1991 LEXUS SC400 check engine

DIAGNOSTIC CHART
DIAGNOSTIC
CHART
WIRING DIAGRAM
Check and replace ECU.Replace air flow meter. Check and replace ECU.
Check voltage of air flow meter power source. Check voltage terminal KS of ECU.
Check for open and short in harness and
connector between air flow meter and ECU. Check voltage of terminal KS.
Repair or repolace harness or
connector
When diag. code 31 is displayed,
check and replace ECU.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±89
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NGOK
OKNG
INSPECTION PROCEDURE
1Check voltage between terminals KS and E1 of engine & ECT ECU connector.
C
OK
PConnect the check harness A.
(See page TR±30)
Measure voltage between terminals KS and E1 of en-
gine & ECT ECU connector while engine is cranked.
Voltage: 2.0 ± 4.0 V (Neither 0 V nor 5 V)
When diag. code 31 is displayed, check and replace en-
gine & ECT ECU.
2Check for open and short in harness and connector between engine & ECT ECU and air flow
meter (See page IN±27).
Repair or replace harness or connector.
TR±90
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ENGINE TROUBLESHOOTING Circuit Inspection
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OKNG
NGOK
3Disconnect air flow meter connector and check voltage between terminals \
KS and E1 of engine
& ECT ECU.
C
OK
PDiconnect the air flow meter connector.
Turn ignition switch on.
Measure voltage between terminals KS and E1 of en-
gine & ECT ECU.
Voltage: 4 ± 6 V
Check and replace ECU.
4Diconnect air flow meter connector and check voltage between terminals VC and E1 \
of engine
& ECT ECU.
C
OK
PDiconnect the air flow meter connector.
Turn ignition switch on.
Measure voltage between terminals VC and E1 of en-
gine & ECT ECU.
Voltage: 4 ± 6 V
Replace air flow meter.
Check and replace engine & ECT ECU.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±91
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DIAGNOSTIC CHART
DIAGNOSTIC
CHART
HINT:
wIf diag. code 41 is displayed, check throttle position sensor circuit; if d\
iag. code 47 is displayed,
check sub±throttle position sensor circuit.
w If diag. codes º22º (water temperature sensor circuit), º24º\
(intake air temperature sensor circuit)
and º41º (throttle position sensor circuit) are output simultaneous\
ly, E2 (sensor ground) may be
open.
WIRING DIAGRAM
Check and replace ECU. Adjust or replace throttle position
sensor.
Check for momentary interruption
(See page
TR±23).
Repair or replace harness or
connector.
Check voltage of throttle position sensor.
Check operation for throttle position sensor.
Check for open and short in harness and
connector between throttle position sensor and ECU.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±95
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NGOK
INSPECTION PROCEDURE
HINT:
wIf diag. code 41 is displayed, check throttle position sensor circuit. If \
diag. code 47 is displayed, check
sub±throttle position sensor circuit.
w If diag. codes º22º (water temperature sensor circuit), º24º\
(intake air temperature sensor circuit) and º41º
(throttle position sensor circuit) are output simultaneously, E2 (sensor ground) may be open.
#Check voltage between terminals VTA1, 2, IDL1, 2 and E2 of engine & ECT ECU connector.
C
OK
Hint
P(2) Connect the Check Harness A.
(See page TR±30).
(2) Turn ignition switch on.
(2) For sub±throttle position sensor, remove intake air duct and disconnect sub±throttle valve step motor
connector.
Measure voltage between terminals VTA1, 2, IDL1, 2
and E2 of engine & ECT ECU connector when the
(sub±) throttle valve is opened gradually from the closed
condition.
The voltage should increase steadily in proportion to the
throttle valve opening angle.
Check for Momentary interruption
(See page TR±23).
Go to step [2].
TR±96±
ENGINE TROUBLESHOOTING Circuit Inspection
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OKNG
OKNG
2Check (sub±) throttle position sensor.
C
OK
Hint
P(2) Remove intake air duct.
(2) Disconnect (sub±) throttle position sensor connec-tor.
Measure resistance between terminals 3, 2, and 1 of
(sub±) throttle position sensor connector when the
throttle valve is opened gradually from the closed condi-
tion.
Resistance between terminals 3 and 1 should increase
gradually in accordance with the throttle valve opening
angle.
Adjust or replace (sub±) throttle position sensor
(See page FI±73).
3Check for open and short in harness and connector between engine & ECT E\
CU and
(sub±) throttle position sensor (See page IN±27).
Repair or replace harness or connector.
Check and replace engine & ECT ECU.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±97
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WIRING DIAGRAM
Check for the test mode.
Check for open in harness and connector
between ECU and relay.Repair or replace harness or
connector.
Proceed to next circuit inspection shown
on matrix chart (See page
TR±39).
Check and replace ECU.
Diag. Code 43Starter Signal Circuit
CIRCUIT DESCRIPTION
When the engine is cranked, the intake air flow is slow, so fuel vaporization is poor. A rich mixture is therefore
necessary in order to achieve good startability. While the engine is being cranked, the battery voltage is applied
to terminal STA of the ECU. The starter signal is mainly used to increase the fuel inj\
ection volume for the starting
injection control and after±start injection control.
DTC No.DTC Detecting ConditionTrouble Area
43No starter signal to ECU.

Open or short in starter signal circuit.

Open or short in ignition switch or starter
relay circuit.

ECU
DIAGNOSTIC CHARTDIAGNOSTIC CHART
HINT: This diagnostic chart is based on the premise that the engine is cranke\
d normally. If the engine is not
cranked, proceed to the matrix chart of problem symptoms on page TR±39.
TR±96
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ENGINE TROUBLESHOOTING Circuit Inspection
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INSPECTION PROCEDURE
Check output condition of diag. code 43
Setting the test mode.
6. Turn ignition switch OFF.
7. Connect terminals TE2 and E1 of TDCL.
8. Turn ignition switch ON.(Don't start the engine)
9. Connect terminals TE1 and E1 of TDCL.
Check if code ª43º is output by the ªCHECKº en-
gine warning light.
Code ª43º is output.
Start the engine.
Check if the code ª43º disappear.
Code ª43º is not output.
Proceed to next circuit inspection shown on
matrix chart (See page
TR±39).
Check for open in harness and connector between engine (& ECT) ECU
and starter relay (See page IN±27)
Repair or replace harness or connector.
Check and replace engine (& ECT) ECU.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±97
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