Lexus NX: System Description

Lexus NX Service Manual / Brake / Brake Control / Dynamic Control Systems / Electronically Controlled Brake System / System Description

SYSTEM DESCRIPTION

FUNCTION DESCRIPTION

(a) Electronically Controlled Brake System

(1) Upon receiving signals from the skid control ECU (brake booster with master cylinder assembly), this system effects hydraulic pressure control at the 4 wheels.

(b) Regenerative Brake Cooperative Control

(1) Controls hydraulic braking in order to recover electrical energy by utilizing the regenerative brake of the hybrid control system as much as possible.

(1) Effects cooperative control with the power steering ECU assembly in order to provide steering assist in accordance with the operating conditions of the vehicle.

(d) Anti-lock Brake System (ABS)

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

(e) Electronic Brake Force Distribution (EBD)

(1) The EBD control utilizes ABS, realizing proper brake force distribution between the front and rear wheels in accordance with the driving conditions. In addition, during braking while cornering, it also controls the brake forces of the right and left wheels, helping maintain vehicle behavior.

(f) Brake Assist (BA)

(1) The primary purpose of brake assist is to provide an auxiliary brake force to assist a driver who cannot generate a large brake force during emergency braking, thus helping ensure the vehicle's braking performance.

(g) Traction Control (TRAC)

(1) The TRAC helps restrain the slippage of the drive wheels if the driver depresses the accelerator pedal excessively when starting off or accelerating on a slippery surface.

(h) Vehicle Stability Control (VSC)

(1) The VSC helps restrain sideways slippage of the vehicle during a strong front wheel skid or strong rear wheel skid, during cornering.

(i) Hill-start Assist Control

(1) When starting uphill, this control maintains the brake hydraulic pressure to the 4 wheels, in order to momentarily prevent the vehicle from descending backward.

(j) Brake Hold System

(1) The brake hold operation occurs when the brake pedal is depressed and the vehicle is fully stopped. Under brake hold, the wheel cylinder oil pressure of the 4 wheels is maintained, and the vehicle's fully stopped condition is sustained. When the acceleration pedal is depressed, the wheel cylinder oil pressure is automatically released. Also, depending on the vehicle conditions, the electric parking brake operates.

(k) Trailer Sway Control

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

(l) Active Cornering Assist

While turning, if the vehicle is understeering and the driver is depressing the accelerator pedal, the brakes of the inside wheels are applied according to the vehicle understeer tendency to ensure excellent turning performance.

FUNCTION OF COMPONENTS

Component		Function
Brake Booster with Master Cylinder Assembly	Brake Actuator	Consists of 4 switching solenoid valves, 2 linear solenoid valves and 8 control solenoid valves. Changes the brake fluid path based on the signals from the skid control ECU during operation of the ABS with EBD, brake assist, TRAC, VSC, hill-start assist control and brake hold in order to control the fluid pressure that is applied to the wheel cylinders.
	Hydraulic Brake Booster	Generates hydraulic pressure in accordance with the amount of effort applied to the brake pedal by the driver. When a malfunction occurs in the brake system, the hydraulic brake booster supplies the fluid pressure (which is generated by brake pedal effort) directly to the wheel cylinders.
	Stroke Simulator	Generates a pedal stroke during braking in accordance with the driver's pedal effort.
Brake Actuator	Skid Control ECU	Monitors the driving conditions of the vehicle in accordance with the signals received from the sensors and through cooperative control with the hybrid vehicle control ECU and power steering ECU assembly calculates the required amount of braking force, and controls the brake actuator. Judges the vehicle driving condition based on signals from each sensor, and controls ABS with EBD, brake assist, TRAC, VSC, hill-start assist control and brake hold. Operates the brake booster pump assembly to control accumulator pressure based on accumulator pressure sensor signal.
	Relief Valve	Returns the brake fluid to the reservoir tank to prevent excessive pressure if the pump operates continuously due to a malfunction of the accumulator pressure sensor.
	Linear Solenoid Valve	Controls the wheel cylinder pressure to generate a braking force responsive to the braking force requirement when the brake pedal is depressed normally.
	Switching Solenoid Valve	Switches the brake hydraulic path when the brake control system is activated.
	Control Solenoid Valve	Controls the wheel cylinder pressure when the ABS with EBD, brake assist, TRAC, VSC, hill-start assist control and brake hold are activated.
	Regulator Pressure Sensor	The regulator pressure sensor converts the fluid pressure generated by the hydraulic brake booster into electrical signals and transmits them to the skid control ECU. Accordingly, the skid control ECU determines the braking force required by the driver.
	Wheel Cylinder Pressure Sensor	This sensor detects the fluid pressure that acts on the respective wheel cylinders and transmits them to the skid control ECU in the form of feedback. Accordingly, the skid control ECU monitors the fluid pressure of the wheel cylinders and controls the control solenoid valve, in order to achieve the optimal wheel cylinder pressures.
	Accumulator Pressure Sensor	The accumulator pressure sensor constantly detects the brake fluid pressure in the accumulator and transmits the signals to the skid control ECU. Accordingly, the skid control ECU controls the pump motor.
Skid Control ECU	Solenoid Relay	Supply or cut off power to solenoid valves in the brake actuator.
Skid Control ECU	Motor Relay	Normally supplies power to the pump motor using 2 out of 3 relays. Ensures power to the pump motor by using the rest relay if the skid control ECU malfunctions.
Brake Booster Pump Assembly		Consists of a pump, pump motor and accumulator. The hydraulic power source portion generates and stores the hydraulic pressure, which the skid control ECU uses for controlling braking.
Speed Sensor		Detects the wheel speed of the 4 wheels.
Brake Pedal Stroke Sensor Assembly		Directly detects the extent of the brake pedal stroke operated by the driver.
Steering Sensor		Detects the steering direction and angle of the steering wheel.
Airbag ECU Assembly	Yaw Rate and Acceleration Sensor	Detects the vehicle's yaw rate. Detects the vehicle's acceleration in the forward, rearward and lateral directions.
Brake Master Cylinder Reservoir Assembly	Brake Fluid Level Warning Switch	Detects a low brake fluid level.
Stop Light Switch Assembly		Detects the brake pedal depressing signal.
Stop Light Control ECU Assembly		Illuminates the stop lights while brake hold control is operating.
Front Door Courtesy Light Switch Assembly (Driver Side)		Detects whether the door is open or closed, sends the information to the skid control ECU.
Integration Control and Panel Assembly	VSC OFF Switch	Switches off the TRAC and VSC operation.
Integration Control and Panel Assembly	Brake Hold Switch	Permits the brake hold operation.
Power Steering ECU Assembly		Operates cooperatively with the skid control ECU to control the steering assist torque.
Hybrid Vehicle Control ECU		Actuates the regenerative brake upon receiving a signal from the skid control ECU. Sends the actual regenerative brake control value to the skid control ECU. Controls the motive force based on an output control request signal received from the skid control ECU while the VSC or TRAC is operating.
Parking Brake ECU Assembly		Sends the electric parking brake condition to the skid control ECU. Controls the parking brake actuator assembly based on the signals received from the skid control ECU.
Combination Meter Assembly	ABS Warning Light	Illuminates to alert the driver when the skid control ECU detects a malfunction in the ABS, EBD or brake assist.
	Brake Warning Light / Red (Malfunction)	Illuminates to alert the driver when the skid control ECU detects a malfunction in the apportioning of the brake. Illuminates to inform the driver when the brake fluid is low.
	Brake Warning Light / Yellow (Minor Malfunction)	Illuminates to alert the driver when a minor malfunction is detected in the brake system, which does not affect the braking force (such as a malfunction in the regenerative brake).
	Slip Indicator Light	Blinks to inform the driver when the ABS, TRAC or VSC is operated. Illuminate to alert the driver in the event of malfunction of the TRAC, VSC or hill-start assist control.
	VSC OFF Indicator Light	Illuminates to inform the driver when VSC off mode is selected.
	Multi-information Display (TRAC OFF Message)	Displays the information message when TRAC off mode is selected.
	Brake Hold Standby Indicator Light	Comes on when the system is in the brake hold control permission state.
	Brake Hold Operated Indicator Light	Comes on when the brake hold is operating. Blinks when a malfunction has occurred during brake hold control selection.
	Master Warning Light	Illuminates to alert the driver when the skid control ECU detects a malfunction in the brake control system.
	Meter Buzzer	This buzzer sounds continuously to inform the driver when there is a malfunction in the hydraulic pressure circuit or a failure in the power supply. Emits a warning sound to inform the driver when hill-start assist control operation is started or finished.

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