Lexus ES300 (1997 year). Service manual — part 12

46

CHASSIS — BRAKES

3. Operation

During Applying

Pressing the parking brake pedal causes the sector’s ratchet to engage with the pawl.
At this time, a clockwise force is applied to the pawl by the pawl spring.
Then, when the pressure on the brake pedal is released, the reaction force of the parking broke and the force
of the pedal return spring cause the pawl and sector, which remain engaged, to return (only for the amount
of the loss stroke). As a result, the parking brake becomes locked.
At this time, a counterclockwise force is applied by the pawl spring to the pawl. However, as the reaction
force of the parking brake and the force of the pedal return spring are stronger, the pawl and the sector do
not become disengaged.

During Releasing

When the parking brake pedal is pressed again, as the reaction force of the parking brake and the force of
the pedal return spring will not be applied to the pawl, the pawl spring causes the pawl to rotate
counterclockwise. As a result, the pawl is released from the sector’s ratchet.
Next, when the parking brake pedal returns to its initial point, the lever that is provided on the pedal will
push the pawl downward. This force, along with the force of the pawl spring, causes the parking brake to
revert to the initial state.

47

CHASSIS — BRAKES

ABS

1. General

As in the previous model, the brake control of each wheel during ABS activation is implemented by the
following three modes: pressure reduction, pressure holding, and pressure increase modes.
During ABS activation, the ECU controls the fluid pressure of each front left and right wheels independently
while the fluid pressure of rear left and right wheels is controlled simultaneously. However, the ABS actuator
has been changed from the 3-position solenoid valve type to the 2-position solenoid valve type for weight
reduction. And, the diagnostic codes have been changed.
For the diagnostic code check method, diagnostic code, and diagnostic code clearance, see the 1997 ES300
Repair Manual (Pub. No. RM511U).
The operation of the ABS on models with ABS and on models with ABS & TRAC system are the same.

2. System Diagram

48

CHASSIS — BRAKES

3. Wiring Diagram

49

CHASSIS — BRAKES

4. Hydraulic Circuit

5. Operation

The hydraulic system of the ABS has 4 circuits. Although the hydraulic circuit described below is 1 circuit,
it is applicable to other circuits as well.

During Normal Braking (ABS not activated)

During normal braking, the ABS is not activated
and the ECU dose not send control signal.
When the brake pedal is depressed, the fluid
pressure in the master cylinder rises, brake fluid
passes from port (A) to port (B), and then flows
to the brake wheel cylinder. When the brake
pedal is released, brake fluid returns from the
brake wheel cylinder to the master cylinder
through port (B) to port (A) and the No. 1 Check
Valve.

Condition of Each Valve and Pump Motor

Part Name

Signal from

ABS ECU

Operation

Pressure Hold-
ing Valve

OFF

Port

(A)

Open

Pressure
Reduction Valve

OFF

Port

(D)

Closed

Pump Motor

OFF

Stop

Hydraulic Circuit