2001 TOYOTA PRIUS Service Repair Manual

CHASSIS ± STEERING
182CH68
Rear Wheel
Speed SensorsBrake ECU
Torque Sensor Signal 1
Torque Sensor Signal 2
EMPS
ECUMeter ECU
Gateway ECUDLC3
Display ECU
Multi-information
Display
123
EMPS (Electric Motor-assisted Power Steering)
1. General
In accordance with sensor information such as that provided by the rear wheel speed sensors, the torque sen-
sor that is mounted on the steering gear, and the EMPS ECU determines the direction and the force of the
power assist and actuates the DC motor that is mounted on the steering gear to provide power assist to the
steering effort.
The EMPS in the Prius the following features:
This system can provide power assist even when the engine is stopped.
This system offers excellent fuel economy characteristics because power assist is provided by the DC mo-
tor that is mounted on the steering gear, and this motor consumes energy only when power assist is re-
quired.
Unlike the conventional hydraulic power steering system, this system excels in serviceability because it
does not require pipes or the power steering fluid.

System Diagram 

CHASSIS ± STEERING
182CH69
Meter ECU
EMPS ECU
Relay
DC MotorRear Wheel Speed
Sensors and Rotors
Brake ECU Multi-information Display 124
2. Layout of Main Components
3. Function of Main Components
ComponentsFunction
St i
Torque Sensor
Detects the twist of the torsion bar, converts the torque that is applied
to the torsion bar into an electrical signal, and outputs this signal to
the ECU.
Steering
Gear
DC MotorGenerates power assist in accordance with a signal received from the
EMPS ECU.
Reduction
MechanismReduces the rotation of the DC motor and transmits it to the pinion
shaft.
EMPS ECU
Actuates the DC motor mounted on the steering gear for providing
power assist, based on the signals received from various sensors and
the rear wheel speed signal.
Meter ECUOutputs a signal for displaying the PS warning on the center display
in case of a malfunction in the system.
Brake ECURear wheel speed signals are outputted to EMPS ECU.
RelaySupplies power to the DC motor for power assist.
Multi-information DisplayDisplays the PS warning in case of a malfunction in the system.

CHASSIS ± STEERING
182CH70
182CH71
182CH72
Torque
Sensor
Rack and
PinionDC Motor
Reduction Mechanism
Motor
Shaft
Stator
Rotor
Pinion Gear
Motor Shaft
DC Motor
Ring Gear
Pinion Shaft125
4. Construction and Operation of Main Components
Steering Gear
1) General
The steering gear consists of the rack and pin-
ion, DC motor, reduction mechanism, and
torque sensor. The front suspension compo-
nents have been optimized to reduce the load
on the DC motor. Also, the components have
been optimally allocated to ensure ample rack
travel and wheel turning angle, resulting in
excellent cornering performance.
2) DC Motor
The DC motor is mounted on the gear hous-
ing. The DC motor consists of the motor shaft
that is integrated with the hypoid pinion to
transmit the torque that has been generated by
the drive force of DC motor to the pinion
shaft, the rotor and stator.
3) Reduction Mechanism
A reduction mechanism that transmits the
rotation of the motor to the pinion shaft has
been mounted on the steering gear. The reduc-
tion mechanism consists of the ring gear that
is secured to the pinion shaft and the pinion
gear that is integrated with the motor shaft.
The power assist of the motor is transmitted
by the reduction mechanism to the pinion
shaft, which provides power assist to the
steering effort.

CHASSIS ± STEERING
182CH73
Contact 1 Resistor 1
Upward
Resistor 2
DownwardContact 2
Contacts Torque
Sensor
Resistor
182CH74
Resistor
Basic
Position
Contact
Straightline 126
4) Torque Sensor
A torque sensor that detects the torque that is input by the steering wheel has been mounted on the pinion
shaft. The torque sensor has been integrated with the pinion shaft, and the pinion shaft's input and output
shafts are linked via the torsion bar. Resistors for the torque sensor are mounted on the input shaft of the
pinion shaft, and contacts for the torque sensor are mounted on the output shaft of the pinion shaft. Oper-
ating the steering wheel causes the torsion bar to twist, creating a displacement between the pinnion
shaft's input and output. Two systems of torque sensors detect this displacement in the form of voltage
changes, which are then output to the EMPS ECU.
i) Straightline Driving
If the vehicle is driven straight and the driv-
er does not turn the steering, torque is not
generated in the pinion shaft's input shaft.
Thus, the torsion bar does not twist, and no
changes in resistance occur in the torque
sensor.

CHASSIS ± STEERING
182CH95
182CH76
Twist Angle
Basic Position
Cornering
Torque
[V]
Voltage
Voltage difference between
outputs 1 and 2 during
straight line driving (0)
Torque Sensor 2
Torque Sensor 1
0 ±+
Right Turn Left Turn127
ii) Steering to the right
When the driver turns the steering to the
right, the steering torque is transmitted to
the pinion shaft's input shaft, causing the
input shaft to rotate. Because the reaction
force of the ground surface acts on the rack
bar, the torsion bar that links the input shaft
and the pinion twists until a torque that
equalizes with the reaction force is gener-
ated. Thus, a relative displacement is
created between the resistor that is secured
onto the input shaft and the contact that is
secured onto the pinion shaft's output shaft.
As a result, the resistance changes, causing
the torque sensor's outputs 1 and 2 to
change as shown in the right diagram. The
EMPS ECU uses this voltage difference to
calculate the power assist torque to drive
the DC motor, thus generating a power as-
sist force in the pinion shaft via the reduc-
tion mechanism.
iii) Steering Hold Condition
The torsion bar shift to a position in which the sum of the driver's steering torque and the motor's assist
torque equalizes with the reaction force of the ground surface in order to maintain the steering holding
condition.
EMPS ECU
1) EMPS Control
The EMPS ECU receives signals from various sensors, judges the current vehicle condition, and deter-
mines the assist ampere to be applied to the DC motor accordingly.
2) Self-Diagnosis
If the EMPS ECU detects a malfunction in the EMPS system, the warning light that corresponds to the
function in which the malfunction has been detected lights up to alert the driver of the malfunction.
The EMPS ECU will also store the codes of the malfunctions. The DTCs (Diagnostic Trouble Codes)
can be accessed through the use of a hand-held tester. For details, see the 2001 Prius Repair Manual (Pub.
No. RM778U).
3) Fail-Safe
If the EMPS ECU detects a malfunction in the EMPS system, the system basically turns OFF the power
to prohibit power assist. As a result, the EMPS system operates the same way as manual steering.
However, depending on the location in which the malfunction occurred, power assist may be provided
by reducing the power assist amperage or by fixing the amount of power assist without relying on the
vehicle speed.