System Description

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: System Description Diagram - ABS (Anti-Lock Brake System)
  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: System Description Diagram - 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 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: System Description Diagram - BA (Brake Assist)
  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: System Description Diagram - 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 hydraulic pressure through the pump and solenoid valve. The SLIP indicator light blinks when the system is operating.
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: System Description Diagram - 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 hydraulic pressure with the pump and solenoid valve. The SLIP indicator light blinks and the buzzer sounds when the system is operating.
ABS with EBD & BA & TRAC & 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 the wheel brake pressure by the ABS & TRACTION actuator assembly) should be implemented.
2. The SLIP indicator blinks and the skid control buzzer sounds to inform the driver 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 FUNCTION
1. When a failure occurs in the ABS with BA & TRAC & VSC systems, the ABS warning light and the VSC warning light come on and ABS with BA & TRAC & 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.
2. If control is prohibited due to a malfunction during operation, control will be disabled gradually. This is to avoid sudden vehicle instability.
INITIAL CHECK
When the vehicle speed first becomes approximately 4 mph (6 km/h) or more after the ignition 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 solenoid valve and motor can be heard from the engine compartment, but this is not a malfunction.

FUNCTION OF COMPONENTS

COMPONENTS FUNCTION

Components	Function
Front and speed sensors	Detect the wheel speed and send the signal to the skid control ECU.
Skid control ECU (housed in ABS & 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 & 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 hydraulic pressure applied to each wheel cylinder. Housed in the skid control ECU. Housed in the skid control ECU
Solenoid relay	Supplies power to each solenoid. Houses the skid control ECU
Motor relay (VSC No. 1 relay)	Supplies power to pump motor. Installed in the engine room R/B No. 3.
Fail-safe relay (VSC No. 2 relay)	Cuts off power to the motor when the pump motor circuit malfunctions. Installed in the engine room R/B No. 3.
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 amount of deflection of beam in the sensor, 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 hydraulic pressure in the master cylinder. Housed in the ABS & TRACTION actuator assembly.
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 or the brake fluid has decreased. Simultaneously comes on with the ABS warning light to inform the driver that both ABS and EBD cannot operate.
VSC warning light	Comes on to inform the driver that a malfunction in the VSC system has occurred. Blinks to output DTC.
SLIP indicator light	Blinks to inform the driver that TRAC and VSC are operating.

SERVICE MODE (VSC and TRAC prohibited mode)
HINT:

VSC and TRAC control can be prohibited by switching to service mode.
1. Procedures for service mode.
HINT:
- Perform step (See procedure "A") to (See procedure "E") within 30 seconds.
- Perform step (See procedure "C") and (See procedure "D") within 15 seconds each.
- Make sure that the shift lever is in the P position.
1. Turn the ignition switch on (IG).
2. Apply the parking brake (Procedure "A").
3. Depress and release the brake pedal twice (Procedure "B").
4. While holding the brake pedal down, release and apply the parking brake twice (Procedure "C").
5. With the parking brake applied, depress and release the brake pedal twice (Procedure "D").
6. Check that the VSC warning light and SLIP indicator light come on (Procedure "E").
HINT:
- If the VSC warning light and SLIP indicator light do not come on, repeat the procedures.
- Turning the ignition switch off ends service mode.