Page 1145 of 1771

System Descriptio n
This powe r steerin g syste m is speed-sensitive . It s mai n component s ar e a mechanica l spee d sensor , a rotar y valve-typ e
gearbox, an d a constan t volume , vane-typ e pump . Th e syste m regulate s th e powe r assis t accordin g to vehicl e speed , roa d
conditions , an d drivin g style . Th e amoun t o f powe r assis t change s linearl y fro m hig h assist , whe n drivin g a t lo w speed s o r
parking , t o lo w assist , whe n drivin g a t hig h speeds .
Fluid Flo w Diagra m
The flui d volum e fro m th e pump , whic h i s drive n b y th e cran k pulley , i s regulate d b y th e flo w contro l valv e buil t int o th e
pump . Th e regulate d flui d i s fe d t o th e valv e bod y uni t locate d outsid e th e pinio n shaf t o f th e steerin g gearbox . Th e flui d i s
regulate d t o th e prope r pressur e b y th e valve s i n th e valv e bod y uni t an d b y th e powe r steerin g spee d senso r befor e it
flow s t o th e powe r cylinder . Th e directio n o f th e flo w is determine d b y th e rotar y valve . Th e flui d the n returns to th e reser -
voi r throug h th e retur n passag e an d i s fe d t o th e pum p again .
To RESERVOI R
VALVE BOD Y UNI T
TORSIO N BA R
INPU T SHAF T
VALVE (SLEEVE )
RIGHT CYLINDE R PASSAG E
POWE R CYLINDE R
Steering Gearbo xLOW SPEE D FEE D PASSAG E
The steerin g gearbo x i s th e rack-and-pinio n type . Th e valv e bod y uni t consist s o f th e valv e bod y unit , wher e th e flui d pas -
sag e i s regulate d b y th e flui d pressure , an d th e assis t contro l valv e uni t whic h regulate s th e flui d passag e t o th e rotar y
valv e accordin g t o vehicl e speed .
ASSIST CONTRO L VALV E UNI T
STEERIN G RAC K VALV
E BOD Y UNI T
ToRESERVOI R
From PUM P
POWE R CYLINDE RCYLINDE
R LINE S ASSIS
T CONTRO L VALV E UNI T
PUM P
To POWE R STEERIN G
SPEE D SENSO R
PUM
P OUTLE T
HIGH-PRESSUR E
HIGH SPEE D
FEE D PASSAG E
POWER STEERIN G
SPEE D SENSO R
RESERVOI R
LEFT CYLINDE R
PASSAG E
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Page 1146 of 1771

Steering Pum p
Constructio n
The pum p is a vane-typ e incorporatin g a flo w contro l valv e (wit h a n integrate d relie f valve ) an d i s drive n b y a POLY-V-bel t
fro m th e cran k pulley . Th e pum p feature s 1 0 vanes . Eac h van e perform s tw o intake/discharg e operation s fo r ever y rota -
tio n o f th e rotor . Thi s mean s tha t th e hydrauli c flui d pressur e puls e become s extremel y smal l durin g discharge .
DRIVE SHAF TFLOW CONTRO L VALV E
CAM RIN G
INLE T POR TVANE
ROTO R
Operatio n
The belt-drive n pulle y rotate s th e roto r throug h th e driv e shaft . A s th e roto r rotates , th e hydrauli c pressur e i s applie d t o
th e van e chambe r o f th e roto r an d th e vane s wil l rotat e whil e bein g pushe d ont o th e inne r circumferenc e o f th e ca m ring .
Th e inne r circumferenc e o f th e ca m rin g ha s a n extende d portio n wit h respec t t o th e cente r o f th e shaft , s o th e vane s
mov e downwar d i n th e axia l directio n a s th e roto r rotates . A s a resul t o f thi s rolle r movement , th e interna l volum e o f th e
van e chambe r wil l change , resultin g i n flui d intak e an d discharge .
START O F FLUI D INTAKE :FLUID INTAKE :FLUID MOVEMENT :FLUID DISCHARGE :
DISCHARGE POR T
The vane s ar e pushe d ont othe inne r circumferenc e o f th ecam ring .
The volum e o f th e van e cham -ber increase s s o tha t flui d i ssucke d in .
Th e sucked-i n flui d move stoward th e discharg e port .As th e vane s retur n t o th ethei r origina l positio n o n th einne r side , th e volum e o f th evan e chambe r decrease s s othe flui d i s discharge d fro mthe discharg e port .
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Page 1147 of 1771

System Descriptio n
Steering Pum p (cont'd )
Flow Contro l
Fluid fro m th e pum p run s throug h a meterin g orific e to
th e valv e bod y unit . Thi s create s a pressur e differenc e
betwee n th e pum p an d valv e bod y uni t side s o f th e ori -
fice. Whe n pressur e in th e pum p sid e is highe r tha n th e
forc e o f th e sprin g holdin g th e flo w contro l valv e closed ,
i t pushe s th e valv e dow n (open) , an d exces s flui d
return s t o th e pum p inlet . Th e combine d effec t o f th e
meterin g orific e an d th e flo w contro l valv e provide s a
relativel y constan t flo w o f flui d t o th e valv e bod y unit .
PUM P
FLO W CONTRO L
VALVE
RELIEF VALV E OPE N
DAMPIN G
ORIFICE
FLOW CONTRO L
VALV E
RELIE F VALV E BAL L
METERIN GORIFICE
To Valv e bod y Uni t
Pressur e Relie f
As pressur e o n th e valv e bod y uni t sid e build s up , i t
pushe s th e relie f valv e bal l (insid e th e flo w contro l
valve ) u p agains t it s spring , an d exces s flui d return s t o
th e pum p inlet . As the pressur e unde r th e flo w contro l
valv e drops , th e relie f valv e bal l i s close d b y it s spring ,
an d th e flo w contro l valv e i s force d dow n again , allow -
in g exces s flui d fro m th e pum p sid e t o retur n t o th e
inlet . Thi s flo w contro l valve-relie f valv e cylinde r keep s
pump outpu t pressur e betwee n 7,00 0 - 8,00 0 kP a (7 0 - 8 0
kgf/cm2, 99 5 - 1,13 8 psi) .
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Page 1148 of 1771
System Descriptio n
Power Steerin g Spee d Senso r
The powe r steerin g spee d senso r i s a trochoid-rotor ,
hydraulic pum p combine d wit h a relie f valv e an d a one -
wa y valve . I t i s drive n b y th e speedomete r gea r shaf t
which in tur n is drive n b y a helica l gea r o n th e differential .
ONE-WAY VALV E
INNER ROTO R
OUTER ROTO R
RELIE F VALV E
SPEEDOMETE R GEA R
The powe r steerin g spee d senso r turn s onl y whe n th e
ca r i s moving , controllin g th e assis t contro l valve .
ProCarManuals.com
Page 1149 of 1771
One-way Valve (In Power Steering Speed
Sensor)
When the vehicle is moving at high speed, negative
pressure develops at the sensor inlet because the power
steering speed sensor is pumping faster than the fluid
can be supplied. To compensate for this, the outlet and
inlet ports are connected internally by a passage con-
taining a one-way valve that lets output fluid recirculate
to the inlet port to equalize pressure.
Driving at High Speed:
ONE-WAY VALVE
RESERVOIR
ROTORS
Relief Valve (In Power Steering Speed Sensor)
When the vehicle is moving in reverse, the power steer-
ing speed sensor also turns backward and pumps fluid
in the opposite direction. To avoid building up pressure
in the reaction chambers that would increase steering
effort while driving in reverse, the inlet and outlet-ports
are connected by a second internal passage containing a
relief valve that allows the fluid to recirculate.
Driving in Reverse:
RESERVOIR
RELIEF VALVEProCarManuals.com
Page 1150 of 1771

System Descriptio n
System Operatio n
Assist Contro l Valv e
The assis t contro l valv e uni t consist s o f th e assis t contro l valve , whic h i s controlle d b y th e powe r steerin g spee d sensor ,
an d th e pressur e contro l valve . Th e pressur e contro l valv e an d th e assis t contro l valve , locate d betwee n th e pum p an d th e
spee d sensor , contro l th e flui d passag e t o th e rotar y valve . Th e flui d pressur e fro m th e pum p i s reduce d t o a "standar d
pressure " b y th e pressur e contro l valve , an d thi s standar d pressur e i s fe d t o th e assis t contro l valve . Th e standar d pres -
sur e i s draw n ou t b y th e rotatio n o f th e spee d sensor , producin g a pressur e ga p betwee n th e pressur e contro l valv e an d
th e assis t contro l valv e whe n th e standar d pressur e flui d passe s th e senso r orifice . Thi s pressur e gap , whic h change s wit h
th e flui d volum e draw n (tha t is , i t change s wit h vehicl e speed) , i s referre d t o a s th e powe r steerin g spee d senso r signa l
pressure . Th e assis t contro l valv e operate s accordin g t o thi s spee d senso r signa l pressur e t o chang e th e flui d passag e t o
th e hig h spee d rotar y valve .
POWE R STEERIN G SPEE D SENSO R
STANDAR D PRESSUR E
PRESSUR E CONTRO L VALV E
SPRING
To RESERVOI R
T o HIG H SPEE D VALV E
ASSIST CONTRO L VALV E POWE
R STEERIN G SPEE D
SENSO R SIGNA L PRESSUR E
ORIFICE
ORIFICE
FEED
SPRIN G
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Page 1151 of 1771

Valve Bod y Uni t
The rotar y valv e bod y uni t ha s tw o valves , a lo w spee d valv e an d a hig h spee d valve , whic h ca n b e identifie d b y th e cham -
fer width.
CHAMFE R O F HIG H SPEE D VALVE : LARG E
HIG H SPEE D VALV E
SLEEV E
CHAMFE R O F LO W SPEE D VALVE : SMAL L
LO W SPEE D VALV E
The openin g are a (th e ga p betwee n th e inpu t shaf t an d th e sleeve ) o f th e tw o valve s change s wit h th e displacemen t angl e
(angl e ga p betwee n th e inpu t shaf t an d th e sleeve ) produce d durin g steering . I n othe r words , th e valv e openin g are a
change s accordin g t o th e intensit y o f th e roa d reactio n (steerin g torque) .
Whe n th e vehicl e i s stopped , th e powe r steerin g spee d senso r function s operat e onl y th e lo w spee d valve .
Becaus e th e lo w spee d valv e ha s th e smal l chamfe r an d thereb y th e smal l openin g area , hig h flui d pressur e ca n b e fe d t o
th e powe r cylinde r wit h th e smal l displacemen t angl e (smal l steerin g torque) , resultin g i n ligh t steerin g (hig h assist) .
FLOW: 0 (ZERO ) OPENIN
G ARE A
HIG H SPEE D
VALV E
SMALL A
t STO P
DISPLACEMEN T ANGL E
When drivin g th e vehicle , th e flui d i s applie d t o th e hig h spee d valve , too , a s th e vehicl e spee d senso r functions . Becaus e
th e hig h spee d valv e ha s a larg e chamfe r an d thereb y th e larg e openin g area , th e openin g are a o f th e tw o valve s i s com -
pounded , eve n thoug h th e displacemen t angl e i s th e sam e a s whe n th e vehicl e i s stopped .
Therefore , th e flui d pressur e i s no t generate d unles s th e displacemen t angl e i s larg e (larg e steerin g torque) , resultin g i n
moderatel y heav y steerin g (mediu m assist) .
From ASSIS T CONTRO L VALV E
HIG H SPEE D
VALV E OPENIN
G ARE A
LARG EAREA
MIDDL EAREA
SMAL LAREA
At STO P
PRESSUR E MIDDL
E SPEE D RANG EPRESSUR E
DISPLACEMEN T ANGL E
T
o RIGH T CYLINDE R
LO
W SPEE D VALV E
HIGH-PRESSUR
E
Fro
m LEF T CYLINDE R HIG
H SPEE D RANG E
PRESSUR E
Fro
m LEF T CYLINDE R
HIGH-PRESSUR E
LO W SPEE D VALV E
T o RIGH T CYLINDE R
INPUT SHAF T
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Page 1152 of 1771

System Description
System Operation (cont'd)
Hydraulic Control at Each Speed Range
When stopped:
When the vehicle is stopped with the engine idling, the fluid flow is shut off because the power steering speed sensor
rotor is not rotating. The standard pressure generated by the pressure control valve is applied to the assist control valve to
move the valve to the left. When the assist control valve is in this position, the orifice valve is closed. Therefore, the fluid
discharged from the pump is fed to the low speed valve of the valve body unit. Because the low speed valve has the small
chamfer and thereby the small opening angle, the large hydraulic pressure (steering rack thrust) results in the smallest
steering force (high assist).
HIGH SPEED FEED PASSAGE
LOW SPEED FEED PASSAGE
ASSIST CONTROL VALVE UNIT
POWER STEERING SPEED
SENSOR SIGNAL PRESSURE
HIGH SPEED
VALVE
SLEEVE
TORSION BAR
INPUT SHAFT
VALVE BODY UNIT
SENSOR ORIFICE
ASSIST VOLUME: LIGHT
STEERING WITH VEHICLE PARKED
ROAD SURFACE RESISTANCE
SMALL
LARGE
FLOW: O (ZERO)
LOW SPEED
VALVE
POWER STEERING SPEED
SENSOR AT A STOP
RESERVOIR
PUMP
FULL CLOSE
PRESSURE CONTROL VALVE
(HIGH)
ASSIST CONTROL
VALVEProCarManuals.com