sensor LEXUS SC400 1991 Service Repair Manual

Trouble Area
ºCHECKº
Engine Warning
Light *

Memory* See PageTrouble Area
Normal
ModeTest
Mode
MemorySee Page
w Open or short in air flow meter circuit.
w Air flow meter
w ECU
ONN.A.TR±88
wECUONONTR±92
wOpen or short in throttle position sensor circuit.
wThrottle position sensorONONTR±94Throttle osition sensor
w ECU
wNo. 1 speed sensor
w Combination meter
w Open or short in No. 1 speed sensor circuit.
w ECU
ONONTR±98
wOpen or short in starter signal circuit.
w Open or short in ignition switch or main relay circuit.
w ECU
N.A.OFFXTR±100
wOpen or short in sub±throttle position sensor circuit.
wSub±throttle position sensorOFFONTR±94Sub throttle osition sensor
wECU
wOpen or short in No. 1 knock sensor circuit.wOen or short in No. 1 knock sensor circuit.
w No. 1 knock sensor (looseness)
ONN.A.No. 1 knock sensor (looseness)
wECU

w ECUONN.A.XTR±102
w Open or short in No. 2 knock sensor circuit.w Oen or short in No. 2 knock sensor circuit.
w No. 2 knock sensor (looseness)
ONN.A. No. 2 knock sensor (looseness)
w ECU

*1, 2: See page TR±20.
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ENGINE TROUBLESHOOTING Diagnostic Code ChartTR±19
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Trouble Area
ºCHECKº
Engine Warning
Light*

Memory* See PageTrouble Area
Normal
ModeTest
Mode
MemorySee Page
w Open or short in EGR step motor circuit.ONNA Oen or short in EGR ste motor circuit.
w ECUONN.A.
w Open in EGR gas temp. sensor circuit.ONON
TR±108 Oen in EGR gas tem . sensor circuit.
w ECUONON
w Open or short in fuel pump ECU circuit.
w Fuel pump ECU
w Engine & ECT ECU power source circuit.
w Fuel pump
w Engine & ECT ECU
ONONTR±114
w A/C switch circuit
w Throttle position sensor IDL circuit
w Neutral start switch circuit.
w Accelerator pedal and cable
w ECU
N.A.OFFXTR±118
*4: This indicates items for which º2 trip detection logicº is used. With this logic, when a lo\
gic malfunction is
first detected, the malfunction is temporarily stored in the ECU memory. If the same case is detected again
during the second drive test, this second detection causes the ºCHECK\
º Engine Warning Light to light up.
The 2 trip repeats the same mode a 2nd time. (However, the IG switch must be turned OFF between the
1st trip and 2nd trip).
In the Test Mode, the ºCHECKº Engine Warning Light lights up the 1st trip a malfunction is detected.
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ENGINE TROUBLESHOOTING Diagnostic Code ChartTR±21
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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±
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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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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.
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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±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±
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Diag. Code 21, 28Main Oxygen Sensor Circuit
CIRCUIT DESCRIPTION
To obtain a high purification rate for the CO, HC and NOx components of th\
e exhaust gas, a three±way catalyst
is used, but for most efficient use of the three±way catalyst, the air±fuel ratio must be \
precisely controlled so
that it is always close to the stoichiometric air±fuel ratio.
The oxygen sensor has the characteristic whereby its output voltage changes\
suddenly in the vicinity of the stoi-
chiometric air±fuel ratio. This characteristic is used to detect the oxygen con\
centration in the exhaust gas and
provide feedback to the computer for control of the air±fuel ratio.
When the air±fuel ratio becomes LEAN, the oxygen concentration in the exh\
aust increases and the oxygen sen-
sor informs the ECU of the LEAN condition (small electromotive force: 0 V)\
.
When the air±fuel ratio is RICHER than the stoichiometric air±fuel rat\
io the oxygen concentration in the exhaust
gas is reduced and the oxygen sensor informs the ECU of the RICH condition \
(large electromotive force: 1V).
The ECU judges by the electromotive force from the oxygen sensor whether th\
e air±fuel ratio is RICH or LEAN
and controls the injection time accordingly. However, if malfunction of the oxygen sensor causes output of ab-
normal electromotive force, the ECU is unable to perform accurate air±fuel \
ratio control.
DTC No.DTC Detecting ConditionTrouble Area
21.
28
(1) Main oxygen sensor signal voltage is reduced to tbetween 0.35 V and 0.70 V for 60 sec. under
conditions (a)
(d). (2 trip detection logic)*
(a) Coolant temp.: Between 805C (176 5F) and
95 5C (203 5F).
(b) Engine speed: 1,500 rpm or more
(c) Load driving (EX. ECT in 4th (5th for M/T) speed, A/C ON, Flat road, 50 mph
(80km/h)).
(d) Main oxygen sensor signal voltage: Alternating above and below 0.45 V.

Main oxygen sensor circuit

Main oxygen sensor
(2) Main oxygen sensor signal voltage exceeds
0.70 V for 3 sec. or more during fuel cut.
*: See page TR±25..
HINT: Diag. trouble code 21 is for the front side main oxygen sensor circuit.\
Diag. trouble code 28 is for the rear side main oxygen sensor circuit.
TR±60
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ENGINE TROUBLESHOOTING Circuit Inspection
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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 co\
nfirming that diag. code isno longer detected.

Disconnect the EFI fuse (15 A) for 10 sec. or more, with IG switch OFF\
.
Initiate test mode (Connect terminal TE2 and E1 of check connector or TDCL with IG
switch OFF).
Start the engine and warm up with all ACC switch OFF.
After the engine is warmed up, let it idle for 3 min.
After performing the idling in , perform gradual acceleration with in th\
e range 1,300
1,700 rpm (centered around 1,500 rpm) with the A/C switch ON and D range fo\
r A/T
(5th for M/T).
(Take care that the engine speed does not fall below 1,200 rpm when shifting.\
Gradu-
ally depress the accelerator pedal and kept it. Steady so that engine br\
aking does not
occur).
Maintain the vehicle speed at 40±50 mph (64±80 km/h).
Keep the vehicle running for 1±2 min. after starting acceleration.
HINT: If a malfunction exists, the ºCHECKº engine warning light will light\
up after approx. 60 sec. from the start of acceleration.
NOTICE: If the conditions in this test are not strictly followed, detection of t\
he malfunction will not be possible.
Malfunction: Main Oxygen Sensor Deterioration
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ENGINE TROUBLESHOOTING Circuit InspectionTR±61
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DIAGNOSTIC CHART
Are there any other codes (Besides 21 or
28) being output?
Replace oxygen sensor.
Go to relevant diag. code chart.
HINT: If diag. code 21 is output, replace the front side main oxygen sensor.If diag. code 28 is output, replace the rear side main oxygen sensor.
WIRING DIAGRAM
TR±62±
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Reference INSPECTION USING OSCILLOSCOPE
wWith the engine racing (4,000 rpm) measure
between terminals OX1, OX2 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.
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ENGINE TROUBLESHOOTING Circuit InspectionTR±63
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