1991 LEXUS SC400 turn signal

[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
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Blower Fan Speed Control System
(When AUTO Switch is ON)
(a) The desired temperature is set using the TEMP switch.
(b) From the input signals (room temperature sensor, ambient temperature sensor and solar sensor) and thetemperature setting, the air conditioner control ECU determines the air flow volume\
and outputs signals
to the power transistor.
(c) When it receives signals from the ECU, the power transistor increases or red\
uces the blower motor speed to control the air flow volume.
(When AUTO Switch is OFF)
The ECU turns the power transistor increasing or reducing the blower mot\
or speed fixing the air flow volume
in accordance with the position of the Manual switch. AC±10
±
AIR CONDITIONING SYSTEM Description
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Diag. Code 23Compressor Lock Sensor Circuit
CIRCUIT DESCRIPTION
The pressure switch sends the appropriate signals to the air conditioner co\
ntrol assembly when the air
conditioner refrigerant pressure drops too low or rises too high. When the air cond\
itioner control assem-
bly receives these signals, it outputs signals via the Engine & ECT ECU to switch off the compressor
relay and turns the magnet clutch off.
Code No.Diag. Code Detecting ConditionTrouble area
23

Open in pressure sensor circuit.

Abnormal refrigerant pressure.
below 206 kPa (2.1 kgf/cm
2, 30 psi)
over 2,650 kPa (27 kgf/cm2, 385 psi)

Pressure switch.

Harness or connector between pressure switch
and ECU. Refrigerant pipe line.

ECU.
DIAGNOSTIC CHART
Check voltage between terminals LP of air
conditioner control assembly and body
ground.
Check pressure switch.
Check harness and connector between air
conditioner control assembly and pressure
switch and body ground
(See page IN-27).
Check and replace air conditioner control
assembly.
Proceed to next circuit inspection
shown on matrix chart (See page
AC-36 ).
Replace pressure switch
Repair or replace harness or
connector.
AC±56±
AIR CONDITIONING SYSTEM Troubleshooting
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Compressor Circuit
CIRCUIT DESCRIPTION
The air conditioner control assembly outputs the magnetic clutch ON signal \
from terminal MGC to the
Engine & ECT ECU.
When the Engine & ECT ECU receives this signal, it sends a signal from terminal A\
CMG and switches
the air conditioner magnetic clutch relay on, thus turning on the air condi\
tioner compressor magnetic
clutch.
The air conditioner control assembly also monitors at terminal A/C IN wheth\
er or not power is being sup-
plied to the magnetic clutch.
DIAGNOSTIC CHART
See next page for the Diagnostic Chart.
WIRING DIAGRAM
AC±84±
AIR CONDITIONING SYSTEM Troubleshooting
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OPERATION
FUNCTION OF COMPONENTS
1. FRONT AIRBAG SENSORA front airbag sensor is mounted inside each of the front
fenders. The sensor unit is a mechanical type. When the sen-
sor detects deceleration force above a predetermined limit in
a collision, the contacts in the sensor make contact, sending
a signal to the center airbag sensor assembly. The sensor
cannot be disassembled.
2. SPIRAL CABLE (in COMBINATION SWITCH) A spiral cable is used as an electrical joint from the vehicle
body side to the steering wheel.
3. STEERING WHEEL PAD (with AIRBAG) The inflater and bag of the airbag system are stored in the
steering wheel pad and cannot be disassembled.
The inflater contains a squib, igniter charge, gas generant,
etc., and inflates the bag in case of a frontal collision.
4. AIRBAG WARNING LIGHT The airbag warning light is located on the combination meter.
It goes on to alert the driver of trouble in the system when a
malfunction is detected in the center airbag sensor assembly
self±diagnosis. In normal operating condition when the igni-
tion switch is turned to the ACC or ON position, the light goes
on for about 6 seconds and then goes off.
5. CENTER AIRBAG SENSOR ASSEMBLY The center airbag sensor assembly is mounted on the floor
inside the center cluster. The center airbag sensor assembly
consists of a center airbag sensor, safing sensors, ignition
control and drive circuit, diagnosis circuit, etc. It receives sig-
nals f rom the airbag sensors, judges whether the airbag must
be activated or not and diagnoses system malfunctions. AB±5
± OperationSRS AIRBAG
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CHECK TERMINAL TT OUTPUT
VOLTAGE
AT00R±01
When a voltmeter is connected to the DLC1, the following
items can be checked.
1. Throttle position sensor signal
2. Brake signal
3. Shift position signal
1. VOLTMETER CONNECTION Connect the positive (+) probe of the voltmeter to terminal T
T
and the negative (±) probe to terminal E1 of the DLC2.
HINT: If a voltmeter with small internal resistance is used, the
correct voltage will not be indicated, so use a voltmeter with
an internal resistance of at least 10 k
.
2. TURN IGNITION SWITCH TO ON (DO NOT START THE
ENGINE)
3. CHECK THROTTLE POSITION SENSOR SIGNAL Check if the voltage changes from approximately 0 V to
approximately 8 V when the accelerator pedal is gradually
depressed from the fully closed position.
4. CHECK BRAKE SIGNAL (LOCK±UP CUT SIGNAL)
(a) Open the throttle valve fully to apply approximately 8 V to terminal T
T.
(b) In this condition, check terminal T
T voltage when the brake
pedal is depressed and released.
TT terminal voltage:
0 V (When brake pedal is depressed)
8 V (When brake pedal is released)
5. START ENGINE
6. CHECK SHIFT POSITION SIGNAL (VEHICLE SPEED ABOVE 60 KM/H OR 37 MPH)
Check up±shifting together with terminal T
T voltage.
HINT: Check for light shocks from up±shifting and for
changes in the tachometer.
 Gear Position Terminal TT output voltage 1st GearBelow 0.5 V 

2nd Gear1.5 ~
2 6 V
 3rd Gear3.5 ~ 4.6 V
 3rd Lock±up4.5 ~ 5.9 V
 O/D5.5 ~ 6.9 V
 O/D Lock±up6.5 ~ 7.9 V
If terminal TT output voltage check cannot be performed, per-
form the check of terminal T
T circuit on page AT±120.
±
A304E (1UZ±FE) AUTOMATIC TRANSMISSION TROUBLESHOOTINGAT±39
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OKNG
OKNG
OKNG
INSPECTION PROCEDURE
1Main throttle position signal check.
C
OK
Hint
PTurn ignition switch ON (Do not start the engine).
Check voltage at the terminal T
T of the DLC2 while grad-
ually depressing the accelerator pedal from the fully
closed position to the fully opened position.
Voltage changes from 0 V to 8 V by stages.
Do not depress the brake pedal during this test. The
voltage will stay at 0 V if depressed.
Proceed to next circuit inspection shown on matrix chart
(See page AT±66).
2Check throttle position sensor.
See engine troubleshooting section in EG section.
Replace throttle position sensor.
3Check harness and connector between ECM and throttle position sensor (S\
ee page IN±33).
Repair or replace harness or connector.
Check and replace ECM.
±
A304E (1UZ±FE) AUTOMATIC TRANSMISSION TROUBLESHOOTINGAT±103
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AT±66
IN±33
WIRING DIAGRAM
Kick±down Switch Circuit
CIRCUIT DESCRIPTION
The kick±down switch is turned on when the accelerator ped-
al is depressed beyond the full throttle opening and sends sig-
nals to the ECM.
When the kick±down switch is turned on, the ECM controls
gear shifting according to the programmed shift diagrams.
If a short circuit develops in the kick±down switch, the ECM
disregards the kick±down signals and controls shifting at the
normal shift points.
DIAGNOSTIC CHARTDIAGNOSTIC CHART
AT±104±
A304E (1UZ±FE) AUTOMATIC TRANSMISSION TROUBLESHOOTING
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