Antilock Brakes / Traction Control Systems: Description and Operation

BRAKE CONTROL: VEHICLE STABILITY CONTROL SYSTEM: SYSTEM DIAGRAM

SYSTEM DIAGRAM













BRAKE CONTROL: VEHICLE STABILITY CONTROL SYSTEM: SYSTEM DESCRIPTION

SYSTEM DESCRIPTION

1. SYSTEM DESCRIPTION

HINT:
- The skid control ECU is built into the hydraulic brake booster.
- The yaw rate sensor and deceleration sensor are combined in a single unit. This unit communicates with the skid control ECU through CAN communication.

(a) ABS with EBD, BA, TRAC and VSC operation
(1) The skid control ECU determines the wheel slippage conditions by receiving signals from each speed sensor, the yaw rate sensor and the steering angle sensor. According to these signals, the ECU controls the engine output torque using electronic throttle control, and the wheel brake pressure using the brake actuator.
(2) The SLIP indicator light blinks and the skid control buzzer sounds to inform the driver that the VSC system and hill-start assist control are operating. The SLIP indicator light also blinks when TRAC and downhill assist control are operating, and the operation being performed is displayed.
2. ACTIVE SAFETY
(a) 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.




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 prevent the wheels from locking by controlling the fluid pressure of each wheel cylinder.
(b) EBD (Electronic Brake force Distribution)
The EBD control utilizes ABS, realizing proper brake force distribution between the front and rear wheels in accordance with the driving conditions.
In addition, when braking while cornering, it also controls the brake forces of the right and left wheels, helping to control the vehicle behavior.




Operation description
The skid control ECU receives the speed signals from each speed sensor to detect the slip condition of the wheels and sends the control signal to the solenoid valve.
The solenoid valve controls the fluid pressure of the rear wheel cylinder and divides the control power appropriately between the front and rear wheels and the right and left wheels.
Both of the ABS and the BRAKE warning lights come on when the EBD system malfunctions.
(c) BA (Brake Assist)
The primary purpose of the brake assist system is to provide an auxiliary brake force to assist drivers who cannot generate a large enough brake force during emergency braking, thus helping to maximize the vehicle's braking performance.




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 necessary, the skid control ECU sends control signals to the pump motor and solenoid valve. The pump motor and solenoid valve then control the pressure applied to each wheel cylinder.
The ABS warning light comes on when the BA system malfunctions.
(d) VSC (Vehicle Stability Control)
The VSC system helps prevent the vehicle from slipping sideways as a result of strong front wheel skid or strong rear wheel skid during cornering.




Operation description
The skid control ECU determines the vehicle condition by receiving signals from the speed sensors, yaw rate sensor and steering angle sensor. The skid control ECU controls the engine torque through the ECM via CAN communication, and the fluid pressure through the pump motor and solenoid valve.
The SLIP indicator light blinks and the skid control buzzer sounds when the system is operating. The VSC OFF indicator light comes on when the VSC system malfunctions.
When the vehicle is stopped in TRAC OFF mode and the VSC OFF switch is pressed for 3 seconds or more, the vehicle changes to VSC OFF mode. During VSC OFF mode, the combination meter's VSC OFF indicator light and slip indicator light illuminate, and VSC, TRAC and AUTO-LSD operations are disabled.
3. DRIVING PERFORMANCE
(a) 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.

HINT: The TRAC system operates when 2WD mode is selected (for 4WD).




Operation description
The skid control ECU detects the vehicle's slip condition by receiving signals from the speed sensors and ECM via CAN communication. The skid control ECU controls the engine torque through the ECM via CAN communication, and the fluid pressure through the pump motor and solenoid valve.
The SLIP indicator light blinks when the system is operating. Both the VSC OFF indicator light and SLIP indicator light come on when the TRAC system malfunctions.
When the vehicle is in normal mode and the VSC OFF switch is pressed for a short amount of time, AUTO-LSD operation becomes possible. During AUTO LSD mode, the combination meter's AUTO LSD indicator light and slip indicator light illuminate, and TRAC operation is disabled.
(b) A-TRAC (Active Traction Control) (for 4WD)
During rugged off-road driving, this function controls the engine output and the brake fluid pressure that is applied to slipping wheels, and distributes the drive force that would have been lost through the slippage to the remaining wheels in order to achieve a LSD (Limited Slip Differential) effect.

HINT: The A-TRAC system operates when 4WD mode is selected.




Operation description
The SLIP indicator light blinks when the system is operating. Both the VSC OFF indicator light and SLIP indicator light come on when the A-TRAC system malfunctions.
(c) AUTO-LSD (Auto Limited Slip Differential)
When the vehicle enters 2WD mode, if the VSC OFF switch is pressed for a short amount of time and the accelerator pedal depressed, the AUTO LSD operates.
The TRAC system restrains brake pressure and reduces differential movement, thus transmitting the drive torque to the other drive wheel to ensure stability under the following conditions:
- Wheels run off the road.
- Drive wheels run idle when starting on a slope with one wheel on snow/ice.

HINT:
- Depressing the brake pedal cancels control of the AUTO LSD system.
- The AUTO LSD system operates when 2WD mode is selected (for 4WD).




Operation description
The SLIP indicator light blinks and the AUTO LSD indicator light comes on when the system is operating.
Both the VSC OFF indicator light and SLIP indicator light come on when the AUTO LSD system malfunctions.
(d) Downhill Assist Control (for 4WD)
When the downhill assist control switch is pressed with the transfer in low (L4) range and without the accelerator and brake pedals in operation, the downhill assist control activates 4-wheel hydraulic pressure control in order to maintain a constant low vehicle speed without causing the wheels to become locked. Thus, the vehicle can descend steep hills in a stable manner.

HINT:
- Depressing the accelerator and brake pedals cancels control of the downhill assist control system.
- The system begins operating when driving down slopes at a speed of 25 km/h (16 mph) or less with the engine brake applied.




Operation description
The SLIP indicator light blinks and the downhill assist control indicator light and stop light come on when the system is operating.
The VSC OFF indicator light comes on and the downhill assist control indicator light blinks when the downhill assist control system malfunctions.
(e) Hill-start Assist Control
When the vehicle starts off on a steep hill, hill-start assist control detects the backward descent of the vehicle and effects 4-wheel hydraulic pressure control to reduce the speed of the vehicle.
For a maximum of 5 seconds after the control starts, fluid pressure is gradually released and the control is complete.

HINT:
- Depressing the brake pedal cancels control of the hill-start assist control system.
- The system does not operate when the shift lever is in the P position or N position, or when the vehicle is running/rolling back up a slope with the shift lever in the R position.




Operation description
The SLIP indicator light blinks, the stop light comes on, and the skid control buzzer sounds when the system is operating.
The VSC OFF indicator light comes on when the hill-start assist control system malfunctions.
4. FUNCTION OF COMPONENTS





5. FAIL-SAFE
(a) When a failure occurs in the ABS with BA, TRAC and VSC systems, the ABS warning light and the brake control warning light come on and ABS with BA, TRAC and VSC operations is prohibited. Then the VSC OFF indicator light blinks to warn the driver. 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.
(b) If control is prohibited due to a malfunction during operation, control will be disabled gradually. This is done to avoid sudden vehicle instability.