System Description

HINT: 

• The skid control ECU is located within the ABS & TRACTION actuator assembly.
• The yaw rate sensor and acceleration sensor are combined and in a signal 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: Anti-lock Brake System - Communication Diagram

   Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
   1. Operation description

      The skid control ECU detects 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 hydraulic 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 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 vehicle behavior.

   Fig 2: Electronic Brake Force Distribution - Communication Diagram

   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 hydraulic pressure of each wheel cylinder and splits the control power properly between the front and rear wheels and the right and left wheels.

      Both of the ABS and BRAKE warning lights come 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: Brake Assist - Communication Diagram

   Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
   1. Operation description

      The skid control ECU receives the hydraulic 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. TRAC (Traction Control)

   The TRAC system helps prevent the drive wheels from slipping if the driver presses down on the accelerator pedal excessively when starting off or accelerating on a slippery surface.

   Fig 4: Traction Control - Communication Diagram

   Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
   1. Operation description

      The skid control ECU detects wheelspin by receiving signals from each speed sensor and the ECM via CAN communication. The skid control ECU controls engine torque with the ECM via CAN communication and brake hydraulic pressure through the pump and solenoid valve.

      The SLIP indicator light blinks when the system is operating. When there is a malfunction in the TRAC system, the SLIP indicator light will come on.

      The VSC OFF switch cancels traction control operation. If TRAC control is turned off, the TRAC OFF indicator light will come on.

5. 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 - Communication Diagram

   Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
   1. Operation description

      The skid control ECU determines the vehicle operating conditions based on signals received from the speed sensors, yaw rate and acceleration sensor, and steering angle sensor. The skid control ECU sends signals via CAN communication to the ECM in order to control engine torque. The skid control ECU controls brake hydraulic pressure using the pump and solenoid valve.

      The SLIP indicator light will blink when the system is operating. If a malfunction occurs in the VSC system, the SLIP indicator light will come on.

      The VSC OFF switch cancels VSC control operation. If VSC control is turned off, the TRAC OFF indicator and VSC OFF indicator lights will come on.

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 the wheel brake pressure by the ABS & TRACTION actuator assembly) should be implemented.
2. The SLIP indicator light blinks to inform the driver that the VSC system is operating. The SLIP indicator light also blinks when traction control is operating, and the operation being performed is displayed.

1. When a failure occurs in the ABS with BA, TRAC or VSC systems, the ABS warning and SLIP indicator lights come on and ABS with BA, TRAC and VSC operations are prohibited. In addition, when there is a failure that disables the EBD operation, the BRAKE warning light also comes on and the EBD operation is prohibited.
2. If control is prohibited due to a malfunction during operation, control will be disabled gradually.

   This is to avoid sudden vehicle instability.

1. When the vehicle speed first becomes approximately 6 km/h (4 mph) or more after the ignition switch is turned on (IG), each solenoid valve and motor of the ABS & TRACTION actuator is sequentially activated to perform an electrical check. During the initial check, the operating sound of solenoid valve and motor can be heard from the engine compartment, but this is not a malfunction.