1991 LEXUS SC300 wire

12. Observe the following precautions to avoid damage to theparts:
(a) Do not open the cover or case of the ECU unless
absolutely necessary (If the IC terminals are touched, the
IC may be destroyed by static electricity.).
(b) To pull apart electrical connectors, pull on the connectoritself, not the wires (See page BE±2).
(c) Be careful not to drop electrical components, such as sensors or relays. It they are dropped on a hard floor,
they should be replaced and not reused.
(d) When checking continuity at the wire connector, insert the tester probe carefully to prevent terminals from
bending.
(e) To disconnect vacuum hoses, pull on the end, not the middle of the hose.
(f) When steam cleaning an engine, protect the distributor,
coil and air filter from water.
(g) Ne ve r use an imp a ct wre n ch to re mo ve or in sta ll temperature switches or temperature sensors.
(h) When using a vacuum gauge, never force the hose onto
a connector that is too large. Use a step±down adapter
instead. Once the hose has been stretched, it may leak.
13. Tag hoses before disconnecting them: (a) When disconnecting vacuum hoses, use tags to identify
how they should be reconnected.
(b) After completing a job, double check that the vacuum hoses are properly connected. A label under the hood
shows the proper layout.
14. Installation of wheels The wheel assemblies are of the uni±directional type and
must be rotated only between the front and rear as illustrated.
Wheels are marked with turning direction indicators.
When installing the wheels, check the marks carefully and
install the wheel correctly.
15. Bleeding of traction control system When repairing the brake master cylinder or TRAC system,
bleed the air out of the TRAC system (See page BR±7).
CAUTION: Since the fluid is under high pressure, it could
spray out with great force, so be careful.
IN±6
±
INTRODUCTION General Repair Instructions
WhereEverybodyKnowsYourName

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 .
±
INTRODUCTION Precautions for Vehicles Equipped with SRS AirbagIN±11
WhereEverybodyKnowsYourName

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.
±
INTRODUCTION How to Troubleshoot ECU Controlled SystemsIN±15
WhereEverybodyKnowsYourName

[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
IN±18±
INTRODUCTION How to Troubleshoot ECU Controlled Systems
WhereEverybodyKnowsYourName

[3] SYMPTOM SIMULATION
The most difficult case in troubleshooting is when there are no problem symptoms occurring. In s\
uch cases, a
thorough customer problem analysis must be carried out, then simulate the same or similar conditions and envi-
ronment in which the problem occurred in the customer's vehicle. No matter now much experience a technician
has, or how skilled he may be, if he proceeds to troubleshoot without confir\
ming the problem symptoms he will
tend to overlook something important in the repair operation and make a wron\
g guess somewhere, which will
only lead to a standstill. For example, for a problem which only occurs when\
the engine is cold, or for a problem
which occurs due to vibration caused by the road during driving, etc., the pr\
oblem can never be determined so
long as the symptoms are confirmed with the engine hot condition or the \
vehicle at a standstill. Since vibration,
heat or water penetration (moisture) are likely causes for problems which are \
difficult to reproduce, the symptom
simulation tests introduced here are effective measures in that the external causes are applied to the vehicle
in a stopped condition.
Important Points in the Symptom Simulation Test
In the symptom simulation test, the problem symptoms should of course be co\
nfirmed, but the problem area
or parts must also be found out. To do this, narrow down the possible problem circuits according to the sym\
ptoms
before starting this test and connect a tester beforehand. After that, carry out the symptom simulation test, judg-
ing whether the circuit being tested is defective or normal and also confir\
ming the problem symptoms at the
same time. Refer to the matrix chart of problem symptoms for each system to \
narrow down the possible causes
of the symptom.
[1] VIBRATION METHOD: When vibration seems to be the major cause.
CONNECTORS
Slightly shake the connector vertically and horizon-
tally.
WIRE HARNESS
Slightly shake the wire harness vertically and horizontally.
The connector joint, fulcrum of the vibration, and body
through portion are the major areas to be checked thor-
oughly.
PARTS AND SENSORS
Apply slight vibration with a finger to the part of the
sensor considered to be the problem cause and
check if the malfunction occurs.
HINT: Applying strong vibration to relays may result
in open relays. IN±20
±
INTRODUCTION How to Troubleshoot ECU Controlled Systems
WhereEverybodyKnowsYourName

Location of Connectors in Engine Compartment
wLocation of connectors
This diagram shows the wire harnesses and connectors used in one system.\
The connector shows the harness side connector, so when checking the part side connector (sensor, ac-
tuator, etc.), be careful not to mistake the terminal positions.
±
INTRODUCTION How to Troubleshoot ECU Controlled SystemsIN±25
WhereEverybodyKnowsYourName

HOW TO USE THE DIAGNOSTIC CHART AND INSPECTION
PROCEDURE
1. For troubleshooting, diagnostic code charts or problem symptom charts are p\
rovided for each circuit withdetailed inspection procedures on the following pages.
2. When all the component parts, wire harnesses and connectors of each circuit \
except the ECU are found to be normal in troubleshooting, then it is determined that the problem \
is in the ECU.
Accordingly, if diagnosis is performed without the problem symptoms occurring, the \
instruction will be to
check and replace the ECU, even if the problem is not in the ECU. So, al\
ways confirm that the problem
symptoms are occurring, or proceed with inspection while using the sympt\
om simulation method.
3. The instructions ºCheck wire harness and connectorº and ºCheck and\
replace ECUº which appear in the inspection procedure, are common and applicable to all diagnostic codes.\
Follow the procedure outlined
below whenever these instructions appear.
Check Wire Harness and Connector
The problem in the wire harness or connector is an open circuit or a short \
circuit.
OPEN CIRCUIT:
This could be due to a disconnected wire harness, faulty contact in the conn\
ector, a connector terminal pulled
out, etc.
HINT:
1. It is rarely the case that a wire is broken in the middle of it. Most cases occur at the connector. Inparticular, carefully check the connectors of sensors and actuators.
2. Faulty contact could be due to rusting of the connector terminals, to foreign materials entering terminals or a drop in the contact pressure between the male and female terminals of th\
e connector.
Simply disconnecting and reconnecting the connectors once changes the conditio\
n of the
connection and may result in a return to normal operation.
Therefore, in troubleshooting, if no abnormality is found in the wire harness and \
connector check, but the prob-
lem disappears after the check, then the cause is considered to be in th\
e wire harness or connectors.
SHORT CIRCUIT:
This could be due to a short circuit between the wire harness and the body ground or t\
o a short inside the switch,
etc.
HINT:
w When there is a short between the wire harness and body ground, check thorou\
ghly whether the wire
harness is caught in the body or is clamped properly.
±
INTRODUCTION How to Troubleshoot ECU Controlled SystemsIN±27
WhereEverybodyKnowsYourName

1. CONTINUITY CHECK (OPEN CIRCUIT CHECK)(1) Disconnect the connectors at both ECU and sensor sides.
(2) Measure the resistance between the applicable terminalsof the connectors.
Resistance: 1
or less
HINT:
w Measure the resistance while lightly shaking the wire
harness vertically and horizontally.
w When tester probes are inserted into a connector, insert
the probes from the back. For waterproof connectors in
which the probes cannot be inserted from the back, be
careful not to bend the terminals when inserting the
tester probes.
2. RESISTANCE CHECK (SHORT CIRCUIT CHECK) (1) Disconnect the connectors at both ends.
(2) Measure the resistance between the applicable terminalsof the connectors and body ground. Be sure to carry out
this check on the connectors on both ends.
Resistance: 1 M
or higher
HINT: Measure the resistance while lightly shaking the wire
harness vertically and horizontally.
3. VISUAL CHECK AND CONTACT PRESSURE CHECK (a) Disconnect the connectors at both ends.
(b) Check for rust or foreign material, etc. on the terminalsof the connectors.
(c) Check crimped portions for looseness or damage and check if the terminals are secured in the lock position.
HINT: The terminals should not come out when pulled lightly.
(d) Prepare a test male terminal and insert it in the female
terminal, then pull it out.
HINT: When the test terminal is pulled out more easily than
others, there may be poor contact in that section.
IN±28
±
INTRODUCTION How to Troubleshoot ECU Controlled Systems
WhereEverybodyKnowsYourName