oxygen sensor LEXUS SC400 1991 Service Repair Manual

OKNG
NGOK
3Check for open and short in harness and connector between EFI main relay and engine (& ECT)
ECU (See page IN±27).
Repair or replace harness or connector.
Check and replace engine (& ECT) ECU.
4Check voltage between terminal HT of engine (& ECT) ECU connector and \
body ground.
C
OK
PWarm up engine to normal operating temperature.
Measure voltage between terminal HT of engine (&
ECT) ECU connector and body ground, when engine is
idling and racing at 3,500 rpm.
Replace sub±oxygen sensor.* *: In this case, oxygen sensor can be deteriorated.
Check and replace engine (& ECT) ECU.
TR±80±
ENGINE TROUBLESHOOTING Circuit Inspection
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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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CIRCUIT DESCRIPTION (Cont'd)
DIAGNOSIS CODE DETECTION DRIVING PATTERN
Purpose of the driving pattern.
(a) To simulate diag. code detecting condition after diag. code is recorded.
(b) To check that the malfunction is corrected when the repair is completed co\
nfirming that diag. codeis no longer detected.
Malfunction: Open or Short in Sub±Oxygen Sensor

Initiate test mode (See page TR±12).
Start engine and warm up.
After engine is warmed up, let it idle for 3 min.
With the A/C ON and the transmission in D range (O/D OFF), drive at 40 ±\
60 mph (64±96
km/h) for 6 min.
After the driving in , stop the vehicle.
Once the vehicle is stopped, within the next minute apply acceleration w\
ith the throttle
fully open for 4 sec.
HINT: If a malfunction exists, the ºCHECKº engine warning light will light\
up after full acceleration for 4 sec.
NOTICE: If the conditions in this test are not strictly followed, detection of the\
malfunction will not be possible.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±83
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DIAGNOSTIC CHART
DIAGNOSTIC
CHART
HINT:
wWhen other codes are output in addition to 27 and 29 at the same time, check the ci\
rcuits for other
codes first.
w If diag. code 27 is displayed, check left bank sub±oxygen sensor circ\
uit, and if 29 is displayed,
check right bank sub±oxygen sensor circuit.
WIRING DIAGRAM
Check voltage of terminals HTL2, HTR2.
Check resistance of sub±oxygen sensor heater.
Check and repair sub±oxygen sensor heater
circuit.
Check operation of sub±oxygen sensor heater.
Check and replace ECU. Replace main oxygen sensor.
*: In this case, oxygen sensor can be deteriorated.
Replace main oxygen sensor.*
TR±84±
ENGINE TROUBLESHOOTING Circuit Inspection
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NGOK
OKNG
INSPECTION PROCEDURE
1Disconnect the engine & ECT ECU connector.
C
OK
P(2) Connect the Check Harness A.(See page TR±30)
(2) Disconnect engine & ECT ECU connector.
(2) Turn ignition switch on.
Measure voltage between terminals HTL2, HTR2 of en-
gine & ECT ECU connector and body ground.
Voltage: 10 ± 14 V
Go to step [3].
2Check sub±oxygen sensor heater.
C
OK
PDisconnect sub±oxygen sensor connector.
Measure resistance between terminals 1 and 2 of sub±
oxygen sensor connector.
Resistance: 5.1 ± 6.3
20 C (68 F)
Replace sub±oxygen sensor.
Check and repair harness or connector between
main relay and sub±oxygen sensor, sub±oxygen
sensor and engine & ECT ECU.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±85
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NGOK
3Check voltage between terminals HTL2, HTR2 of engine & ECT ECU connector and \
body
ground.
C
OK
Hint
P(2) Connect engine & ECT ECU connector.
(2) Warm up engine to normal operating temperature.
Measure voltage between terminals HTL2, HTR2 of en-
gine & ECT ECU connector and body ground, when en-
gine is idling and racing at 4,000 rpm.
In the 4,000 rpm racing check , continue engine racing
at 4,000 prm for approx. 20 seconds or more.
Replace sub±oxygen sensor.**: In this case, oxygen sensor can be deteriorated.
Check and replace engine & ECT ECU.
TR±86±
ENGINE TROUBLESHOOTING Circuit Inspection
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27
Position of Parts in Engine Compartment
I 1 ISC Valve S 3Speed Sensor No. 1
I 2 Igniter S 4 Speed Sensor No. 2 (for ECT)
I 4 Ignition Coil S 6 Starter
I 6 Injector No. 1 S 7 Starter
I 7 Injector No. 2 S 8 Sub Throttle Actuator
I 8 Injector No. 3 S 9 Sub Throttle Position Sensor
I 9 Injector No. 4
I 10 Injector No. 5 T 1 Theft Deterrent Horn
I 11 Injector No. 6 T 2 Throttle Position Sensor T 3 Traction Brake Actuator
K 1 Knock Sensor No. 1 (on Front Side) T 4 Traction Brake Actuator
K 2 Knock Sensor No. 2 (on Rear Side) T 5 Traction pump and Motor
N 1 Neutral Start SW, Back±Up Light SW and A/T Indicator SW V 1 VSV (for EGR) (Exc. USA Spec) (for A/T) V 2 VSV (for EVAP)
N 2 Noise Filter (for Defogger System) V 4 VSV (for Water Valve)
N 3 Noise Filter (for Ignition System) V 7 VSV (for Intake Control Valve)
O 2 Oil Pressure SW W 1 Washer Level SW
O 3 Oxygen Sensor Main No. 1 (on Front Side) W 2 Washer Motor
O 4 Oxygen Sensor Main No. 2 (on Rear Side) W 3 Water Temp. Sender W 5 Wiper Motor
P 1 PPS Solenoid
P 20 Power Steering Pressure SW
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Position of Parts in Body
M 1 Mobilephone R 16 Remote Control Mirror LH
M 2 Moon Roof Control Relay R 17 Remote Control Mirror RH
M 3 Moon Roof Control SW R 18 Remote Control Mirror SW
M 4 Moon Roof Motor and Limit SWS 13 Stereo Power Amplifier
O 8 Oxygen Sensor Sub (for California) S 14 Stereo Power Amplifier
P 4 Power Window Control SW RH T 16 Telephone Transceiver and Speaker Relay
P 5 Power Window Master SW and Door Lock Control SW LH T 17 Telephone Transceiver and Speaker Relay
P 6 Power Window Motor LH T 18 Tension Reducer Solenoid LH
P 7 Power Window Motor RH T 19 Tension Reducer Solenoid RH
P 20 Personal Light V 5 Vanity Light LH
R 10 Rear Combination Light LH V 6 Vanity Light RH
R 11 Rear Combination Light RH
R 12 Rear Speaker LH W 8 Wireless Door Lock ECU
R 13 Rear Speaker RH W 9 Woofer Speaker
R 14 Rear Window Defogger (+) W 10 Woofer Speaker Amplifier
R 15 Rear Window Defogger (±)
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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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67
2. CONTROL SYSTEM
*EFI (ELECTRONIC FUEL INJECTION) SYSTEM
THE EFI SYSTEM MONITORS THE ENGINE CONDITION THROUGH THE SIGNALS INPUT FROM EACH SENSOR (IN\
PUT SIGNALS
FROM (1) TO (13) ETC.) TO THE ECU. THE BEST FUEL INJECTION TIMING IS DECIDED BASED ON THIS DATA AND THE PROGRAM
MEMORIZED BY THE ECU, AND THE CONTROL SIGNAL IS OUTPUT TO TERMINALS #10, #20, #30, #40, #50 AND #60 OF THE ECU TO
OPERATE THE INJECTOR. (INJECT THE FUEL). THE EFI SYSTEM PRODUCES CONTROL OF FUEL INJECTION OPERATION BY THE
ECU IN RESPONSE TO THE DRIVING CONDITIONS.
* ESA (ELECTRONIC SPARK ADVANCE) SYSTEM
THE ESA SYSTEM MONITORS THE ENGINE CONDITION THROUGH THE SIGNALS INPUT TO THE ECU FROM EACH SENSOR (INPUT
SIGNALS FROM (1), (2), (4) TO (13) ETC.). THE BEST IGNITION TIMING IS DECIDED ACCORDING TO THIS DATA AND THE
MEMORIZED DATA IN THE ECU AND THE CONTROL SIGNAL IS OUTPUT TO TERMINAL IGT THIS SIGNAL CONTROLS THE IGNITER
TO PROVIDE THE BEST IGNITION TIMING FOR THE DRIVING CONDITIONS.
* OXYGEN SENSOR HEATER CONTROL SYSTEM (USA SPEC.)
THE OXYGEN SENSOR HEATER CONTROL SYSTEM TURNS THE HEATER ON WHEN THE INTAKE AIR VOLUME IS LOW (TEMP. OF
EXHAUST EMISSIONS IS LOW), AND WARMS UP THE OXYGEN SENSOR TO IMPROVE DETECTION PERFORMANCE OF THE
SENSOR.
THE ECU EVALUATES THE SIGNALS FROM EACH SENSOR (INPUT SIGNALS FROM (1), (2), (4)\
, (9) TO (11) ETC.,) AND OUTPUTS
CURRENT TO TERMINAL HT TO CONTROL THE HEATER.
* ISC (IDLE SPEED CONTROL) SYSTEM
THE ISC SYSTEM (STEP MOTOR TYPE) INCREASES THE RPM AND PROVIDES IDLING STABILITY FOR FAST IDLE±UP WHEN THE
ENGINE IS COLD, AND WHEN THE IDLE SPEED HAS DROPPED DUE TO ELECTRICAL LOAD AND SO ON. THE ECU EVALUATES
THE SIGNALS FROM EACH SENSOR (INPUT SIGNALS FROM (1), (4), (5), (8)\
, (9), (11) ETC.), OUTPUTS CURRENT TO TERMINAL ISC1,
ISC2, ISC3 AND ISC4 TO CONTROL ISC VALVE.
* EGR CONTROL SYSTEM
THE EGR CONTROL SYSTEM DETECTS THE SIGNAL FROM EACH SENSOR (INPUT SIGNALS \
FROM (1), (4), (9), (10) ETC.), AND
OUTPUTS CURRENT TO TERMINAL EGR TO CONTROL THE EGR VALVE.
* FUEL PUMP CONTROL SYSTEM
THE COMPUTER OUTPUTS CURRENT TO TERMINAL FPC AND CONTROLS THE FUEL PUMP ECU AND FUEL PUMP DRIVE SPEED
IN RESPONSE TO CONDITIONS.
* ACIS (ACOUSTIC CONTROL INDUCTION SYSTEM)
ACIS INCLUDES A VALVE IN THE BULKHEAD SEPARATING THE SURGE TANK INTO TWO PARTS. THIS VALE IS OPENED AND
CLOSED IN ACCORDANCE WITH THE DRIVING CONDITIONS TO CONTROL THE INTAKE MANIFOLD LENGTH IN TWO STAGES FOR
INCREASED ENGINE OUTPUT IN ALL RANGES FROM LOW TO HIGH SPEEDS.
THE ECU JUDGES THE VEHICLE SPEED BY THE SIGNALS ((4), (5)) FROM EACH \
SENSOR AND OUTPUTS SIGNALS TO THE
TERMINAL ACIS TO CONTROL THE VSV (FOR OPENING AND CLOSING THE INTAKE CONTROL VALVE).
3. DIAGNOSIS SYSTEM
WITH THE DIAGNOSIS SYSTEM, WHEN THERE IS A MALFUNCTION IN THE ECU SIGNAL\
SYSTEM, THE MALFUNCTIONING SYSTEM IS
RECORDED IN THE MEMORY. THE MALFUNCTIONING SYSTEM CAN BE FOUND BY READING THE CODE DISPLAYED BY THE
CHECKING ENGINE WARNING LIGHT.
4. FAIL±SAFE SYSTEM
WHEN A MALFUNCTION HAS OCCURRED IN ANY SYSTEM, IF THERE IS A POSSIBILITY OF \
ENGINE TROUBLE BEING CAUSED BY
CONTINUED CONTROL BASED ON THE SIGNALS FROM THAT SYSTEM, THE FAIL±SAFE SYSTEM EITHER CONTROLS THE SYSTEM
BY USING DATA (STANDARD VALUES) RECORDED IN THE ECU MEMORY OR ELSE STOPS THE ENGINE.
NOTE: THE SPECIFICATION DESCRIPTIONS ªUSA SPEC.º AND ªEXC. USA SPEC.º USED IN THIS SECTION INDICATE THE
FOLLOWING SPECIFICATIONS.
USA SPEC. : USA (50 STATES) SPECIFICATIONS
EXC. USA SPEC. : USA (EXCEPT CALIFORNIA) AND CANADIAN SPECIFICATIONS.
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