Lexus ES300 (1997 year). Manual — part 13

50

CHASSIS — BRAKES

During Emergency Braking (ABS activated)

1) Pressure Reduction Mode

When the wheel is about to lock, the control
signal from the ECU causes port (A) to close,
port (D) to open, thus engaging the pressure
reduction mode.
At this time, the brake fluid flows from the
wheel cylinder, through ports (C) and (D), to
the reservoir, reducing the wheel cylinder
pressure. At the same time, the brake fluid is
pumped and returned to the master cylinder.

Condition of Each Valve and Pump Motor

Part Name

Signal from

ABS ECU

Operation

Pressure
Holding Valve

ON

Port

(A)

Closed

Pressure
Reduction Valve

ON

Port

(D)

Open

Pump Motor

ON

Rotating

2) Pressure Holding Mode

After the fluid pressure in the wheel cylinder
is reduced or increased to the required
pressure, a control signal from the ECU causes
ports (A) and (D) to close. As a result, the
system engages in the pressure holding mode
to maintain the fluid pressure in the wheel
cylinder.

Condition of Each Valve and Pump Motor

Part Name

Signal from

ABS ECU

Operation

Pressure
Holding Valve

ON

Port

(A)

Closed

Pressure
Reduction Valve

OFF

Port

(D)

Closed

Pump Motor

ON

Rotating

Hydraulic Circuit

Hydraulic Circuit

51

CHASSIS — BRAKES

3) Pressure Increase Mode

When the fluid pressure in the wheel cylinder
needs to be increased in order to apply more
braking force, a control signal from the ECU
causes port (A) to open, port (D) to close, thus
engaging in the pressure increase mode.
Accordingly, the circuit will be in the same
state as in normal braking, in which the brake
fluid is sent from the master cylinder to the
wheel cylinder to increase the fluid pressure in
the wheel cylinder.
The fluid pressure increase rate is controlled
by repetition of the pressure increase and
pressure holding mode.

Condition of Each Valve and Pump Motor

Part Name

Signal from

ABS ECU

Operation

Pressure
Holding Valve

OFF

Port

(A)

Open

Pressure
Reduction Valve

OFF

Port

(D)

Closed

Pump Motor

ON

Rotating

Hydraulic Circuit

52

CHASSIS — BRAKES

ABS & TRAC SYSTEM

1. General

The ABS & TRAC system totally controls the engine torque control through the fuel cutoff and the braking
of the driving wheels (front wheels). It helps to avoid slippage of the driving wheels that tends to happen
during starting and acceleration and to maintain an optimal driving force according to the road surface
conditions. The system eliminates the need for a subtle accelerator pedal operation and ensures vehicle
stability when starting, accelerating or turning on slippery roads.
The operation of the ABS portion of the ABS & TRAC system is the same as that of the models with ABS.

2. System Diagram

53

CHASSIS — BRAKES

3. Wiring Diagram