1991 ACURA NSX Service Repair Manual

System Description

EPS Control Unit

The control unit receives input signals from the car's speed sensors, torque sensor and steering rotation sensor, and
outputs control signals to the power unit. It performs a multitude of control works including a circuit which diagnoses
troubles in each functional part of the system and the control unit itself. The control unit operates the system while the engine is in the process of starting, on receipt of voltage generated by the alternator. The system remains operationalif the engine stalls with the ignition switch in ON position.
< Basic Control >

(1) Travelling speeds are compiled into data, receiving input signals from the car's speed sensors.
(2) The magnitude and direction of torque are compiled into data, receiving input signals from the torque sensor.
(3) The rotational speed and direction of steering are compiled into data, receiving input signals from the steering rota- tion sensor.
(4) Determination of motor torque data from the prescribed assisting force map, based on the car speed data, steering rotational speed data and the steering torque data.
(5) Changeover of control modes and output of control signals to the power unit, based on data on car speed, direction of torque and rotational direction of steering.
Normal Control Mode:
The rotational direction of motor is determined after changing over to the left/right steering mode in accordance with tor-
que direction data. The mode is switched to the straight ahead mode when the output data is zero.
Return Control Mode:
According to torque direction data and steering rotational direction data, the mode is changed over to the return control
mode to improve the steering return characteristic.
Damper Control Mode:
According to car speed data, torque value data and steering rotational speed data, the mode is changed over to the damper
control mode to improve the convergence property of the steering.
Steering rotation sensor
Steering rotation duty Steering torque sensor
Unloader duty

(Reduces over-current flow
at full-lock position)
Output duty

Mode changeover
Normal mode

Return mode
Damper mode
Torque duty
Motor control
signal output To power unit
Base duty

Car speed sensorProCarManuals.com

Self-Diagnosis Function

The EPS control unit monitors the system inputs and outputs, and the driving current of the motor. If there is a problem

in the system, the control unit turns the system off by actuating the relay in the power unit. Power assist stops and

normal manual steering operation resumes. The control unit also turns the EPS indicator light on to alert the driver, and

memorizes the problem in the form of a code. Connecting the terminals of the service check connector with a jumper

wire enables the EPS indicator light to blink the problem code when the ignition switch is turned on.

Unloader Control

If the steering wheel is turned fully and held in the full-lock position, the steering torque reaches the maximum point,

and an over-current flows to the motor and the power unit. The control unit detects this and reduces the current flow

to the motor.

Average Moving Current Control

The electric current flow to the motor is estimated from the current values detected by the current sensor, and the aver-

age current is obtained at two second intervals. The motor driving current is suppressed when the average current value

exceeds a predetermined marginal value. The control unit regurates the motor current during continuous loading to sup-

press any excessive temperature rise in the power unit and the motor.

Over-Voltage Control

If there is an excessive increase in power source voltage due to a poor battery condition, an alternator voltage regulator

problem, etc., the motor assisting force increases, resulting in excessive control. To prevent this, the control unit signals

to the power unit are corrected to ensure that adequate assisting force is generated.ProCarManuals.com

System Description

Power Unit

The power unit consists of a driving circuit, current sensor, field effect transistor (FET) bridge circuit, and two relays.
It receives control signals from the CPU and controls the driving current of the motor. The driving circuit controls the
rotational direction and speed of the motor by driving the FET bridge circuit with a pulse width modulation (PWM) method

on receipt of an input of driving signals from the EPS control unit.

< Rotational Speed Control >
The PWM driving signal is a digital signal repeating the process of voltage ON/OFF at a constant frequency, which changes the ratio of ON time per one cycle of this signal. The ratio is called the duty ratio. When there is a change in duty ratio,
the average voltage changes as smoothly as an analog type. The ratio of digital signal voltage (E) and the average voltage
(Motor rotational speed fast)
ONE CYCLE

AVERAGE

VOLTAGE (60%)

VOLTAGE APPLIED RATIO

IN ONE CYCLE: 60%

CURRENT

SENSOR

POWER

RELAY

FROM

BATTERY

RELAY

CONTROL

SIGNALS

VOLTAGE APPLIED RATIO

IN ONE CYCLE: 40%

AVERAGE

VOLTAGE (40%)

(Motor rotational speed slow)

DIGITAL VOLTAGE
ONE CYCLE

CURRENT
FEED BACK

DRIVE

SIGNAL

< Rotational Direction Control >
Normal Mode Control:
The table below shows the normal control mode to con-
trol the flow of current from the battery:
("PWM" in the table indicates PWM control based on
torque sensor data). Return Control Mode:
Return control mode improves the steering return charac-
teristics. ("PWM" in the table denotes PWM control based on torque sensor data while "PWM-r" PWM con-
trol based on rotation sensor).
POWER UNIT

FET DRIVE

CIRCUIT
GROND


FAIL SAFE

RELAY

Damper Control Mode:
The damper mode control which improves the convergence of steering, is performed with damper mode signals from
the control unit. In this mode, a short current circuit is formed on the motor side by turning off FET (1) and (2), and on FET (3) and (4), which suppresses the returning speed of the steering. < Motor Driving Current Control >

A current sensor, power relay and fail-safe relay are built into the power unit. The current sensor detects motor driving

current and transmits data to the EPS control unit. If there is a problem in the system, a cut-off signal is sent from the EPS
control unit to relay, then the relay cuts off motor current to switch to manual steering operation. (V) is called the duty ratio
of motor is slow. As the duty ratio increases, the rotational speed increases to increase the torque. When the duty ratio is low, the rotational speed
Its relationship is expressed by V = E x
Steering
condition Steering to
rightStraight

ahead

Steering to
left

OFFOFF
ON

FET

(1)
FET

(2)
FET

(3)
FET

(4)

OFF

OFF

OFF

ON
OFF

PWM

OFF
OFF

PWM
Motor operation
Stops
Operates in direction
steering to the left Operates in direction
steering to the right
Steering condition
Return from right steering
to straight ahead
Return from left steering
to straight ahead
FET (1)

PWM-r

OFF
FET (2)

OFF

PWM-r
FET (3)

OFF

PWM
OFF

PWM

FET (4)

MOTORProCarManuals.com

Steering Gearbox

< Motor and Power Assist Mechanism >

A motor is housed inside the gearbox housing. It consists of a yoke with a permanent magnet fixed in it, a rotor with

a field coil, and brushes which pass electricity to rotor commutators. The motor is on the rack shaft of the steering gear-

box. The drive current from the power unit flows through the power relay and fail-safe relay to the brushes of the motor.

The drive current flows to rotor commutators through brushes, exciting the field coil of the rotor and rotates the rotor

with the magnetic action of the permanent magnet inside the yoke. The rotor rotates in the opposite direction when

the direction of drive current is reversed.

The rotation torque of the motor is transmitted to the ball screw throught the helical gear. The rotation torque on the

ball screw is converted into assist thrust in the direction of steering rack by the recirculating ball screw. The recirculating

ball screw acts to reduce the steering thrust put into the steering rack through the joint to lessen the steering force re-

quired at the steering wheel.

< Recirculating Ball Screw >

The recirculating ball screw is constructed so that balls roll between the screw shaft and the nut, and those coming out

of the screw face enter the screw groove again through a tube for recirculation. The rotation torque on the shaft is con-

verted into thrust in axial direction as the slope of the screw shaft presses against the nut through balls. Since the torque

conversion is made in this method through the screw shaft and balls, the friction is small, high transmission efficiency is

obtained for both the forward and backward directions, and the steering feels identical to the ordinary manual steering.

Furthermore, since the force from the backward direction (force pressing the steering rack) is converted into the rotation,

torque of the screw shaft, instantaneous kick-back from road surface will rapidly rotate the motor. In such a rapid rota-

tion, the inertia of the motor increases, which acts to reduce the kick-back. Any great kick-back will be transmitted from

the steering rack to the torque sensor on the pinion, which acts to twist the pinion shaft in the same direction as the input

direction of kick-back. As a result, the motor operates the kick-back to reduce asist force.

BALL SCREW
ROTOR

NUT

TUBE
STEEL BALL
HELICAL GEAR
BALL SCREW
RECIRCULATING

BALL SCREW
RACKProCarManuals.com

Wiring Locations

EPS CONTROL UNIT

MAIN RELAY BOX

POWER UNIT

TORQUE SENSOR 6-P CONNECTOR/

STEERING WHEEL SPEED SENSOR

4-P CONNECTOR
STEERING GEARBOX

(TORQUE SENSOR/STEERING WHEEL

SPEED SENSOR)
UNDER-DASH FUSE/RELAY BOX
TRANSMISSION CONTROL MODULE (TCM)

22-P CONNECTOR
VEHICLE SPEED SENSOR

(VSS)
ALTERNATOR 4-P CONNECTOR
EPS INDICATOR LIGHTProCarManuals.com

Troubleshooting

Troubleshooting Precautions

Always use the test harness to verify the resistance, voltage, current and other electrical characteristics for the EPS
control unit, torque sensor, steering wheel speed sensor, power unit, and their connectors.
The test probe must not make direct contact with the connector terminals. Make sure that the terminals are not bent or damaged in any way when a test harness is installed.
A digital circuit tester must be used to perform the inspections.
An analog circuit tester must be used only when this procedure is indicated in the specification figures.
The EPS system connector should not be disconnected except when necessary.
Inspect the connection condition and ground of each EPS system connector before peforming the troubleshooting
procedures.ProCarManuals.com

Troubleshooting Procedures

Under normal conditions, the EPS indicator light in the gauge assembly comes on when the ignition switch is turned

to the ON position, then goes off after the engine is started. This indicates that the bulb and its circuits are operating

correctly.

EPS INDICATOR LIGHT

When the EPS control unit senses an abnormality in the system, the control unit memorizes the problem, the EPS indica-

tor light blinks at equal intervals, and the power steering assist is turned off.

When the EPS indicator light has been reported on, connect the two terminals of the service check connector with a

jumper wire. Then turn on the ignition switch and observe the blinking frequency.

SERVICE CHECK

CONNECTOR (BLU)

GRN/WHT

SERVICE CHECK CONNECTOR (BLU)
BLUProCarManuals.com

Troubleshooting

Diagnostic Trouble Code (DTC) Display

When the service check connector terminals are connected with a jumper wire, the EPS control unit blinks the EPS indi-
cator light to indicate the DTC. The DTC is indicated by a series of long and short blinks. One long blink equals 10 shortblinks. Add the long and short blinks together to determine the DTC, then refer to the Troubleshooting Chart on page 17-51.

Example 1.3 seconds
0.28 seconds
1.3 seconds

0.36 seconds
The control unit can memorize any number of simultaneous problems. The DTC is repeated sequentially beginning with
the lowest number.
The DTC is stored in the control unit memory even when the ignition switch is turned off. After repairing the EPS system,
disconnect the CLOCK fuse in the relay box for more than 10 seconds to reset the EPS control unit memory.ProCarManuals.com