ACURA NSX 1997 Service Repair Manual

System Description

Function and Operation (cont'd)

EPS Control Unit

The control unit receives input signals from the vehicle speed sensors, torque sensor. It performs a multitude of control

functions, including a circuit which can detect 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 alter-

nator. 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 vehicle'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 calculated data.

(4) Determination of motor torque data from the prescribed assisting force map, based on the vehicle speed data, steer-

ing rotational speed data and the steering torque data.

(5) Changeover of control modes based on data on vehicle 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 vehicle 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.ProCarManuals.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. 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 the SCS service connector (special tool)

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

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. 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 average

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 regulates the motor current during continuous loading to sup-

press any excessive temperature rise in the motor.ProCarManuals.com

System Description

Function and Operation (cont'd)

The power module in the EPS 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 modulation (PWM) method on receipt of an input of driving signals from the CPU.

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 Its relationship is expressed by 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.

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 char-

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

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

trol based on steering rotation speed data).

Damper Control Mode:

The damper mode control, which improves the convergence of steering, is performed with damper mode signals from the

CPU.

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.ProCarManuals.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 gearbox.

The drive current from the control 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 direc-

tion 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 mechanical advan-

tage of the recirculating ball screw, compaired to a conventional rack and pinion, acts to reduce the steering force to the

steering rack through the joint, to lessen the steering effort required at the steering wheel.

Recirculating Ball Screw

The recirculating ball screw is constructed so that steel 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

converted into thrust in an axial direction as the slope of the screw shaft presses against the nut through the steel balls.

Since the torque conversion is made in this method through the screw shaft and the steel 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 the 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 assist force.ProCarManuals.com

Component Location s
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EPS Control Unit Terminal Arrangement

18P CONNECTOR

WIRE SIDE OF FEMALE TERMINALSProCarManuals.com

3P CONNECTOR (LEFT SIDE)

VB: Battery voltage

IG SW: Ignition switch

VB: Battery voltage

IG SW: Ignition switch

PULSE UNIT 7P CONNECTOR (MT)

6P CONNECTOR
3P CONNECTOR (RIGHT SIDE)ProCarManuals.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 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.

• 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

Diagnostic Troubl e Cod e (DTC ) Indicatio n
1. Connec t th e SC S servic e connecto r t o th e servic e chec k connecto r unde r th e glov e box .
2 . Tur n th e ignitio n switc h O N (II) , bu t d o no t star t th e engine .
3 . Recor d th e blinkin g frequenc y o f th e EP S indicato r light . Th e blinkin g frequenc y indicate s th e diagnosti c troubl e cod e
(DTC) .
NOTE : Chec k th e DT C carefull y an d recor d it . Th e memor y o f th e DT C is erase d i f th e connecto r i s disconnecte d fro m
th e EP S contro l unit .
SCS SERVIC E CONNECTO R07PAZ-001010 0
SERVICE CHEC KCONNECTO R (2P )
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