1991 LEXUS SC300 ect

When the negative (±) terminal cable is disconnected from
the battery, memory of the clock and audio systems will be
cancelled. So before starting work, make a record of the con-
tents memorized by each memory system. Then when work
is finished, reset the clock and audio systems as before.
When the vehicle has power tilt and power telescopic steer-
ing, power seat, power outside rear view mirror and power
shoulder belt anchorage, which are all equipped with
memory function, it is not possible to make a record of the
memory contents. So when the operation is finished, it will be
necessary to explain this fact to the customer, and ask the
customer to adjust the features and reset the memory.
To avoid erasing the memory of each memory system, never
use a back±up power supply from outside the vehicle.
3. Even in cases of a minor collision where the airbag does not deploy, the front airbag sensors and the steering wheel pad
should be inspected. (See page AB±9)
4. Never use airbag parts from another vehicle. When replacing parts, replace them with new parts.
5. Before repairs, remove the airbag sensors if shocks are likely
to be applied to the sensors during repairs.
6. The center airbag sensor assembly contains mercury. After
performing replacement, do not destroy the old part. When
scrapping the vehicle or replacing the center airbag sensor
assembly itself, remove the center airbag sensor assembly
and dispose of it as toxic waste.
7. Never disassemble and repair the front airbag sensors, center airbag sensor assembly or steering wheel pad in order
to reuse it.
8. If the front airbag sensors, center airbag sensor assembly or steering wheel pad have been dropped, or if there are cracks,
dents or other defects in the case, bracket or connector,
replace them with new ones.
9. Do not expose the front airbag sensors, center airbag sensor assembly or steering wheel pad directly to hot air or flames.
10. U s e a v o l t / o h m m e t e r w i t h h i g h i m pedance (10 kW/V
minimum) for troubleshooting of the electrical circuit.
11. Information labels are attached to the periphery of the airbag components. Follow the notices.
12. After work on the airbag system is completed, perform the
airbag warning light check (See page AB±30).
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INTRODUCTION Precautions for Vehicles Equipped with SRS AirbagIN±9
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FRONT AIRBAG SENSOR
1. Never reuse the front airbag sensors involved in a collisionwhen the airbag has deployed. (Replace both the left and
right airbag sensors.)
2. Install the front airbag sensor with the arrow on the sensor facing toward the front of the vehicle.
3. The front airbag sensor set bolts have been anti±rust treated. When the sensor is removed, always replace the set bolts
with new ones.
4. The front airbag sensor is equipped with an electrical connection check mechanism. Be sure to lock this
mechanism securely when connecting the connector. If the
connector is not securely locked, a malfunction code will be
detected by the diagnosis system (See page AB±10).
SPIRAL CABLE (IN COMBINATION
SWITCH)
The steering wheel must be fitted correctly to the steering column
with the spiral cable at the neutral position; otherwise cable dis-
connection and other troubles may result. Refer to page AB±15
of this manual concerning correct steering wheel installation.
STEERING WHEEL PAD (WITH AIRBAG)
1. When removing the steering wheel pad or handling a new steering wheel pad, it should be placed with the pad top
surface facing up.
In this case, the twin±lock type connector lock lever should
be in the locked state and care should be taken to place it so
the connector will not be damaged. And do not store a steer-
ing wheel pad on top of another one. (Storing the pad with its
metallic surface up may lead to a serious accident if the air-
bag inflates for some reason.)
2. Never measure the resistance of the airbag squib. (This
may cause the airbag to deploy, which is very
dangerous.)
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INTRODUCTION Precautions for Vehicles Equipped with SRS Airbag
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3. Grease should not be applied to the steering wheel pad andthe pad should not be cleaned with detergents of any kind.
4. Store the steering wheel pad where the ambient temperature remains below 93
C (200
F), without high humidity and
away from electrical noise.
5. When using electric welding, first disconnect the airbag connector (yellow color and 2 pins) under the steering
column near the combination switch connector before
starting work.
6. When disposing of a vehicle or the steering wheel pad alone, the airbag should be deployed using an SST
before disposal (See page AB±91). Perform the
operation in a place away from electrical noise.
CENTER AIRBAG SENSOR ASSEMBLY
The connector to the center airbag sensor assembly should be
connected or disconnected with the sensor mounted on the floor.
If the connector is connected or disconnected while the center air-
bag sensor assembly is not mounted to the floor, it could cause un-
desired ignition of the airbag system.
WIRE HARNESS AND CONNECTOR
The airbag system's wire harness is integrated with the cowl wire
harness assembly. The wires for the airbag wire harness are en-
cased in a yellow corrugated tube. All the connectors for the sys-
tem are also a standard yellow color. If the airbag system wire har-
ness becomes disconnected or the connector becomes broken
due to an accident, etc., repair or replace it as shown on page
AB±22 .
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INTRODUCTION Precautions for Vehicles Equipped with SRS AirbagIN±11
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INSPECTION AND ADJUSTMENT OF
JOINT ANGLE DURING REMOVAL
AND INSTALLATION OF
PROPELLER SHAFT
When performing operations which involve the removal and instal-
lation of the propeller shaft be sure to check the joint angle. Make
adjustment if necessary (See page PR±13).
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INTRODUCTION Inspection
and Adjustment of Joint Angle During
Removal and Installation of Propeller Shaft
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HOW TO TROUBLESHOOT ECU CONTROLLED SYSTEMS
A large number of ECU controlled systems are used in the LEXUS SC400. In \
general, the ECU controlled sys-
tem is considered to be a very intricate system requiring a high level o\
f technical knowledge and expert skill to
troubleshoot. However, the fact is that if you proceed to inspect the circuits one by one, trou\
bleshooting of these
systems is not complex. If you have adequate understanding of the system\
and a basic knowledge of electricity,
accurate diagnosis and necessary repair can be performed to locate and fix the p\
roblem. This manual is de-
signed through emphasis of the above standpoint to help service technici\
ans perform accurate and effective
troubleshooting, and is compiled for the following major ECU controlled \
systems:
Repair ManualSystemPage
1. EngineTR±1
Vl1
2. Automatic TransmissionAT±33
Vol. 13. Anti±Lock BrakeBR±43
4. Traction ControlBR±139
5. Steering Column Electronic ControlSR±22
6. SRS AirbagAB±25
7. Power Seat ControlBE±265
Vl2
8. Door Lock ControlBE±347
Vol. 29. Theft DeterrentBE±381
10. Wireless Door Lock ControlBE±425
11. Cruise ControlBE±459
12. Air ConditionerAC±21
The troubleshooting procedure and how to make use of it are described on\
the following pages.
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INTRODUCTION How to Troubleshoot ECU Controlled SystemsIN±15
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HOW TO PROCEED WITH TROUBLESHOOTING
Carry out troubleshooting in accordance with the procedure on the follow\
ing page. Here, only the basic proce-
dure is shown. Details are provided in each section, showing the most effective methods for each circuit. Confirm
the troubleshooting procedures first for the relevant circuit before beginn\
ing troubleshooting of that circuit.


Ask the customer about the conditions and the
environment when the problem occurred.
\b 
Confirm the symptoms and the problem
conditions, and check the diagnostic codes.
(When the problem symptoms do not appear
during confirmation, use the symptom simu-
lation method described later on.)
  
Check the result obtained in Step \b, then
confirm the inspection procedure for the sys-
tem or the part which should be checked us-
ing the diagnostic code chart or the matrix
chart of problem symptoms.

Check and repair the affected system or part
in accordance with the instructions in Step .

After completing repairs, confirm that the
problem has been eliminated.
(If the problem is not reproduced, perform the
confirmation test under the same conditions
and in the same environment as when it oc-
curred for the first time.)
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INTRODUCTION How to Troubleshoot ECU Controlled Systems
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[1] CUSTOMER PROBLEM ANALYSIS
In troubleshooting, the problem symptoms must be confirmed accurately and \
all preconceptions must be
cleared away in order to give an accurate judgment. To ascertain just what the problem symptoms are, it is ex-
tremely important to ask the customer about the problem and the conditions at t\
he time it occurred.
Important Points in the Problem Analysis
The following 5 items are important points in the problem analysis. Past pr\
oblems which are thought to be unre-
lated and the repair history, etc. may also help in some cases, so as much information as possible s\
hould be
gathered and its relationship with the problem symptoms should be correctly asce\
rtained for reference in trou-
bleshooting. A customer problem analysis table is provided in the troubl\
eshooting section for each system for
your use.
Important Points in the Customer Problem Analysis
wWhat Vehicle model, system name
w When Date, time, occurrence frequency
w Where Road conditions
w Under what conditions? Running conditions, driving con\
ditions, weather conditions
w How did it happen? Problem symptoms
(Sample) Engine control system check sheet.
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INTRODUCTION How to Troubleshoot ECU Controlled SystemsIN±17
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[2] SYMPTOM CONFIRMATION AND DIAGNOSTIC CODE CHECK
The diagnostic system in the LEXUS SC400 fulfills various functions. The fi\
rst function is the Diagnostic Code
Check in which a malfunction in the signal circuits to the ECU is stored\
in code in the ECU memory at the time
of occurrence, to be output by the technician during troubleshooting. Anot\
her function is the Input Signal Check
which checks if the signals from various switches are sent to the ECU correct\
ly.
The air conditioner system has an Actuator Check function whereby the ECU a\
utomatically operates the actua-
tors of the damper and blowermotor, etc. to check the operation. The cruise control system has a Cancel Si\
gnal
Check function which memorizes and displays what sort of signal it was that l\
ast cancelled the cruise control.
By using these check functions, the problem areas can be narrowed down qui\
ckly and troubleshooting can be
performed effectively. Diagnostic functions are incorporated in the following systems in the \
LEXUS SC400.
SystemDiagnostic CodeCheckInput Signal Check(Sensor Check)Other Diagnosis Function
Engine (with Test Mode)
Automatic Transmission (with Test Mode)
Anti±Lock Brake
Traction Control (Option)
SRS Airbag
Power Seat (only vehicles with memory function)
Wireless Door Lock Control
Cruise Control Cancel Signal
Check
Air Conditioner Actuator Check
In diagnostic code check, it is very important to determine whether the pr\
oblem indicated by the diagnostic code
is still occurring or occurred in the past but returned to normal at pre\
sent. In addition, it must be checked in the
problem sym ptom check whether the malfunction indicated by the diagnostic code is d\
irectly related to the prob-
lem symptom or not. For this reason, the diagnostic codes should be chec\
ked before and after the symptom
confirmation to determine the current conditions, as shown in the table below. If this is not done, it may, depend-
ing on the case, result in unnecessary troubleshooting for normally oper\
ating systems, thus making it more diffi-
cult to locate the problem, or in repairs not pertinent to the problem. Ther\
efore, always follow the procedure in
correct order and perform the diagnostic code check.
DIAGNOSTIC CODE CHECK PROCEDURE
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INTRODUCTION How to Troubleshoot ECU Controlled Systems
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