blocked by the closed build valves and check valves.

POWER ASSIST

The boost control valve shuttles between its three positions to provide power assisted braking (Fig. 11).

ABS BRAKING-BUILD PRESSURE

ISOLATION VALVES

Closed, isolating wheel brakes from master cylinder primary and secondary fluid supplies and open to booster servo circuit pressure through open build valves (Fig. 12).

DECAY VALVES

Closed, not allowing the escape of pressurized fluid supply from the hydraulic system (Fig. 12).

BUILD VALVES

Open, allowing booster servo circuit pressure to flow to the wheel brakes through the isolation valves (Fig. 12).

The boost control valve shuttles between its three positions to provide power assisted braking (Fig. 12).

ABS BRAKING-HOLD PRESSURE

For explanation purposes we will assume all speed sensors are sending the same wheel speed information, requiring the same modulation at the same rate.

ISOLATION VALVES

Closed, isolating the wheel brakes from the master cylinder primary and secondary fluid supplies. Build and decay valves are closed preventing any fluid from reaching the open isolation valves (Fig. 13).

DECAY AND BUILD VALVES

Closed, not allowing fluid supply to reach the open isolation valves (Fig. 13).

ISOLATION VALVES

Closed, isolating the wheel brakes from the master cylinder primary and secondary fluid supplies (Fig. 14).

DECAY VALVES

Open, allowing release of fluid pressure through decay valve to the fluid reservoir (Fig. 14).

BUILD VALVE

Closed, blocking booster servo circuit fluid to wheel brakes (Fig. 14).