System, including the components required to support advanced systems that use ABS technology to improve other aspects of vehicle stability and control. The key valves required for basic ABS are the holding valves and the reduction valves. The ABS actuator houses one holding valve and one reduction valve for each wheel circuit. The holding and reduction valves are both 2-position valves that can either block or allow the flow of hydraulic fluid. The key valves required for basic ABS are the holding valves and the reduction valves. The ABS actuator houses one holding valve and one reduction valve for each wheel circuit. The holding and reduction valves are both 2-position valves that can either block or allow the flow of hydraulic fluid.
553 Brake Systems Technician Handbook Pedal Brake Hold Pedal Brake Release SLAFR SLRFR Front Right Rear Left Front Left Rear Right SLARL SLRRL SLRFL SLAFL SLRRR SLARR SMC1 SMC2 SCSS SLAFR SLRFR Front Right Rear Left Front Left Rear Right SLARL SLRRL SLRFL SLAFL SLRRR SLARR SMC1 SMC2 SCSS Technical Training 129
553 Brake Systems Technician Handbook Normal Braking Holding Reduction During normal braking ABS is not activated. The pressure holding valve is open and the pressure reduction valve is closed. When the brake pedal is depressed, fluid passes freely through the holding valve to the wheel cylinder and the reduction valve prevents fluid pressure from escaping to the reservoir. When a wheel begins to slip, the Skid Control ECU initiates hold mode to prevent any additional pressure from reaching the wheel cylinder. • The holding valve closes to block the flow of fluid. • The reduction valve remains closed, preventing hydraulic fluid from going to the reservoir. If the wheel continues to slip in hold mode, the ECU initiates reduction mode to decrease pressure to the wheel cylinder. • The holding valve remains closed to block the flow of fluid. • The reduction valve opens, venting pressure from the wheel cylinder circuit to the reservoir. 130 Technical Training
553 Brake Systems Technician Handbook Wheel Speed Control The graph above shows how increase, holding and reduction modes work during a typical ABS cycle. A – Normal Braking During normal braking there is little difference between wheel speed and vehicle speed. ABS does not activate and master cylinder pressure is transmitted directly to the wheel cylinders. B – Limiting Wheel Slip When a wheel begins to slip, ABS initiates hold mode for that wheel. If the wheel continues to slip ABS briefly moves to reduction mode, decreasing pressure to the brake, then returns to hold mode and reevaluates the wheel’s slip ratio. ABS continues cycling between hold and reduction mode until the wheel’s slip ratio begins to decrease. C – Restoring Target Wheel Speed If brake pressure were to remain low after the wheel regained traction the wheel wouldn’t be able to help stop the vehicle. So as soon as the wheel begins to regain speed ABS starts increasing brake pressure, alternating between hold and increase modes as the wheel returns to target speed.
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- Fall '08
- Brake, Disc brake, Drum brake, Vehicle braking technologies, Brake Systems, disc brakes