1991 LEXUS SC400 turn signal

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
WhereEverybodyKnowsYourName

OKNG
OKNG
OKNG
INSPECTION PROCEDURE
HINT: If diag. code 14 is displayed, check No. 1 igniter circuit. If diag. code 15 is displayed, check No. 2 igniter circuit.
1Check for Spark.
C
OK
Hint
Disconnect the high±tension cord from the distributor,
hold its end about 12.5 mm (1/2º) from the ground, see
if spark occurs while the engine is being cranked.
Spark should be generated.
To prevent excessive fuel injected from the injectors dur-
ing the check. Don't crank the engine for more than 1±2
seconds at a time.
Go to step [4].
2Check for open and short in harness and connector in the IGF and IGT signal c\
ircuit between
engine & ECT ECU and igniter (See page IN±27)
Repair or replace harness or connector.
#Disconnect igniter sonnector and check voltage between terminals IGF1, 2 of engine & ECT
ECU connector and body ground.
C
OK
P(2) Disconnect igniter connector.
(2) Connect the Check Harness A.
(See page TR±30).
(2) Turn ignition switch on.
Measure Voltage between terminals IGF1, 2 or engine
& ECT ECU connector and body ground.
Voltage: 4 ± 6 V
Replace igniter.*1
Check and replace engine & ECT ECU.
*1: When dia. code 14 is displayed, replace the igniter with 5 wire harness. (extra wire is for tachometer). When diag.
code 15 is displayed, replace the igniter with 4 wire har-
ness.
TR±56±
ENGINE TROUBLESHOOTING Circuit Inspection
WhereEverybodyKnowsYourName

Diag. Code 71
EGR System Malfunction (Only for
USA spec.)
CIRCUIT DESCRIPTION
If even one of the following conditions is ful-
filled, the VSV is turned ON by a signal from
the ECU. This results in atmospheric air act-
ing on the EGR valve, closing the EGR valve
and shutting off the exhaust gas (EGR cut±
OFF).
w
Coolant temp. below 505C (122 5F)
w
During deceleration (throttle valve closed)
w
Light engine load (amount of intake air very
small).
w
Engine speed over 5,200 rpm. The EGR system is designed to recirculate the exhaust gas properly controll\
ed according to the driv-
ing condition back into the intake air±fuel mixture. It helps to slow dow\
n combustion in the cylinder
and thus lower the combustion temperature which, in turn, reduces the amoun\
t of NOx emission. The
amount of EGR is regulated by the EGR vacuum modulator according to the engine\
load.
Code No.Diagnostic Code Detecting ConditionTrouble Area
EGR t i 70
5C (158 5F) b l f 1 4
w
Open in EGR
gas temp. sensor circuit.EGR gas temp. is 70 5C (158 5F) or below for 1 4
min. under conditions (a) and (b).
(2 trip detection logic)*
Oen in EGR gas tem. sensor circuit.
w
Short in VSV circuit for EGR.
71
(2 trip detection logic)*
(a) Coolant temp.: 63 5C (145 5F) or more.
(b) EGR operationpossible (EX ECT in 3rd speed
Short in VSV circuit for EGR.
w
EGR hose disconnected, valve stuck.
(b) EGR operation possible (EX. ECT in 3rd speed
(5th for M/T), A/C ON, 60 mph (96 km/h), Flat
road)
EGR hose disconnected, valve stuck.
w
Clogged EGR gas passage.
road).gg g g
w
ECU
*: See page TR±25.
TR±104
±
ENGINE TROUBLESHOOTING Circuit Inspection
WhereEverybodyKnowsYourName

OKNG
OKNG
INSPECTION PROCEDURE
1Check output condition of diag. code 51.
C
OK
Hint
PSetting the test mode.
(2) Turn ignition switch OFF.
(2) Connect terminals TE2 and E1 and TDCL.
(2) Turn ignition switch ON.(For checking terminal A/C, start the engine.)
(2) Connect terminals TE1 and E1 of TDCL.
Check if code ª51º is output by the ªCHECKº engine
warning light.
*: Before the STA signal is input (ST is not ON), diagnostic code 43 is also output.
Diag. code 42 is output with vehicle speed 5 km/h (3
mph) or below
IDL1 .... Go to step 2 NSW ... Go to page TR±22.
A/C ..... Go to step 3
Proceed to next circuit inspection shown on
maxtrix chart (See page TR±35).
2Check throttle position sensor.
C
OK
PDisconnect throttle position sensor connector.
Measure resistance between terminals IDL1 and E2 of
throttle position sensor connector.
Adjust or replace throttle position sensor
See page FI±73).
Check and repair harness or connector between
engine & ECT ECU and throttle position sensor.
TR±120±
ENGINE TROUBLESHOOTING Circuit Inspection
WhereEverybodyKnowsYourName

ECU Power Source Circuit
CIRCUIT DESCRIPTION
When the ignition switch is turned on, battery voltage
is applied to the terminal IGSW of the ECU, and the
main relay control circuit in the ECU sends a signal to
the terminal M±REL of the ECU, switching on the
main relay. This signal causes current to flow to the
coil, closing the contacts of the main relay and supply-
ing power to the terminals +B and +B1 of the ECU.
If the ignition switch is turned off, the ECU continues
to switch on the main relay for a maximum of 2 se-
conds for the initial setting of the ISC valve.
DIAGNOSTIC CHART
See next page for the DIAGNOSTIC CHART.
WIRING DIAGRAM
TR±120±
ENGINE TROUBLESHOOTING Circuit Inspection
WhereEverybodyKnowsYourName

ECU Power Source Circuit
CIRCUIT DESCRIPTION
When the ignition switch is turned on, battery voltage
is applied to the terminal IG SW of the ECU, and the
main relay control circuit in the ECU sends a signal to
the terminal M±REL of the ECU, switching on the
main relay. This signal causes current to flow to the
coil, closing the contacts of the main relay and supply-
ing power to the terminals +B and +B1 of the ECU.
If the ignition switch is turned off, the ECU continues
to switch on the main relay for a maximum of 2 se-
conds for the initial setting of the ISC valve.
DIAGNOSTIC CHARTDIAGNOSTIC CHART
See next page for the DIAGNOSTIC CHART.
WIRING DIAGRAM
TR±124±
ENGINE TROUBLESHOOTING Circuit Inspection
WhereEverybodyKnowsYourName

ISC Valve Circuit
CIRCUIT DESCRIPTION
The ISC valve is provided on the intake air chamber and in-
take air bypassing the throttle valve is directed to the ISC
valve through a hose.
A step motor is built into the ISC valve. It consists of four
coils, the magnetic rotor, valve shaft and valve. When cur-
rent flows to the coils due to signals from the ECU, the rotor
turns and moves the valve shaft forward or backward,
changing the clearance between the valve and the valve
seat.
In this way the intake air volume bypassing the throttle valve
is regulated, controlling the engine speed.
There are 125 possible positions to which the valve can be
opened.
DIAGNOSTIC CHARTDIAGNOSTIC CHART
Check operation for ISC valve.Replace ISC valve.
Repair or replace harness or
connector.
Proceed to next circuit inspection shown on
matrix chart (See page TR±35).
Check harness and connector between EFI
main relay, ISC valve and ECU.
TR±140±
ENGINE TROUBLESHOOTING Circuit Inspection
WhereEverybodyKnowsYourName

HOW TO PROCEED WITH TROUBLESHOOTING
The Engine Control System broadly consists of the sensors, ECU and actuator\
s. The ECU receives signals from
various sensors, judges the operating conditions and determines the opti\
mum injection duration, timing, ignition
timing and idle speed.
In general, the Engine Control System is considered to be a very intricate\
system to troubleshoot. But, the fact
is that if you proceed to inspect the circuit one by one following the p\
rocedures directed in this manual, trouble-
shooting of this system is not complex.
This section explains the most ideal method of troubleshooting and tells how\
to carry out the necessary repairs.
[1] CUSTOMER PROBLEM ANALYSISUsing the customer problem analysis check sheet for reference, ask the c\
ustomer in as much details as
possible about the problem.
[2] CHECK AND CLEAR DIAGNOSTIC CODE (PRECHECK) Before confirming the problem symptom, first check the diagnostic code and mak\
e a note of any malfunc-
tion code which is output, then clear the code.
HINT: Output of the malfunction code indicates that there is a malfunction in\
the circuit indicated. However,
it does not indicate whether the malfunction is still occurring or occur\
red in the past and returned to normal.
In order to determine this, the problem symptoms should be confirmed in 4 \
first and the diagnostic code
be rechecked in [6].
Accordingly, if troubleshooting is begun based on the malfunction code only in diag\
nostic code check in
[2] , it could result in a misdiagnosis, leading to troubleshooting of circuits which are normal \
and making
it more difficult to locate the cause of the problem.
[3] SETTING THE TEST MODE DIAGNOSIS, [4] PROBLEM SYMPTOM CONFIRMATION, [5] SYMPTOM SIMULATION
In order to find out the trouble more quickly, set the diagnosis check in test mode and with higher sensing
ability of the ECU, confirm the problem symptoms. If the trouble does not reappear, use the symptom simu-
lation method to make sure the trouble is reproduced.
[6] DIAGNOSTIC CODE CHECK IN TEST MODE Check the diagnostic code in test mode. If the malfunction code is output, pr\
oceed to ºstep [8] Diagnostic
Code Chartº. If the normal code is output, proceed to ºstep [7] Basic Inspectionº.
[7] BASIC INSPECTION Carry out basic inspection such as the spark check and fuel pressure che\
ck, etc.
[8] DIAGNOSTIC CODE CHART If the malfunction code is displayed, proceed to inspect the circuit ind\
icated by the chart for each code.
[9] MATRIX CHART OF PROBLEM SYMPTOMS If the normal code is displayed in the diagnosis in test mode, perform tro\
ubleshooting according to the in-
spection order in the Matrix Chart of Problem Symptoms.
[10] PARTS INSPECTION When the Matrix Chart of Problem Symptoms instructs to check the parts, proc\
eed to parts inspection sec-
tion included in this manual.
[11] CIRCUIT INSPECTION Determine if the malfunction is the sensor, actuator, wire harness, connector or the ECU.
TR±2
±
ENGINE TROUBLESHOOTING How to Proceed with Troubleshooting
WhereEverybodyKnowsYourName