ACURA NSX 1991 Service Repair Manual

Rotation Sensor

The rotation sensor is housed inside the steering gearbox near the pinion. It converts the rotation speed and direction of

steering into voltage signals. The rotation of the steering wheel is transmitted to the pinion, which drives the sensor by

accelerating its speed through the three pieces of transmission the gears. Acting as a kind of direct current generator, the

sensor generates direct current voltage proportionately with the rotational speed of the pinion. When the steering direc-

tion is reversed, the voltage is generated in the opposite direction. No power is generated when driving straight ahead

without turning the steering wheel.

Interface Circuit

The interface circuit is housed inside the steering gearbox near the pinion. It rectifies and amplifies the signals from the

torque sensor and the rotation sensor, and transmits the steering signals to the EPS control unit. A phase compensating

circuit is also built in, which changes the output signals in accordance with the drive signals input from the EPS control

unit.

TRANSMISSION GEARS

INTERFACE CIRCUIT

ROTATION SENSORProCarManuals.com

System Description

Function and Operation

EPS Control Unit

The control unit receives input signals from the car's speed sensors, torque sensor and rotation sensor. It performs a mul-

titude of control functions, including a circuit which can detect troubles in each functional part of the system and the con-

trol 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 operational if the engine stalls with the ignition switch in ON (II) 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 rotation 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 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

torque 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.

Rotation sensor
Torque sensor

Steering rotation duty

Unloader duty

(Reduces over-current flow

at full-lock position)

Output duty
Torque duty

Mode changeover

• Normal mode

• Return mode

• Damper mode
Motor control

signal output
Base duty
Car speed sensorsProCarManuals.com

Self-Diagnosis Functio n
Th e EP S contro l uni t monitor s th e syste m input s an d outputs , an d th e drivin g curren t o f th e motor . I f ther e i s a proble m in
th e system , th e contro l uni t turn s th e syste m of f b y actuatin g th e relay . Powe r assis t stop s an d norma l manua l steerin g
operatio n resumes . Th e contro l uni t als o turn s th e EP S indicato r ligh t o n t o aler t th e driver , an d memorize s th e proble m in
th e for m o f a code . Connectin g th e terminal s o f th e servic e chec k connecto r wit h th e SC S servic e connecto r (specia l tool )
enable s th e EP S indicato r ligh t t o blin k th e proble m cod e whe n th e ignitio n switc h i s turne d o n (II) .
Unloade r Contro l
I f th e steerin g whee l i s turne d full y an d hel d in th e full-loc k position , th e steerin g torqu e reache s th e maximu m point , an d
a n over-curren t flow s t o th e motor . Th e contro l uni t detect s thi s an d reduce s th e curren t flo w to th e motor .
Averag e Movin g Curren t Contro l
Th e electri c curren t flo w to th e moto r i s estimate d fro m th e curren t value s detecte d b y th e curren t sensor , an d th e averag e
curren t i s obtaine d a t tw o secon d intervals . Th e moto r drivin g curren t i s suppresse d whe n th e averag e curren t valu e
exceed s a predetermine d margina l value . Th e contro l uni t regulate s th e moto r curren t durin g continuou s loadin g t o sup -
press any excessiv e temperatur e ris e in th e motor .
Over-Voltag e Contro l
I f ther e i s a n excessiv e increas e i n powe r sourc e voltag e du e t o a poo r batter y condition , a n alternato r voltag e regulato r
problem , etc. , th e moto r assistin g forc e increases , resultin g i n excessiv e control . T o preven t this , th e contro l signal s ar e
correcte d to ensur e tha t adequat e assistin g forc e is generated .
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System Description

Function and Operation

The control unit consists of a driving circuit, current sensor, field effect transistor (FET) bridge circuit, and two relays. It

receives control signals from the central processing unit (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 modu-

lation (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 (V) is called the duty ratio (8). Its relationship is expressed by V = E x 8. When the duty ratio is low, the rotational

speed of motor is slow. As the duty ratio increases, the rotational speed increases to increase the torque.

(Motor rotational speed slow)

ONE CYCLE
(Motor rotational speed fast)

ONE CYCLE

AVERAGE /

VOLTAGE (40%)
VOLTAGE APPLIED RATIO

IN ONE CYCLE: 40%

RELAY

CONTROL

SIGNALS
VOLTAGE APPLIED RATIO

IN ONE CYCLE: 60%
AVERAGE

VOLTAGE (60%)

FROM

BATTERY
POWER

RELAY
CURRENT

SENSOR

Rotational Direction Control

Normal Mode Control:

The table below shows the normal control mode to con-

trol the flow of current from the battery:

CURRENT

FEED BACK

DRIVE

SIGNAL

FET DRIVE

CIRCUIT
GROUND

FAIL SAFE

RELAY

("PWM" in the table indicates PWM control based on

torque sensor data).

Return Control Mode:

Return control mode improves the steering return char-

acteristics. ("PWM" in the table denotes PWM control

based on torque sensor data while "PWM-r" PWM con-

trol based on rotation sensor).

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 turning 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 control unit. The current sensor detects motor driving

current. If there is a problem in the system, a cut-off signal is sent from the CPU to relay, then the relay cuts off motor cur-

rent to switch to manual steering operation.
CONTROL UNITProCarManuals.com

Steering Gearbo x
Motor an d Powe r Assis t Mechanis m
A moto r i s house d insid e th e gearbo x housing . I t consist s o f a yok e wit h a permanen t magne t fixe d i n it , a roto r wit h a
fiel d coil , an d brushe s whic h pas s electricit y t o roto r commutators . Th e moto r i s o n th e rac k shaf t o f th e steerin g gearbox .
Th e driv e curren t fro m th e contro l uni t flow s throug h th e powe r rela y an d fail-saf e rela y t o th e brushe s o f th e motor .
Th e driv e curren t flow s t o roto r commutator s throug h brushes , excitin g th e fiel d coi l o f th e roto r an d rotate s th e roto r wit h
th e magneti c actio n o f th e permanen t magne t insid e th e yoke . Th e roto r rotate s i n th e opposit e directio n whe n th e direc -
tio n o f driv e curren t i s reversed .
The rotatio n torqu e of th e moto r is transmitte d to th e bal l scre w through the helica l gear . The rotatio n torqu e on th e bal l
scre w is converte d int o assis t thrus t i n th e directio n o f steerin g rac k b y th e recirculatin g bal l screw . Th e mechanica l advan -
tag e o f th e recirculatin g bal l screw , compaire d t o a conventiona l rac k an d pinion , act s t o reduc e th e steerin g forc e t o th e
steerin g rac k throug h th e joint , t o lesse n th e steerin g effor t require d a t th e steerin g wheel .
ROTORRACK
RECIRCULATIN GBALL SCRE W
BAL L SCRE W
HELICA L GEA R
Recirculatin g Bal l Scre w
Th e recirculatin g bal l scre w is constructe d s o tha t stee l ball s rol l betwee n th e scre w shaf t an d th e nut , an d thos e comin g
ou t o f th e scre w fac e ente r th e scre w groov e agai n throug h a tub e fo r recirculation . Th e rotatio n torqu e o n th e shaf t i s
converte d int o thrus t i n a n axia l directio n a s th e slop e o f th e scre w shaf t presse s agains t th e nu t throug h th e stee l balls .
Sinc e th e torqu e conversio n i s mad e i n thi s metho d throug h th e scre w shaf t an d th e stee l balls , th e frictio n i s small , hig h
transmissio n efficienc y i s obtaine d fo r bot h th e forwar d an d backwar d directions , an d th e steerin g feel s identica l t o th e
ordinar y manua l steering .
Furthermore , sinc e th e forc e fro m th e backwar d directio n (forc e pressin g th e steerin g rack ) i s converte d int o th e rotatio n
torqu e o f th e scre w shaft , instantaneou s kick-bac k fro m th e roa d surfac e wil l rapidl y rotat e th e motor . I n suc h a rapi d rota -
tion , th e inerti a o f th e moto r increases , whic h act s t o reduc e th e kick-back . An y grea t kick-bac k wil l b e transmitte d fro m th e
steerin g rac k t o th e torqu e senso r o n th e pinion , whic h act s t o twis t th e pinio n shaf t i n th e sam e directio n a s th e inpu t
directio n o f kick-back . A s a result , th e moto r operate s th e kick-bac k to reduc e assis t force .
BALL SCRE W
TUBESTEE L BAL L
NUT
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Component Location s
GAUGE ASSEMBL Y
UNDER-HOO D FUSE /
RELAY BO X
TRANSMISSIO N CONTRO L
MODUL E (TCM )
22 P CONNECTO R
VEHICL E SPEE D SENSO R
(VSS)
(M/T)PULS E UNI T 7 P CONNECTO R
TORQU E SENSO R 6 P CONNECTOR /
ROTATIO N SENSO R
4 P CONNECTO R
STEERING GEARBO X
(TORQU E SENSOR /
ROTATIO N SENSOR )
UNDER-DASH FUS E BO X
EP
S CONTRO L UNI T
ALTERNATO R4P CONNECTO R
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GAUGE ASSEMBLY

EPS INDICATOR LIGHT

UNDER-HOOD FUSE/RELAY BOX

No. 2 (15 A)
IG SW (50 A)

CLOCK (7.5 A)

E-PS
(50 A)

UNDER-DASH FUSE BOX

No. 5 (15 A)ProCarManuals.com

EPS Control Unit Terminal ArrangementProCarManuals.com

3P CONNECTOR (LEFT SIDE)

WIRE SIDE OF FEMALE TERMINALS
VB: Battery voltage

IG SW: Ignition switch

3P CONNECTOR (RIGHT SIDE)

WIRE SIDE OF FEMALE TERMINALS
VB: Battery voltage

IG SW: Ignition switch

6P CONNECTOR

WIRE SIDE OF FEMALE TERMINALS

4P CONNECTOR

WIRE SIDE OF FEMALE TERMINALS
PULSE UNIT 7P CONNECTOR (MT)

WIRE SIDE OF FEMALE TERMINALSProCarManuals.com

Troubleshooting Precautions

EPS Indicator Light

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

the ON (II) position, then goes off after the engine is started. This indicates that the bulb and its circuits are operating cor-

rectly. If there is any trouble in the system, the EPS indicator light turns on during driving, and the power steering assist is

turned off. When the EPS indicator light comes on, the control unit memorizes the DTC. In this case, the control unit does

not activate the EPS system after the engine starts again but it keeps the EPS indicator light on.

Diagnostic Trouble Code (DTC)

• The lowest DTC is indicated first.

• The DTCs continue blinking until the ignition switch is turned off.

• The DTCs are erased from the control unit when the EPS control unit VBU power supply or connector is disconnected.

• The EPS system can be reset and the control unit's memory can be erased by disconnecting the CLOCK (7.5 A) fuse for

more than ten seconds.

Self-diagnosis:

The CPU (central processing unit) controls the following when it detects a problem during self-diagnosis:

1. Turns the EPS indicator light ON to alert the driver.

2. Power assist stops, and normal manual steering operation resumes (except DTC 21 and DTC 33).

3. The EPS control unit memorizes the diagnostic trouble code (DTC).

4. After the DTC is stored in the control unit, the CPU stops self-diagnosis.

Troubleshooting:

• Before starting the troubleshooting, clear the DTC by disconnecting the CLOCK (7.5 A) fuse for ten seconds or more,

and test-drive the car. Check that the symptom of the trouble appears again, and then troubleshoot. If the problem is an

intermittent problem, the system does not become active after the engine starts even though the problem is solved.

• When both EPS indicator light and A/T "D" indicator light come on, perform the A/T troubleshooting first.

• When the customer's reported problem cannot be verified in the car, ask the customer about the conditions when the

EPS indicator light came ON, then test-drive the car under those conditions, if possible. If the EPS indicator light does

not come ON during the test, check for loose connections or poor contacts at the connectors by wiggling the harness,

etc.

• The connector terminal numbers are viewed from the wire side for the female terminals and from the terminal side for

the male terminals.

• After the repair, test-drive the car and check that the EPS indicator light does not come ON again during the test. (Refer

to the Symptom-to-System Chart for diagnostic period.)ProCarManuals.com