Lexus RX300 (MCU15). Manual - part 55

CH

BO

CHASSIS — BRAKES

Pressure
Reduction Valve

Port A

Pressure Holding Valve

Port B

To Wheel
Cylinder

To 
Reservoir
and Pump

From
Wheel
Cylinder

169CH54

169CH55

169CH56

93

3) ABS with EBD Operation

Based on the signals received from the 4 wheel speed sensors and deceleration sensor, the skid control
ECU calculates each wheel speed and deceleration, and checks wheel slipping condition.  And according
to the slipping condition, the ECU controls the pressure holding valve and pressure reduction valve in
order to adjust the fluid pressure of each wheel cylinder in the following 3 modes: pressure reduction,
pressure holding, and pressure increase modes.

Not Activated

Normal Braking

—

—

Activated

Pressure Increase Mode

Pressure Holding Mode

Pressure Reduction Mode

Hydraulic

Circuit

Pressure

Holding Valve

(Port A)

OFF

(Open)

ON

(Close)

ON

(Close)

Pressure

Reduction Valve

(Port B)

OFF

(Close)

OFF

(Close)

ON

(Open)

Wheel Cylinder

Pressure

Increase

Hold

Reduction

CHASSIS — BRAKES

94

4) TRC Operation

The fluid pressure that is generated by the pump is regulated by the pressure regulator valve to the required
pressure.  Thus, the wheel cylinder of the drive wheels are controlled in the following 3 modes: pressure
reduction, pressure holding, and pressure increase modes, to restrain the slippage of the drive wheels.
The diagram below shows the hydraulic circuit in the pressure increase mode when the TRC system
is activated.
In other operating modes, the pressure holding valve and the pressure reduction valve are turned ON/OFF
according to the ABS operation pattern described on the previous page.

161ES36

Master
Cylinder

(1)

(1)

(2)

(3)

(1)

(4)

(5)

(6)

(7)

(1)

(8)

(9)

(12)

(13)

(14)

(15)

(10)

(11)

Front Right
Wheel Cylinder

Rear Left
Wheel Cylinder

Rear Right
Wheel Cylinder

Front Left
Wheel Cylinder

Reservoirs

Pumps

Port (A)

Port (H)

Port (G)

Port (L)

Port (K)

Port (F)

Port (I)

Port (J)

Port (M)

Port (N)

Port (B)

Port (C)

Port (D)

Port (E)

Increase Mode

" System Diagram A

187CH41

Speed Sensor

Skid

Control

ECU

Engine ECU

Brake

Actuator

Slip Indicator

Light

CH

BO

CHASSIS — BRAKES

Mode

Solenoid Valves

95

TRC Activated

TRC Not

Activated

Pressure

Increase

Mode

Pressure

Holding

Mode

Pressure

Reduction

Mode

(2)

Master Cylinder Cut Solenoid Valve
(Front)

OFF

ON

ON

ON

(7)

Port: (A), (F)

Open

Close

Close

Close

(3)

Master Cylinder Cut Solenoid Valve
(Front, Rear)

OFF

OFF

OFF

OFF

(6)

Port: (B), (E)

Open

Open

Open

Open

(4)

Reservoir Cut
Solenoid Valve

OFF

ON

ON

ON

(5)

Port: (C), (D)

Close

Open

Open

Open

(8)

Pressure Holding Valve
(Front)

OFF

OFF

ON

ON

(11)

Port: (G), (J)

Open

Open

Close

Close

(9)

Pressure Holding Valve
(Rear)

OFF

OFF

OFF

OFF

(11)

Port: (H), (J)

Open

Open

Open

Open

(12)

Pressure Reduction Valve
(Front)

OFF

OFF

OFF

ON

(15)

Port: (K), (N)

Close

Close

Close

Close

(13)

Pressure Reduction Valve
(Rear)

OFF

OFF

OFF

OFF

(14)

Port: (L), (M)

Close

Close

Close

Close

Wheel Cylinder

Front wheels

—

Increase

Hold

Reduction

Wheel Cylinder
Pressure

Rear wheels

—

—

—

—

5) VSC Operation

a. General

The VSC system, by way of solenoid valves, controls the fluid pressure that is generated by the pump
and applies it to the brake wheel cylinder of each wheel in the following 3 modes: pressure reduction,
pressure holding, and pressure increase modes. As a result, the tendency to understeer or oversteer
is restrained.

b. Understeer Restraining Control

In understeer restraining control, the brakes of the front wheels and rear wheel of the inner side of
the turn is applied.
Also, depending on whether the brake is ON or OFF and the condition of the vehicle, there are circum-
stances in which the brake might not be applied to the wheels even if those wheels are targeted for
braking.
The diagram below shows the hydraulic circuit in the pressure increase mode, as it restrains an understeer
condition while the vehicle makes a right turn.
In other operating modes, the pressure holding valve and the pressure reduction valve are turned ON/OFF
according to the ABS operation pattern.

CHASSIS — BRAKES

96

Port (A)

Port (H)

Port (G)

Port (L)

Port (K)

Port (F)

Port (I)

Port (J)

Port (M)

Port (N)

Port (B)

Port (C)

Port (D)

Port (E)

Increase Mode

161ES37

Master
Cylinder

(1)

(1)

(2)

(3)

(1)

(4)

(5)

(6)

(7)

(1)

(8)

(9)

(12)

(13)

(14)

(15)

(10)

(11)

Front Right
Wheel Cylinder

Rear Left
Wheel Cylinder

Rear Right
Wheel Cylinder

Front Left
Wheel Cylinder

Reservoirs

Pumps

" System Diagram A

187CH42

Speed Sensor

Skid

Control

ECU

Engine ECU

Brake 

Actuator

Slip Indicator

Light

Stop Light

Switch

Steering Angle

Sensor

Yaw Rate

Sensor

Deceleration

Sensor

VSC Warning

Light

VSC Warning

Buzzer