Anti-skid control with fail-safe function

A braking system with an anti-skid device for a motor vehicle, comprising:

• a master cylinder operatively coupled to a brake pedal of said vehicle;
• at least two wheel braking cylinders each associated with a wheel of the vehicle and each responsive to a hydraulic pressure developed in said master cylinder for braking at least two wheels of said vehicle;
• hydraulic pressure passage means provided between said master cylinder and said two wheel braking cylinders, said hydraulic pressure passage means including a single directional control valve having at least a first passage open position and a second passage closing position to open and close said hydraulic pressure passage means, respectively, said single directional control valve being arranged to be common for all of said wheel braking cylinders;
• a hydraulic pressure supplying device connected through at least two hydraulic pressure control valves to said at least two wheel braking cylinders, the number of said hydraulic pressure control valves being determined in accordance with the number of said wheel braking cylinders;
• means for detecting a speed of at least one of said vehicle wheels;
• means for detecting an acceleration of said at least one vehicle wheel;
• comparing means for comparing said wheel speed and said wheel acceleration with a predetermined reference speed and a predetermined reference wheel acceleration;
• first control means for determining if anti-skid control for said vehicle should be performed in accordance with said comparing of said comparing means and generating first and second signals, said first control signal being supplied therefrom to said single directional control valve to switch said single directional control valve from said passage-opening position to said passage-closing position and said second control signal being supplied to said hydraulic pressure control valves so that said hydraulic pressure control valves are controlled in response to said second control signals to regulate the hydraulic pressures applied to said wheel braking cylinders to prevent said vehicle wheels from being in a skidding condition, each of said control signals performing a switching operation among pressure-controlling modes of each of said hydraulic pressure control valves so as to cause a hydraulic pressure applied to each of said wheel braking cylinders to approach a predetermined value;
• and
• second control means coupled to said first control means for receiving each of said second control signals to monitor the operating condition of each of said hydraulic pressure control valves and determining whether each of said hydraulic pressure control valves is in an abnormal condition, the abnormality of said hydraulic pressure control valve being determined when said second control signal indicative of a specific pressure-controlling mode being successively supplied to said second control means for a predetermined time period, said second control means generating a third control signal in response to said abnormal condition of said at least one hydraulic pressure control valve, so that said single directional control valve is switched to the passageopening condition.