1991 LEXUS SC400 Service Repair Manual

seats ECO mode radiator cap fog light automatic transmission climate control

Page 2673 of 4087

$LEXUS SC400 1991 Service Repair Manual Diag. Code 21, 28Main Oxygen Sensor Circuit CIRCUIT DESCRIPTION To obtain a high purification rate for the CO, HC and NOx components of th\ e exhaust gas, a three±way catalyst is used, but for most$

Diag. Code 21, 28Main Oxygen Sensor Circuit
CIRCUIT DESCRIPTION
To obtain a high purification rate for the CO, HC and NOx components of th\
e exhaust gas, a three±way
catalyst is used, but for most efficient use of the three±way catalyst, the air±fuel ratio must be \
precisely
controlled so that it is always close to the stoichiometric air±fuel \
ratio.
The oxygen sensor has the characteristic whereby its output voltage chan\
ges suddenly in the vicinity of
the stoichiometric air±fuel ratio. This characteristic is used to det\
ect the oxygen concentration in the ex-
haust gas and provide feedback to the computer for control of the air±\
fuel ratio.
When the air±fuel ratio becomes LEAN, the oxygen concentration in the exh\
aust increases and the oxygen
sensor informs the ECU of the LEAN condition (small electromotive force: O V)\
.
When the air±fuel ratio is RICHER than the stoichiometric air±fuel\
ratio the oxygen concentration in the
exhaust gas is reduced and the oxygen sensor informs the ECU of the RICH condition (large electromotive
force: 1V).
The ECU judges by the electromotive force from the oxygen sensor whether th\
e air±fuel ratio is RICH or
LEAN and controls the injection time accordingly. However, if malfunction of the oxygen sensor causes
output of abnormal electromotive force, the ECU is unable to perform accurate \
air±fuel ratio control.
The main oxygen sensors include a heater which heats the Zirconia element. The heater is controlled by
the ECU. When the intake air volume is low (the temperature of the exhaust\
gas is low) current flows to
the heater to heat the sensor for accurate oxygen concentration detectio\
n.
Code No.Diagnostic Code Detecting ConditionTrouble Area
(1) Open or short in heater circuit of main
oxygen sensor for 0.5 sec. or more.
Open or short in heater circuit of main oxygen sen-
sor.
Main oxygen sensor heater
ECU
21 V
28
(2) Main oxygen sensor signal voltage is reduced to be-
tween 0.35 V and 0.70 V for 60 sec. under conditions
(a) ~ (d). (2 trip detection logic)*
(a) Coolant temp.: Between 70 5C (158 5F) and 95 5C
(203 5F).
(b) Engine speed: 1,500 rpm or more.
(c) Load driving (EX. ECT in 4th speed, A/C ON, Flat road, 50 mph (80km/h)).
(d) Main oxygen sensor signal voltage:
Alternating above and below 0.45 V.
Main oxygen sensor circuit
Main oxygen sensor
*: See page TR±21.
HINT : Diag. code 21 is for the left bank main oxygen sensor circuit. Diag. co\
de 28 is for the right bank main oxy-
gen sensor circuit.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±62
WhereEverybodyKnowsYourName

Page 2674 of 4087

LEXUS SC400 1991 Service Repair Manual
CIRCUIT DESCRIPTION (Contd)
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 re

Page 2675 of 4087

$LEXUS SC400 1991 Service Repair Manual DIAGNOSTIC CHART DIAGNOSTIC CHART WIRING DIAGRAM HINT: If diag. code º21º is output, check the left bank main oxygen sens\ or circuit. If diag. code º28º is output, check the right bank main ox$

Page 2676 of 4087

LEXUS SC400 1991 Service Repair Manual
OKNG
OKNG
INSPECTION PROCEDURE
1Disconnect the engine & ECT ECU connector, check voltage between terminals HTL1, HTR1 of
engine & ECT ECU connector and body ground.
C
OK
P(2) Connect the Check Harnes

Page 2677 of 4087

$LEXUS SC400 1991 Service Repair Manual OKNG 3Check voltage between terminals HTL1, HTR1 of engine & ECT ECU connector and \ body ground. C OK Hint P(1) Connect engine & ECT ECU connector. (2) Warm up engine to normal operating temperature$

Page 2678 of 4087

LEXUS SC400 1991 Service Repair Manual
±MEMO±TR±67
ENGINE TROUBLESHOOTING
WhereEverybodyKnowsYourName

Page 2679 of 4087

LEXUS SC400 1991 Service Repair Manual
Diag. Code 22Water Temp. Sensor Circuit
CIRCUIT DESCRIPTION
The water temperature sensor senses the coolant tem-
perature. A thermistor built in the sensor changes the
resistance value according t

Page 2680 of 4087

$LEXUS SC400 1991 Service Repair Manual DIAGNOSTIC CHART DIAGNOSTIC CHART HINT: If diagnostic codes º22º (water temperature sensor circuit), º\ 24º (intake air temperature sensorcircuit) and º41º (throttle position sensor circuit)$

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