ignition LEXUS SC300 1991 Service Repair Manual

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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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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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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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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OKNG
OKNG
1Check operation of speedometer.
Drive the vehicle and check if the operation of the speedometer in the c\
ombination meter is normal.
The No. 1 speed sensor is operating normally if the speedometer display is \
normal.C
Hint
Check speedometer circuit. See combination meter trou-
bleshooting on page BE±131.
2Check voltage between terminal SPD of engine & ECT ECU connector and bod\
y ground.
C
OK
P(2) Shift the shift level to N range.
(2) Jack up a read wheel on one side.
(2) Connect the Check Harness A.
(See page TR±30)
(2) Turn ignition switch on.
Measure voltage between terminal SPD of engine &
ECT ECU connector and body ground when the wheel
is turned slowly.
Voltage is generated intermittently.
Check speedometer circuit. See combination meter trou-
bleshooting on page BE±131.
Check and replace engine & ECT ECU.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±99
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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.
Code No.Diagnostic Code Detecting ConditionTrouble Area
43No starter signal to ECU

Open or short in starter signal circuit.

Open or short in ignition switch or main relay
circuit.

ECU
DIAGNOSTIC CHARTDIAGNOSTIC CHART
HINT: This diagnostic chart is based on the premise that the engine is cranked nor\
mally. If the engine is not
cranked, proceed to the matrix chart of problem symptoms on page TR±35.
Check for the test mode.
Check for open in harness and connectors
between ECU and relay.
Check and replace ECU. Proceed to next circuit inspection shown
on matrix chart (See page
TR±35).
Repair or replace harness or
connector.
WIRING DIAGRAM
TR±100±
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NGOK
OKNG
v
INSPECTION PROCEDURE
1Check output condition of diag. code 43.
C
OK
P
OK
C
Setting the test mode.
(2) Turn ignition switch OFF.
(2) Connect terminals TE2 and E1 of TDCL.
(2) Turn ignition switch ON.(Don't start the engine)
(2) Connect terminals TE1 and E1 of TDCL.
Check if code ª43º is output by the ªCHECKº engine
warning light.
Code ª43º is output.
Start the engine.
Check if the code ª43º disapears.
Code ª43º is not output.
Proceed to next circuit inspection shown on maxtric
chart (See page TR±35).
2Check for open in harness and connectors between engine & ECT ECU and st\
arter relay
(See page IN±27).
Repair or replace harness or connector.
Check and replace engine & ECT ECU.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±101
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Diag. Code 52,53,55Knock Sensor Circuit
CIRCUIT DESCRIPTION
Knock sensors are fitted one each to the right bank and left bank of the cylinder block to detect\
engine knocking.
This sensor contains a piezoelectric element which generates a voltage when \
it becomes deformed, which oc-
curs when the cylinder block vibrates due to knocking. If engine knocking occurs, ignition timing is retarded to
suppress it.
Code No.Diagnostic Code Detecting ConditionTrouble Area
52No No. 1 knock sensor signal to ECU for 3 crank revolu-
tions with engine speed between 1,600 rpm 5,200 rpm.
Open or short in No. 1 knock sensor circuit.

No. 1 knock sensor (looseness)

ECU
53Engine control computer (for knock control) malfunc-
tion at engine speed between 650 rpm and 5,200 rpm.
ECU
55No No. 2 knock sensor signal to ECU for 3 crank revolu-
tions with engine speed between 1,600 rpm 5,200 rpm.
Open or short in No. 2 knock sensor circuit.

No. 2 knock sensor (looseness)

ECU
If the ECU detects the above diagnosis conditions, it operates the fail sa\
fe function in which the corrective retard
angle value is set to the maximum value. TR±102
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NGOK
OKNG
INSPECTION PROCEDURE
1Check voltage between terminals EGR 1 ~ 4 of engine & ECT ECU connector and b\
ody ground.
C
OK
P(2) Connect the Check Harness A.(See page TR±30).
(2) Turn ignition switch on.
Measure voltage between terminals EGR 1 ~ 4 of en-
gine & ECT ECU connector and body ground.
Voltage: 10 ± 14 V
Go to step 5.
2Check voltage between terminals B1 and B2 of EGR step motor connector an\
d body ground.
C
OK
P(2) Disconnect EGR step motor connector.
(2) Turn ignition switch on.
Measure voltage between terminals B1 and B2 of EGR
step motor connector and body ground.
Voltage: 10 ± 14 V
Check for ECU power source circuit
(See page TR±124).
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±111
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OKNG
NGOK
OKNG
INSPECTION PROCEDURE
1Check fuel pump operation.
C
OK
P1. Turn ignition switch ON.
2. Using SST, connect terminals +B and FP of checkconnector.
SST 09843±18020
Check that there is pressure in the hose from the fuel
filter.
Fuel pressure can be felt.
Go to step
.
2Check for open and short in harness and connector between terminals +B e+B, FP  FP of
the check connector and fuel pump ECU (See page IN±27).
Go to step .
Repair or replace harness or connector.
3Check voltage of terminal +B of check connector.
C
OK
PTurn ignition switch ON.
Measure voltage between terminal +B of check connec-
tor and body ground.
Voltage: 10 ± 14 V
Check for ECU power source circuit (See page TR±120),
and check for open in harness and connector between
terminal +B of check connector and main relay.
TR±112
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