LEXUS SC400 1991 Service Repair Manual

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±
ENGINE TROUBLESHOOTING Circuit Inspection
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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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CIRCUIT DESCRIPTION (Cont'd)
DIAGNOSIS CODE DETECTION DRIVING PATTERN
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 repair is completed c\
onfirming that diag. code is nolonger detected.
Malfunction: Open or Short in Knock Sensor

Start engine and warm up.
After engine is warmed up, let it idle for 3 min.
With the A/C ON, perform quick racing (5,000 rpm) three times.
(Rapidly depress the accelerator pedal and suddenly release it.)
HINT: If a malfunction exists, the ºCHECKº engine warning light will lig\
ht up when sudden racing is performed.
NOTICE: If the conditions in this test are not strictly followed, detection of t\
he malfunction will
not be possible.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±103
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DIAGNOSTIC CHART
HINT: If diag. code 52 is displayed, check No. 1 knock sensor (for left bank)\
circuit.If diag. code 55 is displayed, check No. 2 knock sensor (for right bank\
) circuit.
If diag. code 53 is displayed, replace engine & ECT ECU.
Does malfunction disappear when a good knock
sensor is installed? Repair or replace harness or
connector.
Replace knock sensor.
Check no continuity of knock sensor circuit.
Check knock sensor.
Check for open and short in harness and
connector between knock sensor and ECU.
Replace knock sensor.
Check and replace ECU.
WIRING DIAGRAM
TR±104±
ENGINE TROUBLESHOOTING Circuit Inspection
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NGOK
OKNG
INSPECTION PROCEDURE
1Check continuity between terminals KNK1, KNK2 of engine & ECT ECU connector and body
ground.
C
OK
P(2) Connect the Check Harness A.(See page TR±30)
(2) Disconnect the engine & ECT ECU connector.
Measure resistance between terminals KNK1, KNK2 of
engine & ECT ECU connector and body ground.
Resistance: 1 M
or higher
Go to step 3.
2Check knock sensor.
C
OK
PDisconnect knock sensor connector.
Measure resistance between the knock sensor terminal
and body.
Resistance: 1 M
or higher
Replace knock sensor (See page FI±108).
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±105
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OKNG
NOYES
3
Check for open and short in harness and connectors between engine & ECT ECU a\
nd knock
sensor (See page IN±27)
Repair or replace harness or connector.
4Does malfunction disappear when a good knock sensor is installed?
Replace knock sensor (See page FI±108).
Check and replace engine & ECT ECU.
Reference | INSPECTION USING OSCILLOSCOPE
w With the engine racing (4,000 rpm) measure
between terminals KNK1, KNK2 of engine & ECT
ECU and body ground.
HINT: The correct waveform appears as shown in the Illustration on the left.
w Spread the time on the horizontal axis, and confirm
that period of the wave is 151 sec.
(Normal mode vibration frequency of knock sensor:
6.6 KHz).
HINT: If normal mode vibration frequency is not 6.6 KHz, the sensor is malfunctioning.
TR±106
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ENGINE TROUBLESHOOTING Circuit Inspection
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±MEMO±
ENGINE TROUBLESHOOTINGTR±107
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Diag. Code 71
EGR System Malfunction (Only for USA
spec.)
CIRCUIT DESCRIPTION
The EGR system recirculates exhaust gas, which is controlled to the proper quantity to suit the driv\
ing condi-
tions, into the intake air mixture to slow down combustion, reduce the combust\
ion temperature and reduce NOx
emissions.
The lift amount of the EGR valve is controlled by a step motor operated \
by the ECU. The ECU operates the motor
only a preprogrammed amount in response to the engine operating conditions (\
engine rpm, intake air volume
to adjust the EGR volume to the target valve.)
Under the following conditions, EGR is cut to maintain driveability.
wCoolant temp. below 535C (127.45F)
w During deceleration (throttle valve
closed)
w Light engine load (amount of intake air
very small)
w Engine speed over 4,000 rpm
w Engine idling
w Neutral start switch on
Code No.Diagnostic Code Detecting ConditionTrouble Area
(1) Open or short in EGR step motor circuit for 1 sec.
or more.
Open or short in EGR step motor circuit

ECU
71
(2) EGR gas temp. is 65 5C (149 5F) or below for
1 ~ 4 min. under conditions (a) and(b).
(2 trip detection logic)*
(a) Coolant temp.: 65 5C (149 5F) or more.
(b) EGR operation possible (EX. ECT in 3rd speed, A/C ON, 60 mph (96 km/h), Flat road).

Open in EGR gas temp. sensor circuit

ECU
*: See page TR±21.
TR±108
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CIRCUIT DESCRIPTION (Cont'd)
DIAGNOSIS CODE DETECTION DRIVING PATTERN
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 repair is completed c\
onfirming that diag. code is nolonger detected.

Initiate test mode (See page TR±12).
Start engine and warm up.
After engine is warmed up, let it idle for 3 min.
With the A/C ON and transmission in D range (O/D OFF), drive at 40±\
70 mph for 4 min.
HINT: If a malfunction exists, the ºCHECKº engine warning light will light up at 1±4 min. of driving at 40±70 mph (64±112 km/h).
NOTICE: If the conditions in this test are not strictly followed, detection of the\
malfunction will not be possible.
Malfunction: Open in EGR gas temp. sensor circuit
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±109
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