Lexus ES300 (2002 year). Manual — part 106

CHASSIS – BRAKE

CH-59

208CH37

Brake Fluid Level
Warning Switch

Stop Light Switch

Master Cylinder Pressure Sensor

Speed
Sensors

Brake
Actuator

Relays

Speed
Sensors

Combination Meter

Skid
Control
ECU

ECM

Steering Angle Sensor

Yaw Rate Sensor

Deceleration Sensor

TRAC OFF Switch

Parking Brake Switch

DLC3

VSC Warning Buzzer

Speedometer

ABS Warning Light
VSC Warning Light

Slip Indicator Light

Brake System Warning Light

TRAC OFF Indicator Light

ABS with EBD & BRAKE ASSIST & TRAC & VSC SYSTEM

1. General

The primary purpose of the ABS and TRAC system has been to help the vehicle’s behavior during braking

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

The TRAC system controls driving wheels, therefore, it controls the front wheels.

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 tendencies. In such situations, the VSC system dampens the strong understeer or oversteer
to help maintain vehicle behavior.

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.

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 behavior.

2. System Diagram

CHASSIS – BRAKE

CH-60

170CH18

Braking
Force

With Brake Assist System

Without Brake Assist System

Time

a

b

c

d

Service Tip

When this system is activated, the brake pedal could shudder, which is a normal occurrence of the system
in operation and should not be considered a malfunction.

3. Outline of Brake Assist System

Brake Assist system interprets a quick push of the brake pedal as emergency braking and supplements the
braking power applied if the driver has not stepped hard enough on the brake pedal.
In emergencies, drivers, especially inexperienced ones, often panic and do not apply sufficient pressure on
the brake pedal.
Brake Assist system measures the speed and force with which the brake pedal is pushed to determine whether
the driver is attempting to brake rapidly, and applies additional pressure to increase braking performance.
A key feature of Brake Assist system is that the timing and the degree of braking assistance are designed to
ensure that the driver does not discern anything unusual about the braking operation.
When the driver intentionally eases up on the brake pedal, the system reduce the amount of assistance it
provides.

– REFERENCE –

Effectiveness of the Brake Assist Operation:

a. During emergency braking, an inexperienced driver, or a driver in a state of panic might not be able to

firmly depress the brake pedal, although driver can depress it quickly. As a result, only a small amount
of brake force is generated.

b. The pedal effort of this type of driver might weaken as time passes, causing a reduction in the braking

force.

c. Based on how quickly the brake pedal is depressed, the Brake Assist operation assesses the intention of

the driver to apply emergency braking and increases the brake force.

d. After the Brake Assist operation, if the driver intentionally releases the brake pedal, the assist operation

reduces the amount of Brake Assist in order to reduce the feeling of uneasiness.

CHASSIS – BRAKE

CH-61

189CH100

151CH17

Strong Understeer Tendency

Strong Oversteer Tendency

151CH19

Actual Locus of Travel
(Actual Yaw Rate)

Locus of Travel
Based on the
Target Yaw Rate

4. Outline of VSC System

General

The followings are two examples that can be considered as circumstances in which the tires over their lateral
grip limit.
VSC system is to help control the vehicle behavior by applying the engine output control and each wheels
brake control when the vehicle is under the condition indicated below.

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

CHASSIS – BRAKE

CH-62

151CH18

Direction of Travel of the Vehicle’s
Center of Gravity

Slip Angle

Movement of
Vehicle

161ES30

Making a Right Turn

Braking
Force

Understeering
Control Moment

Braking Force

Braking Force

2) Determining Oversteer

Whether or not the vehicle is in the state of
oversteer is determined by the values of the
vehicle’s slip angle and the vehicle’s slip
angular velocity (time-dependent changes in
the vehicle’s slip angle). When the vehicle’s
slip angle is large, and the slip angular
velocity is also large, the ECU determines that
the vehicle has a large oversteer tendency.

Method of VSC Operation

When the skid control ECU determines that the vehicle exhibits a tendency to understeer or oversteer, it
decreases the engine output and applies the brake of a front or rear wheel to control the vehicle’s yaw
moment.
The basic operation of the VSC is described below. However, the control method differs depending on the
vehicle’s characteristics and driving conditions.

1) Dampening a Strong Understeer

When the skid control ECU determines that
the vehicle exhibits a strong tendency to
understeer, depending on the extent of that
tendency, it decreases the engine output and
applies the brakes of the front wheels and
inside rear wheel, thus providing the vehicle
with an understeer control moment, which
helps dampen its tendency to understeer.
Also, depending on whether the brakes are
ON or OFF and the condition of the vehicle,
there are circumstances in which the brakes
might not be applied to the wheels even if
those wheels are targeted for braking.