System Description

HINT:

• The skid control ECU is located within the brake actuator assembly.
• The yaw rate sensor and acceleration sensor are combined in 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: Identifying Anti-Lock Brake System Function Diagram

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

      The skid control ECU detects wheel lock based on speed signals received from the wheel speed sensors. Based on this information, the skid control ECU controls the pump motor and solenoid valves. The pump motor and solenoid valves are used to prevent wheel lock by controlling the hydraulic pressure applied to the brakes at each wheel. The ABS warning light will come on when the system is malfunctioning.

      HINT:

      For vehicles with the multi-information display, the master caution indicator light will come on and a check ABS message will be displayed on the multi-information display if the ABS control cannot be performed.

2. EBD (Electronic Brake force Distribution)

   The EBD control utilizes ABS, and performs proper brake force distribution between the front and rear wheels in accordance with driving conditions. When braking while cornering, it also controls the brake force distribution between the right and left wheels, helping to maintain vehicle behavior.

   Fig 2: Identifying Electronic Brake Force Distribution Function Diagram

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

      The skid control ECU receives a speed signal from each wheel speed sensor, and uses these signals to detect locking of the wheels. The ECU uses this information in order to determine appropriate control of the solenoid valves. The solenoid valves control the hydraulic pressure applied to the brake cylinder at each wheel. In this way, the solenoid valves are used to control the brake power split between the front and rear, and left and right wheels. The ABS waning and brake warning lights will come on if there is a malfunction in the EBD system.

      HINT:

      For vehicles with the multi-information display, the master caution indicator light will come on and a brake malfunction message will be displayed on the multi-information display if the EBD system is malfunctioning.

3. BA (Brake Assist)

   The primary purpose of the brake assist system is to provide auxiliary brake force to assist a driver who cannot generate a large enough brake force during emergency braking, thus helping to maximize the brake performance of the vehicle.

   Fig 3: Identifying Brake Assist Function Diagram

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

      The skid control ECU receives a speed signal from each speed sensor and a fluid pressure signal from the master cylinder pressure sensor to determine whether brake assist is necessary. If brake assist is necessary, the skid control ECU sends control signals to the pump motor and solenoid. The pump and the solenoid valves then control the pressure applied to each wheel cylinder.

      The ABS warning light will come on to indicate a malfunction in the BA system.

      HINT:

      For vehicles with the multi-information display, the master caution indicator light will come on and a check ABS message will be displayed on the multi-information display if the BA system is malfunctioning.

4. TRAC (Traction Control)

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

   Fig 4: Identifying Traction Control Function Diagram

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

      The skid control ECU detects wheel spin by receiving signals from each speed sensor and the ECM via CAN communication. 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 valves.

      The slip indicator light blinks when the system is operating. When there is a malfunction in the TRAC system, both the VSC OFF indicator light and slip indicator light will come on.

      The VSC OFF switch stops traction control operation.

      HINT:

      For vehicles with the multi-information display, the master caution indicator light will come on and a check VSC system message will be displayed on the multi-information display if the TRAC system is malfunctioning.

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: Identifying Vehicle Stability Control Function 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 wheel 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 valves.

      The slip indicator light blinks, and the skid control buzzer sounds when the system is operating. If a malfunction occurs in the VSC system, both the VSC OFF indicator light and slip indicator light will come on.

      The VSC OFF switch stops VSC control operation.

      HINT:

      For vehicles with the multi-information display, the master caution indicator light will come on and a check VSC system message will be displayed on the multi-information display if the VSC cannot be performed.

1. The downhill assist control generates four-wheel hydraulic pressure when the downhill assist control switch is pressed and the accelerator and brake pedals are not depressed in order to maintain a constant low vehicle speed while preventing wheel lock.

   HINT:

   The downhill assist control operates when all of the following conditions:

   • Downhill assist control switch is ON.
   • Shift lever is in the S1 or R position.
   • Accelerator pedal and brake pedal are not depressed.
   • Descending a hill at a vehicle speed of 25 km/h (16 mph) or less.

1. The hill-start assist control generates four-wheel hydraulic pressure when the brake pedal is further depressed while the vehicle is stationary in order to prevent the vehicle from moving backward.

   HINT:

   The hill-start assist control will operate when the brake pedal is further depressed while all of the following conditions are met:

   • Shift lever position is in a position other than P.
   • Accelerator pedal is not depressed.
   • Vehicle is at standstill.
   • Parking brake is not applied.

Enhanced-VSC performs coordinated control consisting of VSC and electronic power steering. By integrating these preventive safety functions, Enhanced-VSC ensures driving stability and maneuverability of the vehicle.

If the vehicle loses stability due to wheel slippage, this function performs brake control by applying brake pressure to the wheels. At the same time, electronic power steering provides steering torque assist control to facilitate the driver's steering maneuvers.

1. Braking control on split friction roads
   1. When braking on a split friction road, the vehicle tends to deflect toward the higher friction side due to the difference between the braking forces on the left and right sides. With Enhanced-VSC, the power steering ECU receives command signals from the skid control ECU. Based on these signals, the power steering ECU operates the motor for the electronic power steering to reduce the effect of the difference between the braking forces on the left and right sides, assisting steering operation. This enables the driver to make steering corrections more easily.
2. Accelerating when surface resistance differs between both sides of wheels
   1. When accelerating on a split friction road, the vehicle tends to deflect toward lower friction side due to the driving force difference between the left and right sides. With Enhanced-VSC, the skid control ECU performs braking control of the drive wheel on the low friction side (traction control function) and transmits command signals to the power steering ECU. Based on these signals, the power steering ECU operates the motor for the electronic power steering to reduce the effect of the driving force difference between the left and right sides, assisting steering operation. As a result, proper motive force and vehicle stability is ensured.
3. Front wheel skid control
   1. When front wheel skidding is detected, engine output is limited and braking control is performed based on the amount of understeer tendency. Accordingly, a moment of force is generated in the vehicle turning direction to limit front wheel skid tendency.
   2. In the case of a front wheel skid tendency, steering torque assist is provided to inform the driver of the front wheel skid. If the driver turns the steering wheel excessively, steering torque assists provided. This enables the driver to prevent the steering wheel excessive operation.
4. Rear wheel skid control
   1. When rear wheel skidding is detected, brake force is applied to mainly the outer wheels according to the amount of oversteer tendency. Accordingly, an anti-spin moment is generated to limit the rear wheel skid tendency.
   2. With Enhanced-VSC, the power steering ECU receives command signals from the skid control ECU. Based on these signals, the power steering ECU operates the motor for the electronic power steering to provide steering assist to help the driver compensate for the rear wheel skid tendency. This enables the driver to operate the steering wheel more easily.

1. The skid control ECU calculates vehicle stability tendency based on 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 brake 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.

1. When a failure occurs in the ABS with BA, TRAC and VSC systems, the ABS warning and slip indicator lights come on and the VSC OFF indicator light blinks, and the ABS with BA, TRAC and VSC operations are prohibited. In addition, when there is a failure that disables EBD operation, the brake warning light also comes on and 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 reaches approximately 6 km/h (4 mph) after the ignition switch is turned on (IG), each solenoid valve and motor of the brake actuator are sequentially activated to perform an electrical check. During the initial check, the operating sound of the solenoid valves and motor can be heard from the engine compartment, but this is not a malfunction.

1. TRAC and VSC operation can be disabled by operating Techstream (See PRECAUTION ).