LEXUS SC400 1991 Service Repair Manual

Disconnect distributor connector.
Measure resistance between each terminal shown in
table below.
Check resistance of each picup coils in distributor.

During cranking or idling, check between terminals G1,
G2, NE and G
of engine (&ECT) ECU.
HINT: The correct waveforms appears as shown in the illustration on the left.
Replace distributor.
Check for open and short in harness and connector between engine
(&ECT) ECU and distributor (See page IN±27
Repair or replace harness or connector.
INSPECTION PROCEDURE
TR±48±
ENGINE TROUBLESHOOTING Circuit Inspection
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Remove distributor cap & rotor.
Using SST (G1 and G2 pickups) and a thikness gauge
(NE pickup). measure the air gap between the signal
rotor projection and pickup coil.
SST 09240±00020 fro G1 and G2 pickups
Air gap: 0.2±0.4 mm (0.008±0.016 in.)
Replace distributor.
Check and replace engine (& ECT) ECU.
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ENGINE TROUBLESHOOTING Circuit InspectionTR±49
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CIRCUIT INSPECTIONDiag. Code 12
RPM Signal Circuit (No.1)
CIRCUIT DESCRIPTION
Cam position sensors (G1 and G2 signals) and engine speed sensor (NE sig\
nal) consist of a signal plate
and a pick up coil.
The G1, G2 signal plates have one tooth each on its outer circumference and\
are mounted on the left and
right bank camshafts.
When the camshafts rotate, the protrusion on the signal plate and the air gap on the pick up coil change,
causing fluctuations in the magnetic field and generating an electromotive forc\
e in the pick up coil.
The NE signal plate has 12 teeth and is mounted on the crankshaft. The NE s\
ignal sensor generates 12
NE signals per engine revolution. The ECU detects the standard crankshaf\
t angle based on the G1, G2
signals, and the actual crankshaft angle and the engine speed by the NE \
signals.
Code No.Diagnostic Code Detecting ConditionTrouble Area
wOpen or short in engine speed sensor, No. 1,w Oen or short in engine s eed sensor, No. 1,
No. 2 cam position sensor circuit
12No ºNEº or ºG1º and ºG2º signal to ECU
No. 2 cam osition sensor circuit
w Engine speed sensor12No NE or G1 and G2 signal to ECU
within 2 sec. after cranking.
wEngine s eed sensor
wNo. 1, No. 2 cam position sensorgNo. 1, No. 2 cam osition sensor
wStarter
wECU
TR±48±
ENGINE TROUBLESHOOTING Circuit Inspection
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DIAGNOSTIC CHART
WIRING DIAGRAM
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ENGINE TROUBLESHOOTING Circuit InspectionTR±49
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OKNG
INSPECTION PROCEDURE
1Check engine speed sensor, No. 1, No. 2 cam position sensor
C
OK
PFor engine speed sensor.
(2) Remove engine under cover.
(2) Disconnected engine speed sensor connector.
For No.1, No. 2 cam position sensor,
(2) Disconnect No. 1, No. 2 cam poisition sensor con-nectors.
Measure resistance of engine speed sencor, No. 1 and
No. 2 cam position sensor.
Replace engine speed sensor, No. 1, No. 2 cam poistion
sensor.
TR±50
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ENGINE TROUBLESHOOTING Circuit Inspection
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OKNG
OKNG
2Check for open and short in harness and connector between engine & ECT ECU an\
d each sen-
sor (See page IN±27).
Repair or replace harness or connector.
3Inspect sensor installation and teeth of signal plate.
Tighen the sensor.
Replace signal plate.
Check and replace engine and ECT ECU.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±51
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Diag. Code 13RPM Signal Circuit (No. 2)
CIRCUIT DESCRIPTION
Refer to RPM signal circuit (No. 1) on page TR±46.
Code No.Diagnostic Code Detecting ConditionTrouble Area
13
No NE signal to ECU for 0.1 sec. or more at
1,000 rpm or more.

Open or short in NE circuit

Distributor13
No 12 pulses of NE to ECU during the interval

Distributor

ECU
No 12 ulses of NE to ECU during the interval
between G1 and G2 pulses.

ECU
DIAGNOSTIC CHART
This code indicates that a momentary interruption of the RPM signal from the\
distributor to the ECU has
occurred, but that it is returned to normal. Note that although this problem may \
not necessarily appear
at the time of inspection, it cannot be ignored because this diagnostic co\
de is output, indicating that there
is or was a malfunction in the RPM signal circuit; this ºmalfunctionº\
is usually a loose connector.
The distributor connector and the NE terminal of the ECU connector must the\
refore be checked for the
following:
1. Loose connectors
2. Dirty connector terminals
3. Loose connector terminals
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Diag. Code 13RPM Signal Circuit (No.2)
CIRCUIT DESCRIPTION
Refer to RPM signal circuit (No. 1) on page TR±48.
Code No.Diagnostic Code Detecting ConditionTrouble Area
No NE signal to ECU for 0.1 sec. or more at
1,000 rpm or more.
wOpen or short in engine speed sensor
w circuit.
w Engine speed sensor
w ECU
13No 12 pulses of NE to ECU during the interval
between G1 and G2 pulses.
wOpen or short in engine speed sensor
w circuit.
w Mechanical system malfunction (skipping
teeth of timing belt, belt stretched)
w Engine speed sensor
w ECU
Deviation in G (G1, G2) and NE signal contin-
ues for 1 sec. during idling (throttle fully
closed) after engine warmed up.wMechanical system malfunction (skipping
teeth of timing belt, belt stretched)
w No. 1, No. 2 cam position sensor
w ECU
DIAGNOSTIC CHARTDIAGNOSTIC CHART
Inspect sensor installation. Check if any teeth of NE
signal plate are broken
Check valve timing (Check for loose and jumping teeth
of timing belt (See page EM±51)). Tighten the sensor.
Replace signal plate.
Adjust valve timing
(Repair or replace timing belt).
HINT: Perform troubleshooting of diag. code 12 first. If no trouble is found, trouble\
shoot the following mechanical systems.
Check for momentary interruption
(See page TR±23).
TR±52±
ENGINE TROUBLESHOOTING Circuit Inspection
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MEMO ±
ENGINE TROUBLESHOOTINGTR±53
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Diag. Code 14, 15Ignition Signal Circuit
CIRCUIT DESCRIPTION
The ECU determines the ignition timing, turns on Tr1 at a predetermined angle (5CA) before the desired
ignition timing and outputs an ignition signal (IGT) º1º to the \
igniter.
Since the width of the IGT signal is constant, the dwell angle control circuit in the ignit\
er determines the
time the control circuit starts primary current flow to the ignition coil bas\
ed on the engine rpm and ignition
timing one revolution ago, that is, the time the Tr2 turns on.
When it reaches the ignition timing, the ECU turns Tr1 off and outputs the IGT signal ºOº.
This turns Tr2 off, interrupting the primary current flow and generating a high voltage i\
n the secondary coil
which causes the spark plug to spark. Also, by the counter electromotive force\
generated when the primary
current is interrupted, the igniter sends an ignition confirmation signal (IGF\
) to the ECU.
The ECU stops fuel injection as a fail safe function when the IGF signal\
is not input to the ECU.
Code No.Diagnostic Code Detecting ConditionTrouble Area
14No IGF1 signal to ECU for 8±11 consecutive
IGT1 signal.
w Open or short in IGF1 and IGT1 circuit from
No. 1 igniter ± ECU.
w No. 1 igniter
w ECU
15No IGF2 signal to ECU for 8±11 consecutive
IGT2 signal.
wOpen or short in IGF2 and IGT2 circuit from
No. 2 igniter ± ECU.
w No. 2 igniter
w ECU
HINT: No. 1 igniter is for left bank and No. 2 igniter is for right bank.
WIRING DIAGRAM
TR±54±
ENGINE TROUBLESHOOTING Circuit Inspection
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