1991 LEXUS SC400 engine coolant

Diag. Code 27, 29Sub±Oxygen Sensor Circuit
CIRCUIT DESCRIPTION
The sub±oxygen sensor is installed on the exhaust pipe. Its construction\
and operation is the same as the main
oxygen sensor on page TR±62.
Code No.Diagnostic Code Detecting ConditionTrouble Area
(1) Open or short in heater circuit of sub±oxygen
sensor for 0.5 sec. or more.

Open or short in heater circuit of sub±oxygen
sensor

Sub±oxygen sensor heater.

ECU
27


29(2) Main oxygen sensor signal is 0.45 V or more and sub±oxygen sensor signal is 0.45 V or less
under sonditions (a) and (b).
(2 trip detection logic) *
(a) Coolant temp.: 80 5C (176 5F) or more.
(b) Accel. pedal: Fully depressed for 2 sec. or more.

Open or short in sub±oxygen sensor circuit.

Sub±oxygen sensor

ECU
*: See page TR±21.
HINT: Diag. code 27 is for the left bank sub±oxygen sensor circuit.
Diag. code 29 is for the right bank sub±oxygen sensor circuit.
TR±82
±
ENGINE TROUBLESHOOTING Circuit Inspection
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Diag. Code 71
EGR System Malfunction (Only for
USA spec.)
CIRCUIT DESCRIPTION
If even one of the following conditions is ful-
filled, the VSV is turned ON by a signal from
the ECU. This results in atmospheric air act-
ing on the EGR valve, closing the EGR valve
and shutting off the exhaust gas (EGR cut±
OFF).
w
Coolant temp. below 505C (122 5F)
w
During deceleration (throttle valve closed)
w
Light engine load (amount of intake air very
small).
w
Engine speed over 5,200 rpm. The EGR system is designed to recirculate the exhaust gas properly controll\
ed according to the driv-
ing condition back into the intake air±fuel mixture. It helps to slow dow\
n combustion in the cylinder
and thus lower the combustion temperature which, in turn, reduces the amoun\
t of NOx emission. The
amount of EGR is regulated by the EGR vacuum modulator according to the engine\
load.
Code No.Diagnostic Code Detecting ConditionTrouble Area
EGR t i 70
5C (158 5F) b l f 1 4
w
Open in EGR
gas temp. sensor circuit.EGR gas temp. is 70 5C (158 5F) or below for 1 4
min. under conditions (a) and (b).
(2 trip detection logic)*
Oen in EGR gas tem. sensor circuit.
w
Short in VSV circuit for EGR.
71
(2 trip detection logic)*
(a) Coolant temp.: 63 5C (145 5F) or more.
(b) EGR operationpossible (EX ECT in 3rd speed
Short in VSV circuit for EGR.
w
EGR hose disconnected, valve stuck.
(b) EGR operation possible (EX. ECT in 3rd speed
(5th for M/T), A/C ON, 60 mph (96 km/h), Flat
road)
EGR hose disconnected, valve stuck.
w
Clogged EGR gas passage.
road).gg g g
w
ECU
*: See page TR±25.
TR±104
±
ENGINE TROUBLESHOOTING Circuit Inspection
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Diag. Code 71
EGR System Malfunction (Only for USA
spec.)
CIRCUIT DESCRIPTION
The EGR system recirculates exhaust gas, which is controlled to the proper quantity to suit the driv\
ing condi-
tions, into the intake air mixture to slow down combustion, reduce the combust\
ion temperature and reduce NOx
emissions.
The lift amount of the EGR valve is controlled by a step motor operated \
by the ECU. The ECU operates the motor
only a preprogrammed amount in response to the engine operating conditions (\
engine rpm, intake air volume
to adjust the EGR volume to the target valve.)
Under the following conditions, EGR is cut to maintain driveability.
wCoolant temp. below 535C (127.45F)
w During deceleration (throttle valve
closed)
w Light engine load (amount of intake air
very small)
w Engine speed over 4,000 rpm
w Engine idling
w Neutral start switch on
Code No.Diagnostic Code Detecting ConditionTrouble Area
(1) Open or short in EGR step motor circuit for 1 sec.
or more.
Open or short in EGR step motor circuit

ECU
71
(2) EGR gas temp. is 65 5C (149 5F) or below for
1 ~ 4 min. under conditions (a) and(b).
(2 trip detection logic)*
(a) Coolant temp.: 65 5C (149 5F) or more.
(b) EGR operation possible (EX. ECT in 3rd speed, A/C ON, 60 mph (96 km/h), Flat road).

Open in EGR gas temp. sensor circuit

ECU
*: See page TR±21.
TR±108
±
ENGINE TROUBLESHOOTING
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227
4. OPERATION OF AIR MIX CONTROL SERVO MOTOR
WHEN THE TEMPERATURE SW IS TURNED TO THE ªCOOLº SIDE, THE CURRENT FLOWS FROM TERMINAL MC OF A/C CONTROL
ASSEMBLY " TERMINAL 4 OF AIR MIX CONTROL SERVO MOTOR " MOTOR " TERMINAL 5 " TERMINAL MH OF A/C CONTROL
ASSEMBLY " GROUND AND THE MOTOR ROTATES. THE DAMPER OPENING ANGLE AT THIS TIME IS INPUT FROM TERMINAL 1 OF
SERVO MOTOR TO TERMINAL TP OF A/C CONTROL ASSEMBLY, THIS IS USED TO DETERMINE THE DAMPER STOP POSITION AND
MAINTAIN THE SET TEMPERATURE.
WHEN THE TEMPERATURE CONTROL SW IS TURNED TO THE ªHOTº SIDE, THE CURRENT FLOWS FROM SERVO MOTOR "
TERMINAL MH OF A/C CONTROL ASSEMBLY " TERMINAL 5 OF AIR MIX CONTROL SERVO MOTOR " MOTOR " TERMINAL 1 "
TERMINAL MC OF A/C CONTROL ASSEMBLY, ROTATING THE MOTOR IN REVERSE AND SWITCHING THE DAMPER FROM ªCOOLº TO
ªHOTº SIDE.
5. AIR CONDITIONER OPERATION
THE A/C CONTROL ASSEMBLY RECEIVES VARIOUS SIGNALS, I.E., THE ENGINE RPM FROM THE IGNITER, OUTLET TEMPERATURE
SIGNAL FROM THE A/C THERMISTOR, COOLANT TEMPERATURE FROM THE WATER TEMP. SENSOR AND THE LOCK SIGNAL FROM
THE A/C COMPRESSOR, ETC.
WHEN THE ENGINE IS STARTED AND THE A/C SW (A/C CONTROL ASSEMBLY) IS ON, A SIGNAL IS INPUT TO THE ECU. (BUILT IN THE
A/C CONTROL ASSEMBLY).
AS A RESULT, THE GROUND CIRCUIT IN A/C CONTROL ASSEMBLY IS CLOSED AND CURRENT FLOWS FROM HEATER FUSE (10A) TO
TERMINAL 2 OF MAGNETIC CLUTCH RELAY " TERMINAL 3 " TERMINAL ACMG OF ENGINE AND ECT ECU " TERMINAL E OF A/C
CONTROL ASSEMBLY " GROUND , TURNING THE MAGNETIC CLUTCH RELAY ON, SO THAT THE MAGNETIC CLUTCH IS ON AND THE
A/C COMPRESSOR OPERATES.
AT THE SAME TIME, THE ENGINE AND ECT ECU DETECTS THE MAGNETIC CLUTCH IS O\
N AND THE A/C COMPRESSOR OPERATES.
OPEN DIRECTION TO AVOID LOWERING THE ENGINE RPM DURING A/C OPERATING.
WHEN ANY OF THE FOLLOWING SIGNALS ARE INPUT TO THE A/C CONTROL ASSEMBLY, THE CONTROL ASSEMBLY OPERATES TO
TURN OFF THE AIR CONDITIONING.
* ENGINE HIGH TEMP. SIGNAL IS HIGH.
* COOLANT HIGH TEMP. SIGNAL IS HIGH.
* A SIGNAL THAT THE TEMPERATURE AT THE AIR OUTLET IS LOW.
* A SIGNAL THAT THERE IS A LARGE DIFFERENCE BETWEEN ENGINE RPM AND COMPRESSOR RPM.
* A SIGNAL THAT THE REFRIGERANT PRESSURE IS ABNORMALLY HIGH OR LOW.
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66
.
ENGINE CONTROL
THIS SYSTEM UTILIZES AN ECU AND MAINTAINS OVERALL CONTROL OF THE ENGINE, TRANSMISSION AND SO ON. AN OUTLINE O\
F
THE ENGINE CONTROL IS EXPLAINED HERE.
1. INPUT SIGNALS
(1) WATER TEMP. SIGNAL CIRCUITTHE WATER TEMP. SENSOR DETECTS THE ENGINE COOLANT TEMP. AND HAS A BUILT±IN THERMISTOR WITH A RESISTANCE
WHICH VARIES ACCORDING TO THE WATER TEMP. THE WATER TEMP. IS INPUT INTO TERMINAL THW OF ENGINE CONTROL
ECU AS A CONTROL SIGNAL.
(2) INTAKE AIR TEMP. SIGNAL CIRCUIT THE INTAKE AIR TEMP. SENSOR IS INSTALLED IN THE AIR FLOW METER AND DETECTS THE INTAKE AIR TEMP., WHICH IS INPUT
AS A CONTROL SIGNAL TO TERMINAL THA OF ENGINE CONTROL ECU.
(3) OXYGEN SENSOR SIGNAL CIRCUIT THE OXYGEN DENSITY IN THE EXHAUST EMISSION IS DETECTED AND INPUT AS A CONTR\
OL SIGNAL FROM THE OXYGEN
SENSOR NO. 1 AND NO. 2 TO TERMINALS OX1, OX2 OF THE ECU AND FROM THE OXYGEN SENSOR SUB (FOR CALIFORNIA) TO
TERMINAL OX3 OF THE ECU.
TO STABILIZE DETECTION PERFORMANCE BY THE OXYGEN SENSOR SUB (FOR CALIFORNIA)\
IS WARMED. THIS HEATER IS
ALSO CONTROLLED BY THE ECU (HT).
(4) RPM SIGNAL CIRCUIT CRANKSHAFT POSITION IS DETECTED BY THE PICK±UP COIL INSTALLED INSIDE THE DISTRIBUTOR. CRANKSHAFT POSITION
IS INPUT AS A CONTROL SIGNAL TO TERMINALS G1 AND G2 OF THE ECU, AND RPM IS INPUT TO TERMINAL NE.
(5) THROTTLE SIGNAL CIRCUIT THE THROTTLE POSITION SENSOR DETECTS THE THROTTLE VALVE OPENING ANGLE AS A CONTROL SIGNAL, WHICH IS
INPUT INTO TERMINAL VTA1 OF THE ECU. WHEN THE VALVE IS COMPLETELY CLOSED, THE CONTROL SIGNAL IS INPUT INTO
TERMINAL IDL1 .
(6) VEHICLE SPEED CIRCUIT THE VEHICLE SPEED IS DETECTED BY SPEED SENSOR NO. 1 INSTALLED IN THE TRANSMISSION AND THE SIGNAL IS INPUT TO
TERMINAL SPD OF THE ECU VIA THE COMB. METER.
(7) NEUTRAL START SIGNAL CIRCUIT THE NEUTRAL START SW DETECTS WHETHER THE SHIFT POSITION IS IN NEUTRAL OR NOT, AND THE SIGNAL IS INPUT INTO
TERMINAL NSW OF THE ECU.
(8) AIRCONDITIONING SW SIGNAL CIRCUIT THE OPERATING VOLTAGE OF THE A/C MAGNETIC CLUTCH IS DETECTED AND THE SIGNAL IS INPUT INTO TERMINAL ACMG OF
ECU AS A CONTROL SIGNAL.
(9) BATTERY SIGNAL CIRCUIT VOLTAGE IS CONSTANTLY APPLIED TO TERMINAL BATT OF THE ECU. WITH THE IGNITION SW TURNED ON, THE VOLTAGE FOR
ECU START±UP POWER SUPPLY IS APPLIED TO TERMINALS +B AND +B1 OF ECU VIA EFI MAIN RELAY.
THE CURRENT FLOWING THROUGH THE IGN FUSE FLOWS TO TERMINAL IGSW OF THE ECU.
(10) INTAKE AIR VOLUME SIGNAL CIRCUIT INTAKE AIR VOLUME IS DETECTED BY THE AIR FLOW METER AND THE SIGNAL IS INPUT TO TERMINAL KS OF THE ECU AS A
CONTROL SIGNAL.
(11) STOP LIGHT SW SIGNAL CIRCUIT THE STOP LIGHT SW IS USED TO DETECT WHETHER OR NOT THE VEHICLE IS BRAKING AND THE SIGNAL IS INPUT INTO
TERMINAL STP OF THE ECU AS A CONTROL SIGNAL.
(12) STA SIGNAL CIRCUIT TO CONFIRM WHETHER THE ENGINE IS CRANKING, THE VOLTAGE APPLIED TO THE STARTER MOTOR DURING CRANKING IS
DETECTED AND THE SIGNAL IS INPUT INTO TERMINAL STA OF THE ECU AS A CONTROL SIGNAL.
(13) ENGINE KNOCK SIGNAL CIRCUIT ENGINE KNOCKING IS DETECTED BY KNOCK SENSOR NO. 1 AND NO. 2 AND THE SIGN\
AL IS INPUT INTO TERMINALS KNK1 AND
KNK2 AS A CONTROL SIGNAL.
SYSTEM OUTLINE
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74
ENGINE CONTROL
EFI MAIN RELAY2 ± 4: CLOSED WITH IGNITION SW AT ON OR ST POSITION
W 4 WATER TEMP. SENSOR
1 ± 2: APPROX. 16.2K (±20 °C, ±4°F)
: APPROX. 2.45K (20 °C, 68°F)
: APPROX. 0.32K (80 °C, 176 °F)
E 3 EGR GAS TEMP. SENSOR (FOR USA SPEC.)
1 ± 2: APPROX. 78.30K (50 °C, 122 °F)
APPROX. 13.06K (100 °C, 212 °F)
APPROX. 3.16K (150 °C, 302 °F)
E 9(A), E10(B) ENGINE AND ECT ECU, E11(A), E12(B) ENGINE ECU
(VOLTAGE AT ENGINE AND ECT ECU WIRING CONNECTORS)
BATT ± E1 : ALWAYS APPROX. 12 VOLTS
IGSW ± E1 : APPROX. 12 VOLTS WITH IGNITION SW ON
+B, +B1 ± E1 : APPROX. 12 VOLTS WITH IGNITION SW ON
VC ± E2 : 4±6 VOLTS WITH IGNITION SW ON
IDL1, IDL2 ± E2 : 0±1 VOLTS WITH IGNITION SW ON AND THROTTLE VALVE FULLY CLOSED
: 10±14 VOLTS WITH IGNITION SW ON AND THROTTLE VALVE FULLY OPEN
VTA1, VTA2 ± E2 : 0.1±1.0 VOLTS WITH IGNITION SW ON AND THROTTLE VALVE FULLY CLOSED
: 3±6 VOLTS WITH IGNITION SW ON AND THROTTLE VALVE FULLY OPEN
THA ± E2 : 1±3 VOLTS WITH IGNITION SW ON AND INTAKE AIR TEMP. 20°C ( 68°F)
THW ± E2 : 0.1±1.0 VOLTS WITH IGNITION SW ON AND COOLANT TEMP. 80°C ( 176 °F)
STA ± E1 : 6±14 VOLTS WITH ENGINE CRANKING
#10, #20, #30, #40, #50, #60 ± E1 : APPROX. 12 VOLTS WITH IGNITION SW ON
IGF ± E1 : 0±1 VOLTS WITH ENGINE IDLING
MREL ± E1 : APPROX. 12 VOLTS WITH IGNITION SW ON
DI ± E1 : 7.5±14 VOLTS WITH ENGINE IDLING
ISC1, ISC2, ISC3, ISC4 ± E1 : APPROX. 12 VOLTS WITH IGNITION SW ON
HTL1, HTR1, HTL2, HTR2 ± E01 : 0±2 VOLTS WITH ENGINE IDLING
EGR ± E1 : APPROX. 12 VOLTS WITH ENGINE IDLING
NSW ± E1 : 0±1 VOLTS WITH IGNITION SW ON AND SHIFT LEVER P OR N RANGE
: 10±14 VOLTS WITH IGNITION SW ON AND SHIFT EXCEPT P OR N RANGE
TE1 ± E1 : APPROX. 12 VOLTS WITH IGNITION SW ON
TE2 ± E2 : APPROX. 12 VOLTS WITH IGNITION SW ON
W ± E1 : APPROX. 12 VOLTS WITH ENGINE IDLING
0±2 VOLTS WITH IGNITION SW ON
A/C ± E1 : 0±2 VOLTS WITH AIR CONDITIONER ON
10±14 VOLTS WITH AIR CONDITIONER OFF
ACMG ± E1 : 0±2 VOLTS WITH AIR CONDITIONER ON
: 10±14 VOLTS WITH AIR CONDITIONER OFF
TR ± E1 : APPROX. 12 VOLTS WITH IGNITION SW ON
VTA1, VTA2 ± E2 : 0±1 VOLTS WITH IGNITION SW ON AND THROTTLE VALVE FULLY CLOSED
3±5.5 VOLTS WITH IGNITION SW ON AND THROTTLE VALVE FULLY OPEN
NE ± E1 : 4±6 VOLTS WITH IGNITION SW ON
(RESISTANCE OF ENGINE AND ECT ECU WIRING CONNECTORS)
+B, +B1 ± #10, #20, #30, #40, #50, #60 : 13.2±14.2
+B, +B1, ISC1, ISC2, ISC3, ISC4 : 10±30
+B, +B1 ±EGR: 33±39
+B, +B1, HT : 5.1±6.3
+B, +B1 ±EVAP : 30±34
IDL1, IDL2 E2 : INFINITY WITH THROTTLE VALVE OPEN 0±0.5WITH THROTTLE VALVE FULLY CLOSED
VTA1, VTA2 E2 : 2.4±10.7WITH THROTTLE VALVE FULLY OPEN
0.3±5.9 WITH THROTTLE VALVE FULLY CLOSED
VCC ± E2 : 3±7K
THW ± E2 : 200±400 WITH COOLANT TEMP. 80°C ( 176 °F)
THA ± E2 : 2±3K WITH INTAKE AIR TEMP. 20°C ( 68°F)
THG ± E2 : 69.4±88.5K WITH EGR GAS TEMP. 50°C ( 11 2 °F)
G1, G2 ±G± : 0.835±13.5K
NE± G± : 0.835±13.5K
SERVICE HINTS
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567 8
21 LEXUS SC300(
Cont ' d )
AIR BAG
CHA RGE
OIL
OIL LEVEL COOLANT
LEVEL REAR
LI GHT
PWR
ABS TRAC
TRAC OFF
CRUISE WASHER
O/ D OFF
FUEL
SEAT BELTP
R N D 2 L
A
C
C A
A
B
B
B
C
C
C
C
C
C
C
B C
C
B
B
BA
B
A
A
C
CA A A A A A A A
C
C
C
C C C A C C B B B B
B A A C C
10 : 00
BK 1E
18 3D
10
3A
1
1J
1 1K
12
IH IF ED
IK1
8
IK2 IK1
9
1
E
3
E
2
E
1
D
BU110
BU1
3
BU1
4 11
11
1
14
19
8
18
11
13 6
18
17
16
20
19
15
13
3
15 5
1
20 9
18 8
11
87 1 2 3 4 5 16 17
2
3
4
5 10 9
12 23 12 4 17 16 7
6
15
10 22 21
W± R
RY
B±O
Y±B
V± R
V±Y
R±W
L±R
W
R± Y B
V±W V
R
L±R
P±L
Y±B
B±Y
B± W
W±B BR
B
W± B W± B BR±B
Y± G
B
Y±G
BR±B
W± B
W±B
W±B
BR±B W± B BR
Y±G
WATER
TEMP.HI GH BEAM TURN
LH
RH
BRAKE DOOR
TACHO
TRI P
SPEED
ILLUMINATIONCHECK±
ENGINE
FUEL
See Taillight and
St op Light Sy stem
<4±6><5±6>
SeeEngineControlSystem
<2±3>
Se e ABS Sy st e m< 8± 7> < 9± 3>
See Cruise Control System
< 16±3>Se e ABS an d Tr act io n
Control System< 8±7>
SeeECTandA/TIndicator
System< 3±5>
D
F1 7 , F18 E
FUEL SENDER Y±B
B±Y
B±W 24
See SRS Airbag System< 15±4>
See Charging Sy stem< 1±2>
See Headlight Sys tem< 4±1> < 5±1>
See Turn Signal and
Hazard Warning
Light Sy stem< 7±3>
See ECT System< 3±5>
From Neutr al St art SW< 1±3>
See Interier Light System< 6±3> From Igniter º TACº < 1±5>
To Engine and ECT ECU< 2±7>
To Cruise Control ECU< 16±3>
To PPS ECU< 15±2> See Illumination System
<4±7><5±7>
See ECT and A/T Indic ator
Syst em< 3±6>
See Unlock and Seat Belt
Warning Syst em< 4±4> < 5±4>
W3
WATER TEMP.
SENDER
Combination Meter
1
COMBINATION METER C12 , C13 , C14
A
AB IG1
8
Under the
c ent er
pillar LH Under the
as htr ay LH
Lef t
kick panelRear side of
c ylinder head RH
C
2
G 2
F
1
F 14
L±R
P
P
SH OR T C ON NE CT OR
S2 2 G , S23 F L±R
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47
216182
216
178
216
129 104 144 10260
90
216 62
68
11 2
148
104
111 200 148 148 190
D1
C16 C24
C15 C19 C23 C22
C12
C13
C14C14
C13
C12
Airbag W arning Light
Charge Warning Light
Dimmer SW [Comb. SW]Cruise Control Indicator Light
[Comb. Meter]A/T Indicator Light
[Comb. Meter]
Combination Meter
Rear Lights Warning Light Turn Signal Indicator Light
[Comb. Meter] High Beam Indicator Light
[Comb. Meter]
Check Engine Warning Light
Open Door Warning Light
Traction Indicator Light
ABS Warning Light
[Comb. Meter] Seat Belt Warning LightABS Warning Light and T raction
Off Indicator Light [Comb. Meter]
Coolant Level Warning LightCruise Control ECU Turn Signal SW
[Comb. SW]
Wiper and Washer SW
[Comb. SW]
Choke Coil
Crutch Start SW
Cruise Control Clutch SW
Diode (for Brake Fluid Level)Light Control SW
[Comb. SW] Horn SW [Comb. SW]Combination MeterCornering Light SW
[Comb. SW]
C12 80
82
216
216 216 98
86
216134
216
216 216 184
216
80
90
102 80
90
102
68
94
134
148
216
(4)
: R/B No. 5 (See page ) 22
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