Toyota 4Runner: System Description

Toyota 4Runner Service Manual / Brake / Brake Control / Dynamic Control Systems / Vehicle Stability Control System(for Hydraulic Brake Booster) / System Description

SYSTEM DESCRIPTION

1. SYSTEM DESCRIPTION

HINT:

The skid control ECU is built into the hydraulic brake booster.

(a) ABS (Anti-lock Brake System)

The ABS helps prevent the wheels from locking when the brakes are applied firmly or on a slippery surface.

(b) Multi-terrain Anti-lock Brake System (Multi-terrain ABS)

The multi-terrain ABS operates in conjunction with the multi-terrain select. This function detects off-road driving in sand, mud, gravel, dirt and so on based on signals from the yaw rate and acceleration sensor, speed sensor and ECM, and then automatically switches to the ABS control most appropriate for the road condition.

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

(d) BA (Brake Assist)

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

(e) TRAC (Traction Control)

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

(f) VSC (Vehicle Stability Control)

The VSC system helps prevent the vehicle from slipping sideways when front or rear wheel skidding occurs while cornering.

(g) A-TRAC (Active Traction Control)

During rugged off-road driving, this system controls the engine output and the brake fluid pressure that is applied to the slipping wheel and distributes the drive force that would have been lost through the slippage to the remaining wheels in order to achieve an LSD (Limited Slip Differential) effect.

(h) AUTO LSD (Auto Limited Slip Differential)

The AUTO LSD operates with the VSC OFF switch on and the accelerator pedal depressed.

The AUTO LSD system is used to restrain brake pressure and reduce differential movement, thus transmitting drive torque to the other drive wheel to ensure startability under the following conditions:

Wheels run off the road.
Drive wheels slip when starting on a slope with one wheel on snow/ice.

(i) Downhill Assist Control

When the downhill assist control switch is pressed with the transfer in the low (L4) range and without the accelerator and brake pedals in operation, 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. Therefore, 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.

(j) 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 performs 4-wheel hydraulic pressure control to reduce the speed of the vehicle.

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 P or N, or when the vehicle is running/rolling back up a slope with the shift lever in R.

(k) CRAWL (Crawl Control)

Crawl control is a system that provides assistance when driving on bumpy off-road surfaces, slippery road surfaces, etc. When crawl control is operating, the engine output and braking force are controlled automatically to enable driving while maintaining a constant speed. As a result, the driver can concentrate on steering the vehicle without having to operate the accelerator and brake pedals very much.

(l) Multi-terrain Select Control

When multi-terrain select control is active, A-TRAC brake control and engine output control most appropriate for the selected road mode are performed automatically. In this way, superior off-road driveability is achieved.

(m) Trailer Sway Control

If the vehicle is towing a trailer with an inappropriate hitch load, trailer sway could be caused by crosswinds, imbalance caused by load, or the driver's steering. Trailer sway control reduces trailer sway by controlling the engine output and the braking of each wheel.

2. FUNCTION OF COMPONENTS

Components	Functions
Speed Sensor	Detects the wheel speed of each wheel.
Hydraulic Brake Booster	Upon receiving a brake control signal from the skid control ECU, the hydraulic brake booster controls the hydraulic circuit and sends hydraulic pressure to each wheel. The master cylinder solenoid (skid control ECU) is removable.
Master Cylinder Solenoid (Skid Control ECU)	Detects the vehicle condition based on signals from the sensors and sends a brake control signal to the hydraulic brake booster.
Motor Relay	Supplies a power source to the pump motor.
Solenoid Relay	Supplies a power source to each solenoid.
Steering Angle Sensor (Spiral Cable Sub-assembly)	Detects the steering angle and amount of movement of the steering wheel, and then sends signals to the skid control ECU.
Yaw Rate and Acceleration Sensor	Detects the vehicle's yaw rate and acceleration and sends a signal to the skid control ECU.
Master Cylinder Pressure Sensor	Built into the master cylinder solenoid. Detects the pressure inside the master cylinder.
Accumulator Pressure Sensor	Built into the master cylinder solenoid and detects the accumulator pressure.
ECM	Sends sensor signals (throttle opening angle, engine speed) to the skid control ECU. The engine is controlled based on a traction control signal from the skid control ECU.
VSC OFF Switch	Enables the driver to select normal mode, TRAC OFF mode or VSC OFF mode.
A-TRAC Switch*1	Allows the driver to turn A-TRAC on and off.
Downhill Assist Control Switch*2	Allows the driver to turn downhill assist control on and off.
Crawl Control Switch*3	The ON/OFF switch of the crawl control switch turns crawl control on and off and the speed selector switch of the crawl control switch changes the control mode.
Multi-terrain Select Switch*3	The ON/OFF switch of the multi-terrain select switch turns multi-terrain select control on and off and the mode selector switch of the multi-terrain select switch selects the road surface mode.
ABS Warning Light	Illuminates to inform the driver that a malfunction in the ABS has occurred. w/ Rear Differential Lock: When the rear differential lock is in the lock state, ABS is also prohibited and the ABS warning light turns on.
BRAKE Warning Light	Illuminates to inform the driver that the parking brake is applied when the system is normal, and when the amount of brake fluid has decreased. Illuminates to inform the driver that a malfunction in the EBD has occurred.
TRAC OFF Indicator Light	Illuminates to inform the driver when the vehicle is in TRAC OFF mode.
VSC OFF Indicator Light	Illuminates to inform the driver that VSC OFF mode is entered using the VSC OFF switch.
Slip Indicator Light	Illuminates to inform the driver that a malfunction has occurred in the TRAC or VSC system. Blinks to inform the driver that TRAC, A-TRAC, VSC, hill-start assist control, CRAWL or trailer sway control is operating.
AUTO LSD Indicator Light*4	Illuminates to inform the driver that AUTO LSD is ready to operate.
A-TRAC Indicator Light*1	Illuminates to inform driver that A-TRAC is ready to operate.
Downhill Assist Control Indicator Light*2	Illuminates to inform the driver when downhill assist control operation is possible.
Crawl Indicator Light*3	Illuminates to inform driver that crawl control is ready to operate.
Multi-terrain Select Indicator Light*3	Illuminates to inform driver that multi-terrain select control is ready to operate.
Meter buzzer	Intermittently sounds to inform the driver if the temperature of the brake actuator has increased excessively due to continuous operation of the downhill assist control (for 4WD) or hill-start assist control.

*1: for 4WD

*2: w/ Downhill Assist Control

*3: w/ Crawl Control

*4: for 2WD

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