ECU LEXUS SC300 1991 Service Repair Manual

ABBREVIATIONS USED IN THIS
MANUAL
ABS Anti±Lock Brake System J/B Junction Block
A/C Air Conditioner LED Light Emitting Diode
ALR Automatic Locking Retractor LH Left±Hand
APPROX. Approximation LHD Left±Hand Drive
A/T, ATM Automatic Transmission LLC Long Life Coolant (Year Around Coolant)
ATF Automatic Transmission Fluid Max. Maximum
BTDC Before Top Dead Center Min. Minimum
BVSV Bimetal Vacuum Switching Valve MP Multipurpose
CALIF. Vehicles Sold in California N Neutral
CB Circuit Breaker OD, O/D Overdrive
DOHC Double Over Head Cam O/S Oversize
DP Dash Pot P & BV Proportioning and Bypass Valve
ECT Electronic Controlled Transmission PCV Positive Crankcase Ventilation
ECU Electronic Controlled Unit PS Power Steering
EFI Electronic Fuel Injection RH Right±Hand
EGR Exhaust Gas Recirculation RL Rear Left
ELR Emergency Locking Retractor RR Rear Right
EPR Evaporator Pressure Regulator RR, Rr Rear
ESA Electronic Spark Advance SRS Supplemental Restraint System
EVAP Evaporative (Emission Control) SSM Special Service Materials
EX Exhaust (manifold, valve) SST Special Service Tools
Ex. Except STD Standard
FIPG Formed in Place Gasket SW Switch
FL Front Left TDC Top Dead Center
FL Fusible Link TDCL Total Diagnostic Communication Link
FPU Fuel Pressure Up Temp. Temperature
FR Front Right TRAC Traction Control System
FR, Fr Front TWC Three±Way Catalyst
HAC High Altitude Compensation VSV Vacuum Switching Valve
IG Ignition VTV Vacuum Transmitting Valve
IN Intake (manifold, valve) w/ With
INT Intermittent w/o Without
ISC Idle Speed Control
IN±34±
INTRODUCTION Abbreviations Used in This Manual
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IDENTIFICATION INFORMATION
VEHICLE IDENTIFICATION NUMBER
This number is stamped on the vehicle identification number plate
at the top of the left instrument panel and is also on the certification
regulation label.A: Vehicle Identification Number
B: Vehicle Identification Number Plate
C: Certification Regulation Label
ENGINE SERIAL NUMBER
The engine serial number is stamped on the engine block as
shown.
GENERAL REPAIR INSTRUCTIONS
1. Use fender seat and floor covers to keep the vehicle clean
and prevent damage.
2. During disassembly, keep parts in the appropriate order to facilitate reassembly.
3. Observe the following:
(a) CAUTION: W ork must be started after approx. 20 seconds
or longer from the time the ignition switch is turned to the
ºLOCKº position and the negative (±) terminal cable is
disconnected from the battery (See page AB±2).
(b) B e fo re pe rfo rmin g ele ctrica l wo rk, disc onnect the
negative cable from the battery terminal.
(c) If it is necessary to disconnect the battery for inspection or repair, always disconnect the cable from the negative
(±) terminal which is grounded to the vehicle body.
(d) To prevent damage to the battery terminal post, loosen the terminal nut and raise the cable straight up without
twisting or prying it.
(e) Clean the battery terminal posts and cable terminals with a shop rag. Do not scrape them with a file or other
abrasive object.
(f) Install the cable terminal to the battery post with the nut loose, and tighten the nut after installation. Do not use
a hammer to tap the terminal onto the post.
(g) Be sure the covers for the positive (+) terminal and negative (±) terminal are properly in place.
4. Check hose and wiring connectors to make sure that they are
secure and correct.
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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.
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INTRODUCTION General Repair Instructions
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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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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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[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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Taking into account the above points, a flow chart showing how to proceed with\
troubleshooting using the diag-
nostic code check is shown below. This flow chart shows how to utilize the diagnostic code check effectively,
then by carefully checking the results, indicates how to proceed either to d\
iagnostic code troubleshooting or to
troubleshooting of problem symptoms.
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[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
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INTRODUCTION How to Troubleshoot ECU Controlled Systems
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