A conventional vehicle motion control system in disclosed in Japanese Laid-Open Publication No. 8-142846. The system includes a braking device for applying a braking force to each wheel of the vehicle. The system further includes a lateral acceleration sensor for sensing the lateral acceleration of the vehicle and a yaw rate sensor for sensing the rate of the vehicle. A vehicle slip angular velocity is calculated on the basis of output signals of the lateral acceleration sensor and the yaw rate sensor and then a vehicle slip angle is calculated on the basis of the vehicle slip angular velocity. A vehicle motion is determined based on the vehicle slip angular velocity and the vehicle slip angle. That is, it is determined whether or not an excessive oversteer of the vehicle exists based on the vehicle slip angular velocity and the vehicle slip angle.
A motion control device actuates the braking device to apply a braking force to at least one of the wheels on the basis of the vehicle motion and irrespective of depression of the brake pedal in order to maintain stability of the vehicle in motion. More particularly, when excessive oversteer occurs during cornering, a braking force is applied to a front wheel on the outside of the curve in order to restrain the oversteer.
However, since the system does not detect abnormal conditions of the lateral acceleration sensor and the yaw rate sensor, the vehicle motion is inaccurately controlled when at least one of the sensors is experiencing an abnormal condition. Therefore, it is necessary to determine whether or not an abnormal condition exists in at least one of the lateral acceleration sensor and the yaw rate sensor.