System Description

SYSTEM DESCRIPTION
HINT:
- The skid control ECU is located within the ABS and TRACTION actuator assembly.
- The yaw rate sensor and acceleration sensor are combined into a single unit. This unit communicates with the skid control ECU through CAN communication.
1. ABS (Anti-lock Brake System
  The ABS helps prevent the wheels from locking when the brakes are applied firmly or when braking on a slippery surface.
  
  Fig 1: Vehicle Stability Control System - System Description (CAN Communication)
  Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
  1. Operation description
    The skid control ECU detects the wheel lock condition by receiving vehicle speed signals from each speed sensor, and sends control signals to the pump motor and solenoid valve.
    
    The pump motor and solenoid valve avoid wheel lock by controlling the brake fluid pressure of each wheel cylinder.
    
    The ABS warning light comes on when the ABS system is malfunctioning.
2. EBD (Electronic Brake Force Distribution)
  The EBD control utilizes the ABS, realizing proper brake force distribution between the front and rear wheels in accordance with driving conditions. In addition, when braking while cornering, it also controls the brake forces of the right and left wheels, helping to maintain proper vehicle control.
  
  Fig 2: Vehicle Stability Control System - System Description (EBD (Electronic Brake Force Distribution))
  Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
  1. Operation description
    The skid control ECU receives the speed signal from each speed sensor to detect the slip condition of the wheels and sends the control signal to the solenoid.
    
    The solenoid valve controls the fluid pressure of each wheel cylinder and splits the control power properly between the front and rear wheels and the right and left wheels.
    
    The BRAKE warning light comes on to indicate a malfunction in the EBD system.
3. BA (Brake Assist)
  The primary purpose of the brake assist system is to provide auxiliary brake force to assist the driver who cannot generate a large enough brake force during emergency braking, thus helping to maximize the vehicle's brake performance.
  
  Fig 3: Vehicle Stability Control System - System Description (BA (Brake Assist))
  Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
  1. Operation description
    The skid control ECU receives the speed signal from each speed sensor and the fluid pressure signal from the master cylinder pressure sensor to determine whether brake assist is necessary or not. If brake assist is deemed necessary, the skid control ECU sends control signals to the pump motor and solenoid. The pump and the solenoid valve then control the pressure applied to each wheel cylinder.
    
    The ABS warning light comes on to indicate a malfunction in the BA (brake assist) system.
4. Regarding changing of TRAC and VSC operation permission / disablement through VSC OFF switch
  1. Changing modes
    Through the VSC OFF switch, the vehicle can be changed to normal mode, TRAC OFF mode and VSC OFF mode.
    1. Normal mode
      When the vehicle is in TRAC OFF mode or VSC OFF mode, and the VSC OFF switch is pressed for a short amount of time, the vehicle changes to normal mode. During normal mode, TRAC and VSC operation is possible. After the ignition switch is turned ON, the vehicle is in normal mode.
    2. TRAC OFF mode
      When the vehicle is in normal mode and the VSC OFF switch is pressed for a short amount of time, the vehicle changes to TRAC OFF mode. During TRAC OFF mode, the combination meter's slip indicator light illuminates, and TRAC operation is disabled. During TRAC OFF mode, if the vehicle speed increases, the vehicle automatically returns to normal mode.
    3. VSC OFF mode
      When the vehicle is stopped and the VSC OFF switch is pressed for 3 seconds or more, the vehicle changes to VSC OFF mode. During VSC OFF mode, the multi-information display shows a VSC OFF display, and the combination meter's slip indicator light illuminates. Also, VSC and TRAC operation is disabled. During VSC OFF mode, even if the vehicle speed increases, the vehicle does not return to normal mode.
5. TRAC (Traction Control)
  The TRAC system helps prevent the drive wheels from slipping when the driver depress the accelerator pedal excessively when starting off or accelerating on a slippery surface.
  
  Fig 4: Vehicle Stability Control System - System Description (TRAC (Traction Control))
  Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
  1. Operation description
    The skid control ECU detects vehicle's slip condition by receiving signals from each speed sensor and ECM via CAN communication. The skid control ECU controls engine torque with the ECM via CAN communication and oil pressure through the pump and solenoid valve.
    
    The SLIP indicator light blinks when the system is operating. If a malfunction occurs in the TRAC system, the VSC warning light and SLIP indicator light will come on and a DTC will be displayed on the multi-information display.
    
    The VSC OFF switch stops traction control operation.
    
    HINT:
    
    When the VSC OFF switch is pressed, the vehicle changes to VSC OFF mode, and TRAC operation is disabled. During TRAC OFF mode, if the VSC OFF switch is pressed again or the vehicle speed increases to 50 km/h (13.8 mph), TRAC operation becomes possible. During TRAC OFF mode, the SLIP indicator light illuminates.
6. VSC (Vehicle Stability Control)
  The VSC system helps prevent the vehicle from slipping sideways as a result of strong front or rear wheel skid during cornering.
  
  Fig 5: Vehicle Stability Control System - System Description (VSC (Vehicle Stability Control))
  Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
  1. Operation description
    The skid control ECU determines vehicle condition by receiving signals from the speed sensor, yaw rate and acceleration sensor and steering angle sensor. The skid control ECU controls engine torque with the ECM via CAN communication and fluid pressure with the pump and solenoid valve.
    
    The SLIP indicator light blinks and the skid control buzzer sounds when the system is operating. If a malfunction occurs in the VSC system, the VSC warning light and SLIP indicator light will come on and a DTC will be displayed on the multi-information display.
    
    The VSC OFF switch stops VSC operation.
    
    HINT:
    
    When the VSC OFF switch is pressed for 3 seconds or more with the vehicle stopped, the VSC system changes to VSC OFF mode. If the VSC OFF switch is pressed again with the system in VSC OFF mode, the TRAC and VSC systems can be operated. When the system is in VSC OFF mode, the SLIP indicator light and VSC OFF indicator light illuminate.
ABS with EBD, BA, TRAC and VSC OPERATION
1. The skid control ECU calculates vehicle stability tendency based on the signals from the speed sensor, yaw rate and acceleration sensor and steering angle sensor. In addition, it evaluates the results of the calculations to determine whether any control actions (control of the engine output torque by electronic throttle control and of the wheel brake pressure by the ABS and TRACTION actuator assembly) should be implemented.
2. The SLIP indicator blinks and the skid control buzzer sounds to inform the drive that the VSC system is operating. The SLIP indicator also blinks when traction control is operating, and the operation being performed is displayed.
FAIL-SAFE
1. When a failure occurs in the ABS with BA, TRAC and VSC systems, the ABS and VSC warning lights, and SLIP indicator light come on and ABS with BA, TRAC and VSC operations are prohibited. In addition to this, when there is a failure that disables the EBD operation, the BRAKE warning light also comes on and the EBD operation is prohibited (see DIAGNOSIS SYSTEM ).
2. If control is prohibited due to a malfunction during operation, control will be disabled gradually. This is done to avoid sudden vehicle instability.
INITIAL CHECK
1. When the vehicle speed first becomes approximately 4 mph (6 km/h) or more after the engine switch is turned on (IG), each solenoid valve and motor of the brake actuator is sequentially activated to perform an electrical check. During the initial check, the operating sound of the solenoid valve and motor can be heard from the engine compartment, but this is not a malfunction.
SERVICE MODE
1. VSC operation can be disables by operating the intelligent tester.
  HINT:
  
  Refer to the intelligent tester operator's manual for further details.

FUNCTION OF COMPONENTS

Components	Function
Front speed sensor (Semiconductor speed sensor, magnetic sensor rotor)	Detect the wheel speed and sends the signal to the skid control ECU. The front speed sensor (semiconductor speed sensor) with integrated bearing and the sensor rotor (magnetic sensor rotor) are installed on the front axle hub. Magnetic sensor rotor is equipped with north and south poles (48 poles each) which are evenly located in a circumferential direction. The magnetic field changes when the magnetic sensor rotor rotates. The semiconductor speed sensor detects the change and outputs it as the vehicle speed pulse.
Rear speed sensor (Semiconductor speed sensor, magnetic sensor rotor)	The rear speed sensor (semiconductor speed sensor) and the sensor rotor (magnetic sensor rotor) are installed on the rear axle carrier. The rear speed sensor has the same structure and operation as the front speed sensor.
Skid control ECU (housed in ABS and TRACTION actuator assembly)	Processes the signals sent from each sensor to control the ABS, BA, TRAC, and VSC. Sends and receives the control signals to or from the ECM, yaw rate and acceleration sensor, and steering angle sensor, etc. via CAN communication.
ABS and TRACTION actuator assembly	Consists of the master cylinder cut solenoid valve, retention solenoid valve, pressure reduction solenoid valve, pump motor, and reservoir, and adjusts the brake fluid pressure applied to each wheel cylinder. Houses the skid control ECU.
Solenoid relay	Supplies power to each solenoid. Housed in the skid control ECU.
Motor relay (ABS MOTOR1 relay)	Supplies power to the pump motor. Installed in the engine room No.3 R/B.
Fail-safe relay (ABS MOTOR2 relay)	Cuts off power to the motor when the pump motor circuit malfunctions. Installed in the engine room No.3 R/B.
Steering angle sensor	Installed in the combination switch. Detects the steering amount and direction and sends the signals to the skid control ECU via CAN communication. Has the magnetic resistance element which detects the rotation of the magnet housed in the detection gear in order to detect the changes of magnetic resistance and the steering amount and direction.
Yaw rate and acceleration sensor	Acceleration sensor measures the capacity of the condenser that changes the distance between the electrodes depending on G force, which occurs when the vehicle is accelerated, and converts the measured value into electrical signals. Yaw rate sensor detects the vehicle's angular velocity (yaw rate) in the vertical direction based on the amount and direction of the piezoelectric ceramics deflection. Sends signals to the skid control ECU via CAN communication.
Master cylinder pressure sensor	Detects the fluid pressure in the master cylinder. Housed in the ABS and TRACTION actuator assembly.
Brake pedal load sensing switch	Installed on the brake pedal. Turned on when the brake pedal is depressed. The brake pedal load sensing switch signals are used as control information for BA (Brake Assist).
ECM	Controls the engine output when TRAC and VSC are operating with the skid control ECU via CAN communication.
Skid control buzzer	Intermittently sounds to inform the driver that the VSC is operating.
ABS warning light	Comes on to inform the driver that a malfunction in the ABS or BA (Brake Assist) has occurred. Blinks to output DTC.
BRAKE warning light	Comes on to inform the driver that the parking brake is ON when the system is normal, the brake fluid has decreased, or the master cylinder pressure is lost as the brake pedal load sensing switch is pressed. Comes on to inform the driver that a malfunction in the EBD has occurred.
VSC warning light	Comes on to inform the driver that a malfunction in the VSC system has occurred.
SLIP indicator light	Blinks to inform the driver that TRAC and VSC are operating. The indicator light comes on when a malfunction is detected. Come on when the VSC OFF switch is ON to inform the driver of the TRAC or VSC prohibited state.
VSC OFF indicator light	Comes on when the VSC OFF switch is ON to inform the driver of the VSC prohibited state.
Multi-information display	Displays to inform the driver that a malfunction in the VSC system has occurred. Displays to output DTC.

HILL-START ASSIST CONTROL
1. When the vehicle starts off on a steep hill, hill-start assist control detects the backward descent of the vehicle and applies hydraulic pressure to the vehicle's brakes to reduce the reversing speed of the vehicle.
  After a maximum of 5 seconds, fluid pressure is gradually released and control will be completed.
DYNAMIC RADAR CRUISE CONTROL
1. This system is designed to maintain an appropriate distance to the vehicle ahead, which is proportional to the vehicle speed. The distance to the vehicle ahead is measured based on signals sent from the millimeter wave radar sensor. To maintain the appropriate distance, this system controls the engine output, and also applies the brake if the distance to the vehicle ahead is too short.
PRE-COLLISION CONTROL
1. The pre-collision system predicts frontal collisions with other vehicles or obstructions. By winding up the seat belts prior to collision, and performing brake control and brake assist control, it lessens the impact during the collision.
2. The extremely high frequency radar recognizes if there is a vehicle or obstruction on the road ahead, and the distance control ECU makes a judgment from the position of the object, the speed and the road surface whether a collision is unavoidable.
3. The distance control ECU sends this information via a CAN communication to the seat belt control ECU, suspension control ECU and skid control ECU, to operate each control element of the pre-collision system.