To prevent the tires from locking and the loosing the ability to turn the steering wheel during emergency braking, it is effective to repeatedly depress and release the brake. There is no time to do this, however, during emergency braking. The ABS uses a computer to determine the rotating condition of the four wheels during braking and can automatically depress and release the brake. The difference in ratio between the vehicle’s body speed and the speed of the wheels is known as the slip ratio. When the difference between the wheel speed and vehicle speed becomes too large, slipping occurs between the tires and the road surface. This also creates friction and may eventually act as a braking force and slow the speed of the vehicle.
The relationship between braking force and the slip ratio can better understood from the graph at the left. Braking force is not proportional to the slip ratio, and is at its maximum when the slip ratio is between 1and 30%. Beyond 30%, the braking force gradually declines. Therefore, in order to maintain a maximum level of braking force, the slip ratio should be maintained within the range of 1to 30% at all times. In addition, it is also necessary to keep cornering force at a high level to maintain directional stability. In order to do this, the ABS is designed to maximize brake performance by utilizing a slip ratio of 10-30% regardless of road conditions, while also keeping cornering force as high as possible to maintain directional stability.
When the Skid Control ECU has Failed
1. On slippery road surfaces having a low coefficient of friction (), since braking distance increases in comparison with road surfaces having a high value of even if the ABS is activated, speed must be reduced when traveling over those road surfaces.
2. On rough roads, or on gravel or road with fresh snow, operation of the ABS may result in a longer stopping distance than for vehicles not fitted with an ABS. In addition, sound and vibrations are generated when the
ABS is activated informing the driver that the ABS is in operation.
The ABS consists of the following parts.
1. Skid Control ECU This determines the amount of slip between the wheel and the road surface based on the signals from the sensors, and controls the brake actuator. Recently, some models have the Skid Control ECU built in the brake actuator.
2. Brake actuator The brake actuator controls the hydraulic pressure of the wheel cylinders by the Skid Control ECU output signal.
3. Speed sensor The speed sensor detects the wheel speeds of each of the four wheels and sends a signal to the Skid Control ECU.
4. Combination meter (1) ABS warning light When the ECU detects a malfunction in the ABS or brake assist system, this light turns on to warn the driver. (2) Brake system warning light When this turns on at the same time as the ABS warning light, it warns the driver that there is a malfunction in the ABS and EBD system.
Normally, the warning light is kept off by the signal sent to the combination meter or the ABS control relay by the ECU. If the ECU fails and there is no signal, the ABS warning light, Brake system warning light, TRC OFF indicator light (vehicles equipped with TRC), and VSC warning light (vehicles equipped with VSC) always turn on.
5. Stop light switch This detects that the brake pedal has been depressed and sends a signal to the Skid Control ECU. The ABS uses a stop light switch signal. However, even if the stop light switch signal is not input because of the time that the stop light switch has failed, the ABS control is performed when the tires become to be locked up. In this case, control begins after the slip rate has become high (wheels tend to lock up) than when the stop light switch is functioning normally.
6. Deceleration sensor(Some models only.) The Deceleration sensor senses the vehicle’s rate of deceleration and sends signals to the Skid Control ECU. The ECU judges the precise road surface conditions using these signals and takes appropriate control measures.