Lexus RX300 (MCU15). Service manual - part 53

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CHASSIS — BRAKES

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JFRONT AND REAR BRAKES

D The front brakes use 16-inch ventilated disc brakes.

D The rear brakes use 15-inch solid disc brakes.

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Front Brake

Rear Brake

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JABS with EBD & BRAKE ASSIST & TRC & VSC SYSTEM

1. General

D The primary purpose of the ABS and TRC system has been to help the vehicle’s stability during braking

and acceleration.  In contrast, the purpose of the VSC system is to help the vehicle’s stability during
cornering.

D Ordinarily, the vehicle corners in a stable manner in accordance with the steering operation.  However,

depending on the unexpected situations or external elements such as the ground surface conditions, vehicle
speed, and emergency avoidance maneuvers, the vehicle may exhibit strong understeer or oversteer ten-
dencies.  In such situations, the VSC system dampens the strong understeer or oversteer to help vehicle
stability.

D The primary purpose of the Brake Assist system is to provide an auxiliary brake force assist to the driver

who cannot generate a large brake force during emergency braking, thus maximizing the vehicle’s brake
performance.

D The EBD control utilizes ABS, realizing the proper brake force distribution between front and rear wheels

in accordance with the driving conditions.
In addition, during cornering braking, it also controls the brake forces of right and left wheels, helping
to maintain the vehicle stability.

CHASSIS — BRAKES

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2. System Diagram

Brake Fluid Level
Warning Switch

Stop Light Switch

Brake
Actuator

Front Speed
Sensors

Relays

Rear Speed
Sensors

Speedometer
ABS Warning Light
VSC Warning Light
Slip Indicator Light

Brake System Warning Light

VSC Warning Buzzer

Skid
Control
ECU

Engine
ECU

Shift Position Switch

Steering Angle Sensor

Yaw Rate Sensor

Decelerationn Sensor

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3. Layout of Main Components

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Brake
Actuator

Skid Control ECU

Front Speed Sensors

Stop Light Switch

Engine ECU

Yaw Rate
Sensor

Rear Speed Sensors

Combination Meter

S Slip Indicator Light

S ABS Warning Light

S VSC Warning Light

S Brake System Warning Light

Deceleration Sensor

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CHASSIS — BRAKES

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4. Function of Main Components

Components

Function

ABS Warning Light

Lights up to alert the driver when the ECU detects the
malfunction in the ABS or Brake Assist System.

Warning Light

VSC Warning Light

Lights up to alert the driver when the ECU detects the
malfunction in the VSC system.

Warning Light
and Indicator
Light

Slip Indicator Light

Blinks to inform the driver when the TRC system or
the VSC system is operated.

Brake System
Warning Light

Lights up together with the ABS warning light to
alert the driver when the ECU detects the malfunc-
tion not only in the ABS but also in the EBD control.

Engine ECU

Sends the throttle valve opening angle signal, shift
position signal, etc., to the skid control ECU.

Skid Control ECU

Judges the vehicle driving condition based on sig-
nals from each sensor, and sends brake control signal
to the brake actuator.
Also transmits the control information to the engine
ECU.

Speed Sensors

Detect the wheel speed of each of four wheels.

Brake Actuator

Changes the fluid path based on the signals from the
skid control ECU during the operation of the ABS
with EBD & Brake Assist & TRC & VSC system, in
order to control the fluid pressure that is applied to
the wheel cylinders.

Master Cylinder Pressure
Sensor

Assembled in the brake actuator and detects the mas-
ter cylinder pressure.

Control Relay

Pump Motor Relay

Supply power to the pump motor in the actuator.

Control Relay

Solenoid Relay

Supply power to the solenoid valves in the actuator.

Blake Fluid Level Warning Switch

Detects the brake fluid level.

VSC Warning Buzzer

Emits an intermittent sound to inform the driver that
the ECU detects the strong understeer tendency or
strong oversteer tendency.

Stop Light Switch

Detects the brake depressing signal.

Yaw Rate Sensor

Detects the vehicle’s yaw rate.

Deceleration Sensor

Detects the vehicle’s acceleration in the lateral direc-
tions.

Steering Angle Sensor

Detects the steering direction and angle of the steer-
ing wheel.

CHASSIS — BRAKES

Locus of Travel
Based on the
Target Yaw
Rate

Actual Locus of
Travel (Actual Yaw Rate)

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5. Outline of VSC System

General

The followings are two examples that can be considered as circumstances in which the tires overcome
their lateral grip limit.

D When the front wheels lose grip in relation to the rear wheels (strong understeer tendency).

D When the rear wheels lose grip in relation to the front wheels (strong oversteer tendency).

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Strong Understeer Tendency

Strong Oversteer Tendency

Method for Determining the Vehicle Condition

To determine the condition of the vehicle, sensors detect the steering angle, vehicle speed, vehicle’s yaw
rate, and the vehicle’s lateral acceleration, which are then calculated by the skid control ECU.

1) Determining Understeer

Whether or not the vehicle is in the state of under-
steer is determined by the difference between the
target yaw rate and the vehicle’s actual yaw rate.
when the vehicle’s actual yaw rate is smaller than
the yaw rate (a target yaw rate that is determined
by the vehicle speed and steering angle) that
should be rightfully generated when the driver op-
erates the steering wheel, it means the vehicle is
making a turn at a greater angle than the loss of
travel.  Thus, the ECU determines that there is
a large tendency to understeer.