TOYOTA PRIUS 2001 Service Repair Manual

CHASSIS ± BRAKES
182CH48
Rubber
Reaction
DiscReaction
RodSpool ValveReturn Spring
RegulatorMaster Cylinder Piston
Power Piston
Operating Rod
Regulator
Master Cylinder Operating Portion
152CH24
Reaction RodTo
ReservoirFrom
Accumulator
Regulator
From
Reservoir Tank
Power Piston
Operating Rod
Rubber Reaction Disc
Spool Valve
Rear Brake Front Brake
Master Cylinder Piston109
2) Master Cylinder and Brake Booster
a. Construction
This construction enables the hydraulic pressure that is generated by the brake booster to be applied
directly to the rear brakes.
The master cylinder is the center port type single master cylinder, which is used for the front brakes
only.
The brake booster is integrated with the master cylinder. The operating portion, master cylinder, and
regulator are positioned coaxially to achieve a simple and compact construction.
The operating rod and the power piston are linked directly to transmit the pedal effort that is applied
to the brake pedal.
The regulator and the spool valve are linked directly. A forward (leftward) force generated by the
master cylinder pressure and a rearward (rightward) force generated by the power assist of the boost-
er are applied to the regulator. Both forces maintain a balance.
A return spring is provided for the regulator to ensure the return of the spool valve.

Cross-Sectional Drawing 


Simplified Drawing 

CHASSIS ± BRAKES
152CH25
Reaction RodTo
ReservoirFrom
Accumulator
Spool Valve
Regulator
From
Reservoir Tank
Power Piston
Rubber
Reaction
DiscOperating Rod
ªAº
ªBº
Return
Spring
To Rear
BrakeTo Front
BrakeReturn
Spring
Master Cylinder PistonBooster Chamber
152CH26
Reaction Rod
To
ReservoirFrom
Accumulator
Regulator
From
Reservoir Tank
Power Piston
Rubber
Reaction
Disc
Spool Valve
To Rear
BrakeTo Front
Brake
Master Cylinder PistonBooster Chamber 11 0
b. Operation
i) Pressure Increase (Low Pressure)
The pedal effort that is applied to the brake pedal is transmitted via the operating rod, power piston,
and master cylinder piston. However, because the load setting of the master cylinder's return spring
is higher than that of the regulator piston's return spring, the regulator piston gets pushed before the
volume in the master cylinder becomes compressed. As a result, the spool valve moves forward.
The spool valve closes the port ªAº between the reservoir and the booster chamber (behind the power
piston) and opens the port ªBº between the accumulator and the booster chamber. Then, the pressur-
ized brake fluid is introduced into the booster chamber to provide a power assist to the pedal effort.
When the pressure is introduced into the booster chamber, the power assist overcomes the force of the
master cylinder's return spring. This causes the volume in the master cylinder to become compressed
and increases the pressure that is applied to the front brakes. At the same time, the pressure in the boost-
er chamber increases the pressure that is applied to the rear brakes.
During the initial stage of the brake operation, the booster pressure that is applied to the rubber reaction
disc is small. Therefore, a return force in the rightward direction does not apply to the spool valve via
the reaction rod.
ii) Pressure Increase (High Pressure)
In contrast to the time when the pressure is low, when the pressure is high, the booster pressure that
is applied to the rubber reaction disc increases. Accordingly, the rubber reaction disc deforms and
causes a return force in the rightward direction to be applied to the spool valve via the reaction rod.
Therefore, in contrast to the time when the pressure is low, a greater reaction force is transmitted to
the brake pedal.
As a result, a variable servo mechanism is realized, in which the servo ratio is lower during high pres-
sure than during low pressure.

CHASSIS ± BRAKES
182CH82
To
ReservoirFrom
AccumulatorRegulatorFrom
Reservoir Tank
ªBº
ªAºTo Rear
BrakeTo Front
Brake
Spool Valve
152CH28
To
Reservoir
ªAº
From
AccumulatorRegulatorTo
Reservoir Tank
Spool Valve
From Rear
BrakeFrom Front
Brake
Booster Chamber111
iii) Holding
This is a state in which the force that is applied via the brake pedal and the master cylinder pressure
are in balance.
The forces that are applied to the front and the rear of the regulator piston, in other words, forces that
are generated by the master cylinder pressure and the regulator pressure become balanced. This causes
the spool valve to close both port ªBº from the booster chamber to the accumulator and port ªAº to
the reservoir. As a result, the brake system is in the holding state.
iv) Pressure Reduce
When the force that is applied to the brake pedal is relaxed, the master cylinder pressure decreases.
Then, the regulator's return (rightward) force becomes relatively greater, causing the regulator to re-
tract and the spool valve to also retract. As a result, the port ªAº between the reservoir and the booster
chamber opens.
The booster pressure becomes reduced in this state, creating a balance that corresponds to the force
that is newly applied via the brake pedal. This process is performed repetitively to reduce the booster
pressure and the master cylinder pressure in accordance with the force that is applied via the brake
pedal.

CHASSIS ± BRAKES
152CH29
From
Reservoir
To Front
Brake
182CH49
To Wheel Cylinders
From
Master
Cylinder
From Hydraulic
Brake Booster
To Front
Wheel
Cylinders
To Rear Wheel
Cylinders
SMC1 and SMC2 SS 11 2
v) During Power Supply Malfunction
If the accumulator pressure is affected due to some type of malfunction, no pressure will be supplied
by the regulator. Then, a power assist cannot be provided to the force that is applied via the brake pedal
and the pressure to the rear brakes cannot be increased.
The pressure to the front brakes will be increased by the master cylinder in accordance with the pedal
effort applied to the brake pedal.
Brake Actuator
1) Switching Solenoid Valves
The control signals from the brake ECU cause the valves to open and close to switch the passages of the
brake fluid. There are 3 switching solenoid valves (SMC1, SMC2, and SS) and they switch the passages
to effect conventional brake control and ABS control.

CHASSIS ± BRAKES
182CH50
To Wheel Cylinder From Hydraulic
Brake BoosterFrom Wheel
CylinderTo Reser-
voir Tank
Pressure Holding Valve
(SFRH, SFLH, SRrH)Pressure Reduction Valve
(SFRR, SFLR, SRrR)
From Hydraulic
Brake Booster
Pressure Holding
Valve
To Wheel
Cylinder
Pressure
Reduction
Valve
Increase HoldingFrom
Wheel
Cylinder
To Reservoir Tank
Decrease
182CH51
To Wheel Cylinders
From
Hydraulic
Brake
Booster
To Built-In Reservoir
From
Wheel
Cylinders
SLA SLR
11 3
2) ABS Solenoid Valves
The control signals from the brake ECU cause the valves to open and close to switch the passages of the
brake fluid. There are 2 (pressure holding valve, pressure reduction valve) ABS solenoid valves pro-
vided. The pressure increases if no current is applied to either valve. The pressure is held if current is
applied only to the pressure holding valve. The pressure decreases if current is applied to both the pres-
sure holding valve and pressure reduction valve. To effect control independently to the front wheels, si-
multaneously to both rear wheels, 6 solenoid valves are provided.
3) Linear Solenoid Valves
The control signals from the brake ECU cause the valves to open and close to regulate the volume of the
brake fluid that flows into the wheel cylinders. There are 2 types of linear solenoids, the SLA for pressure
increase and the SLR for pressure decrease, and they regulate the wheel cylinder pressure in accordance
with the fluctuations in the regenerative brake force. In addition, the SLA contains a relief function to
provide hydraulic pressure to the wheel cylinders in the event of a brake ECU failure.

CHASSIS ± BRAKES
182CH52
Spring
for 1st
Stage
Spring
for 2nd
StageTo
ReservoirStroke
Simulator
Full Stroke
Pedal Stroke 2nd Stage
Spring
Characteristics
1st Stage +
2nd Stage
Spring
Characteristics
Master Cylinder Hydraulic Pressure From
Master
Cylinder
182CH53
From wheel Cylinder (SLR)
182CH54
11 4
4) Stroke Simulator
The stroke simulator generates a pedal stroke in accordance with the driver's pedal effort during braking.
Containing 2 types of coil springs with different spring constants, the stroke simulator provides pedal
stroke characteristics in 2 stages in relation to the master cylinder pressure.
5) Reservoir
Temporarily stores the brake fluid to absorb
the pressure when regulating the wheel cylin-
der pressure.
6) Pressure Sensors
Mounted on the brake actuator, the pressure
sensor linearly detects the pressure that is gen-
erated in the master cylinder, regulator, and
the front and rear wheel cylinders and outputs
them to brake ECU.

CHASSIS ± BRAKES
182CH55
Speed Sensors
Combination Meter
Speedometer
ABS Warning Light
Brake System Warning
Light
Brake Warning Buzzer
Stop Light Switch
ECM
HV ECU
Inverter
Battery ECU
Brake
ECU
Solenoid Relay
Motor Relay
Brake Actuator
ABS Solenoid
Valves
Switching Solenoid Valve
Linear Solenoid Valve
Master Cylinder Pressure
Sensor
Regulator Pressure Sensor
Front Wheel Pressure Sensor
Rear Wheel Pressure Sensor
Pump Motor
High Pressure Switch
Low Pressure Switch
Reservoir Level Switch
Hydraulic Brake Booster11 5
Brake ECU
1) General
Based on the signals received from the sensors the communication it maintains with the HV ECU, the
brake ECU effects conventional brake control, ABS with EBD control, and regenerative brake coopera-
tive control.
2) Fail Safe
If a malfunction occurs in the brake ECU, in the input signals from the sensors, or in actuator system,
this function prohibits the current from flowing to the brake actuator.
As a result, the solenoids in the brake actuator turn off, enabling the braking force of the hydraulic brake
to take effect. Furthermore, by illuminating the ABS warning light or the brake system warning light,
this function alerts the driver that a malfunction exists in the system.
Only if the regenerative brake system cannot be used, such as in the case of a malfunction in the commu-
nication with the HV ECU, this function switches controls to enable the hydraulic brake to generate the
entire brake force.
3) Warning Light Check Function
The ABS warning light and brake system warning light turns on for about 3 seconds after the ignition
switch is turned on to check the circuit.

CHASSIS ± BRAKES
182CH79
Port A
Pressure
Holding
Valve
To
Wheel
Cylinder
Port B
Pressure
Preduction
ValveTo
Reservoir
and Pump
From Wheel Cylinder
182CH80 182CH81
11 6
4) Self-Diagnosis
If the brake ECU detects a malfunction in the brake system, the ABS warning light and brake system
warning light will light up and alert the driver that a malfunction has occurred. The ECU will also store
the codes of malfunctions. See the 2001 Prius Repair Manual (Pub. No. RM778U) for the DTC (Diagnos-
tic Trouble Code) check method, DTC and DTC clearance.
Brake System Control
1) ABS with EBD control
a. General
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 front wheels, helping to maintain the vehicle stability.
The distribution of the brake force is performed under electrical control of the brake ECU, which pre-
cisely controls the brake force in accordance with the vehicle's driving conditions.
b. Operation
Based on the signals received from the 4 wheel speed sensors, the brake ECU calculates each wheel
speed and deceleration, and checks wheel slipping condition. And according to the slipping condition,
the ECU controls the solenoid valves in order to adjust the fluid pressure of each wheel cylinder in the
following three modes: pressure reduction, pressure holding and pressure increase modes.
Not Activated
Normal Braking±±
ActivatedPressure Increase ModePressure Holding ModePressure 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
PressureIncreaseHoldReduction

CHASSIS ± BRAKES
182CH57
To Reservoir TankFrom Master Cylinder
Pressure
SensorPressure
Sensor
SLR
OFF
SLA
ON
SMC1
ON ONSMC2
Stroke
Simulator
ReservoirSS
ON
Pressure
SensorPressure
Sensor
SRrH
ONSFRH
ON
SRrR
ONSFRR
OFFSFLH
OFF
SFLR
OFF
Rear Wheel CylindersFront Wheel Cylinders P & B Valve11 7

Sample of ABS control 

CHASSIS ± BRAKES
Battery
Acceptance
Capacity
182CH58
Braking
Force
Regenerative
Braking ForceHydraulic
Braking Force
Vehicle SpeedRegenerative
Braking ForceHydraulic
Braking Force
Time
Changes in Regenerative Braking Force Changes in Braking Force Apportionment
Braking
Force
Driver's DemandDriver's Demand 11 8
2) Regenerative Brake Cooperative Control
a. General
The regenerative brake cooperative control uses the switching valves and linear solenoid valves to regu-
late the hydraulic pressure that is supplied to the wheel cylinders. It also operates cooperatively with
the regenerative braking force that is generated in the MG2 in accordance with the master cylinder pres-
sure.
b. Apportioning of the Brake Force
The apportioning of the brake force between the hydraulic brake and the regenerative brake varies by
the vehicle speed and time.
The apportioning of the brake force between the hydraulic brake and the regenerative brake is con-
trolled so that the total brake force of the hydraulic brake and the regenerative brake matches the brake
force that the driver requires.

Imagery Drawing 
c. Operation
Regenerative brake cooperative control is executed when the vehicle is driven in the shift position ªDº
or ªBº.
The master cylinder pressure that is generated when the driver presses on the brake pedal is detected
by the pressure sensor, and the brake ECU calculates the brake force request factor. A portion of the
brake force request factor is transmitted to the HV ECU in the form of a regenerative brake activation
request factor. The HV ECU executes generative braking by commanding the electric motor to generate
negative torque.
The brake ECU controls the opening of the linear solenoid valves, which are used for increasing / de-
creasing the hydraulic pressure, to regulate the wheel cylinder hydraulic pressure in relation to the mas-
ter cylinder hydraulic pressure, thus compensating for the brake force that is not provided sufficiently
by the regenerative brake.
While the regenerative brake cooperative control is being prohibited due to an abnormality in the sys-
tem, or when the shift lever is in a position other than D or B, the regenerative braking force is not gener-
ated. At this time, only the hydraulic braking force is applied by turning ON (opening) the linear sole-
noid valve SLA and turning OFF (closing) the SLR.