ECU 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 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±
ENGINE TROUBLESHOOTING Circuit Inspection
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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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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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NGOK
OKNG
OKNG
INSPECTION PROCEDURE
1Check voltage between terminals THA 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 THA and
E2 and engine & ECT ECU connector.
Check for momentary Intertuption
(See page TR±23).
2Check intake air temp. sensor.
C
OK
PDisconnect the air flow meter connector.
Measure resistance between terminals 1 and 2 of air
flow meter connector.
Resistance is within Acceptable Zone on chart.
Replace intake air temp. sensor (Replace air flow meter).
3
Check for open and short in harness and connector between engine & ECT ECU an\
d intake air
temp. sensor (See page IN±27).
Repair or replace harness or connector.
Check and replace engine & ECT ECU.
TR±73±
ENGINE TROUBLESHOOTING Circuit Inspection
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DIAGNOSTIC CHART
DIAGNOSTIC
CHART
WIRING DIAGRAM
Refer to page TR±64 for the WIRING DIAGRAM.
Check and replace ECU. Repair or replace harness or
connector.
Check compression Check and replace ECU.
Does malfunction disappear when a good main
oxygen sensor is installed?
Check compression.
Check each item found to be a possible cause
of problem. Check for open and short in harness and
connector.
Check voltage of terminal OX.
Check voltage of terminal VF.
Replace main oxygen sensor. Repair or replace.
Does malfunction disappear when a good main
oxygen sensor is installed? Check each item found to be a possible cause
of problem. Replace main oxygen sensor.
Repair or replace.
Repair or replace.
Repair or replace.
TR±77±
ENGINE TROUBLESHOOTING Circuit Inspection
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OKNG
OKNG
OKNG
NOYES
3
Check for open and short in harness and connector between engine & ECT ECU an\
d main oxy-
gen sensor, engine & ECT ECU and check connector (See page IN±27).
Repair or replace harness or connector.
4Check each item found to be a possible cause of problem.
Check each circuit found to be a possible cause of trouble according to the results of the check in \
1 or
2 . The numbers in the table below show the order in which the checks sho\
uld be preformed.
TR±132
TR±6
TR±51
TR±13
TR±114
TR±88
TR±68
TR±72
Repair or replace.
5Check comparession (See page EM±30).
Repair or replace.
6Does malfunction disappear when a good main oxygen sensor is installed?
Replace main oxygen sensor.
check and replace engine & ECT ECU.
TR±79±
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OKNG
OKNG
NOYES
7Check each item found to be a possible cause of problem.
Check each ciruit found to be a possible cause of trouble according to the re\
sults of the check in 1. The
numbers in the table below show the order in the which the checks should be per\
formed.
TR±132
TR±6
TR±51
TR±13
TR±114
TR±88
TR±68
TR±72
TR±72
Repair or replace.
8Check compression (See page EM±30).
Repair or replace.
9Does malfunction disappear when a good main oxygen sensor is installed?
Replace main oxygen sensor.
Check and replace engine & ECT ECU.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±80
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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
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ENGINE TROUBLESHOOTING Circuit Inspection
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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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