1991 LEXUS SC400 engine

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 Harness A.(See page TR±30)
(2) Disconnect engine & ECT ECU connector.
(2) Turn ignition switch on.
Measure voltage beween terminals HTL1, HTR1 of en-
gine & ECT ECU connector and body ground.
Voltage: 10 ± 14 V
Go to step [3].
2Check main oxygen sensor heater.
C
OK
PDisconnect main oxygen sensor connector.
Measure resistance between terminals 1 and 2 of main
oxygen sensor connector.
Resistance: 5.1 ± 6.3
at 20 C (68 F)
Replace main oxygen sensor.
Check and repair harness or connector between
main relay and main oxygen sensor, main oxygen
sensor and engine & ECT ECU.
TR±65±
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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.
Measure voltage between terminals of HTL1, HTR1 of
engine & ECT ECU connector and body ground, when
engine is idling and racing at 4,000 rpm.
In the 4,000 rpm racing check, continue engine racing
at 4,000rpm for approx. 20 seconds or more.
Replace main oxygen sensor.**: In this case, oxygen sensor can be deteriorated.
Check and replace engine & ECT ECU.
Reference | INSPECTION USING OSCILLOSCOPE
w With the engine racing (4,000 rpm) measure between
terminals OXL1, OXR2 and E1 of engine & ECT ECU.
HINT: The correct waveform appears as shown in the
illustration on the left, oscillating between approx.
0.1 V and 0.9 V
If the oxygen sensor is deteriorated, the amplitude
of the voltage is reduced as shown on the left.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±66
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±MEMO±TR±67
ENGINE TROUBLESHOOTING
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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 to the coolant temperature.
The lower the coolant temperature, the greater the
thermistor resistance value, and the higher the coolant
temperature, the lower the thermistor resistance value
(See Fig. 1.).
The water temperature sensor is connected to the ECU
(See next page). The 5 V power source voltage in the
ECU is applied to the water temperature sensor from the
terminal THW via a resistor R. That is, the resistor R and
the water temperature sensor are connected in series.
When the resistance value of the water temperature
sensor changes in accordance with changes in the cool-
ant temperature, the potential at the terminal THW also
changes. Based on this signal, the ECU increases the
fuel injection volume to improve driveability during cold
engine operation. If the ECU records the diagnostic
code 22, it operates the fail safe function, keeping the
coolant temperature at a constant 80 5C (176 5F).
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±68
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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) are output\
simultaneously, E2 (sensor ground)
may be open.
WIRING DIAGRAM
Check and replace ECU. Repair or replace harness or con-
nector.
Replace water temp. sensor.
Check voltage of sensor.
Check resistance of sensor.
Check for open and short in harness and
connector between ECU and water temp. sensor. Check for momentary interruption
(See page
TR±23).
TR±69±
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NGOK
OKNG
OKNG
INSPECTION PROCEDURE
HINT: If diagnostic codes º22º (water temperature sensor circuit), º\
24º (intake air temperature sensor circuit)and º41º (throttle position sensor circuit) are output simultane\
ously, E2 (sensor ground) may be open.
1Check voltage between terminals THW and E2 of engine & ECT ECU connector\
.
C
OK
P(2) Connect the Check Harness A.(See page TR±30)
(2) Turn ignition switch on.
Measure voltage between terminals THW and E2 and
engine & ECT ECU connector.
Check for momentary interruption
(See Page TR±23).
2Check water temp. sensor.
C
OK
PDisconnect the water temp. sensor connector.
(See page FI±99).
Measure resistance between terminals.
Resistance is within Acceptable Zone on chart.
Replace water temp. sensor.
3
Check for open and short in harness and connector between engine & ECT ECU and\
water temp.
sensor (See page IN±27).
Repair or replace harness or connector.
Check and replace engine & ECT ECU.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±70
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±MEMO±TR±71
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Diag. Code 24Intake Air Temp. Sensor Circuit
CIRCUIT DESCRIPTION
The intake air temp. sensor is built into the air flow meter and senses the\
intake air temperature.
The structure of the sensor and connection to the ECU is the same as in the\
water temp. sensor shown on page
TR±68.
If the ECU records the diagnostic code º24º, it operates the fail \
safe function, keeping the intake air temperature
at a constant 20 5C (68 5F).
Code No.Diagnostic Code Detecting ConditionTrouble Area
24Open or short in intake air temp. sensor circuit
for 0.5 sec. or more.
Open or short in intake air temp. sensor circuit.

Intake air temp. sensor

ECU
DIAGNOSTIC CHARTDIAGNOSTIC CHART
HINT: If diagnostic codes º22º (water temperature sensor circuit), º\
24º (intake air temperature sensor
circuit) and º41º (throttle position sensor circuit) are output simultaneousl\
y, E2 (sensor ground) may
be open.
WIRING DIAGRAM
Check and replace ECU. Replace water temp. sensor.
Check for momentary interruption
(See page
TR±23).
Repair or replace harness or
connector.
Check voltage of sensor.
Check resistance of sensor.
Check for open and short in harness and
connector between ECU and water temp. sensor.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±72
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