ect LEXUS SC300 1991 Service Repair Manual

GENERAL DESCRIPTION
The LEXUS SC400/300 is equipped with an SRS (Supplemental Restraint System\
) such as the driver
airbag and front passenger airbag. Failure to carry out service operations in \
the correct sequence
could cause the SRS to unexpectedly deployed during servicing, possibly leadin\
g to a serious acci-
dent. Further, if a mistake is made in servicing the supplemental restraint system, i\
t is possible the
SRS may fail to operate when required. Before performing servicing (inc\
luding removal or installa-
tion of parts, inspection or replacement), be sure to read the followin\
g items carefully, then follow
the correct procedure described in the repair manual.
1. Malfunction symptoms of the supplemental restraint system are difficult to confirm, so the diagnostic trouble codes become the most important source of information when troubleshooting. When
troubleshooting the supplemental restraint system, always inspect the diagnostic trouble codes before
disconnecting the battery (See page RS±59).
2. Work must be started after 90 seconds from the time the ignition switch turn\
ed to the LOCK position
and the negative (±) terminal cable is disconnected from the battery. (The supplemental restraint
system is equipped with a back±up power source so that if work is sta\
rted within 90 seconds of
disconnecting the negative (±) terminal cable of the battery, the SRS may be deployed.)
When the negative (±) terminal cable is disconnected from the battery, memory of the clock and audio sys-
tems will be canceled. So before starting work, make a record of the contents memo\
rized by the audio
memory system. When work is finished, reset the audio systems as before and ad\
just the clock. When the
vehicle has power tilt and power telescopic steering, 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 work is finished, it will be necessary\
to explain this fact to the custom-
er, 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 SRS does not deploy, the steering wheel pad, front passenger
airbag assembly, front airbag sensors and center airbag sensor assembly should be inspe\
cted (See pages
RS±16 , 28 , 41 and 44).
4. Never use SRS parts from another vehicle. When replacing parts, replace \
them with new parts.
5. Before repairs, remove the airbag sensor if shocks are likely to be applied to the sensors during r\
epairs.
6. Never disassemble and repair the steering wheel pad, front passenger airbag a\
ssembly, front airbag sensors or center airbag sensor assembly in order to reuse it.
7. If the steering wheel pad, front passenger airbag assembly, front airbag sensors or center airbag sensor assembly have been dropped, or if there are cracks, dents or dents or ot\
her defects in the case, bracket
or connector, replace them with new ones.
8. Do not expose the steering wheel pad, front passenger airbag assembly, front airbag sensors and center airbag sensor assembly directly to hot air or flames.
9. Use a volt/ohmmeter with high impedance (10 k
/V minimum) of troubleshooting of the system's electrical
circuit.
10. Information labels are attached to the periphery of the SRS components. \
Follow the instructions on the
notices.
11. After work on the supplemental restraint system is completed, perform the SRS\
warning light check (See page RS±59 ).
12. If the vehicle is equipped with a mobile communication system, refer to \
the precaution in the IN section. RS±2
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SUPPLEMENTAL RESTRAINT SYSTEM GENERAL DESCRIPTION
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STEERING WHEEL PAD (with AIRBAG)
RS00Q±07
1. When removing the steering wheel pad or handling a new steering wheel pad, it should be placedwith 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 ac\
cident if the airbag inflates for
some reason.
2. Never measure the resistance of the airbag squib. (This may cause the airbag\
to deploy, which isvery dangerous.)
3. Grease should not be applied to the steering wheel pad and the pad should not \
be cleaned withdetergents of any kind.
4. Store the steering wheel pad where the ambient temperature below 93
C (200
F), without high
humidity and away from electrical noise.
5. When using electric welding, first disconnect the SRS connector (yellow\
color and 2 pins) under the steering column near the combination switch connector before startin\
g work.
6. When disposing of a vehicle or the steering wheel pad alone, the airbag shou\
ld be deployed using
an SST before disposal (See page RS±21).
Perform the operation in a place away from electrical noise.
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SUPPLEMENTAL RESTRAINT SYSTEM GENERAL DESCRIPTIONRS±3
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SPIRAL CABLE (in COMBINATION SWITCH)
RS00P±06
The steering wheel must be fitted correctly to the steering column with the\
spiral cable at the neutral posi-
tion, otherwise cable disconnection and other troubles may result.
Refer to page RS±19 of this manual concerning correct steering wheel installation.
FRONT PASSENGER AIRBAG ASSEMBLY
RS00N±07
1. Always store a removed or new front passenger airbag assembly with the a\
irbag door facing up.
Store the airbag assembly with the airbag door facing down could cause a seri\
ous accident if the
airbag inflates.
2. Never measure the resistance of the airbag squib. (This may cause the airbag\
to deploy, which isvery dangerous.)
3. Grease should not be applied to the front passenger airbag assembly and \
the door should not be
cleaned with detergents of any kind.
4. Store the front passenger airbag assembly where the ambient temperature remai\
ns below 93
C
(200
F), without high humidity and away from electrical noise.
5. When using electric welding, first disconnect the SRS connector (yellow col\
or and 2 pins) installed
on the glove compartment door finish plate at upper side of the glove co\
mpartment assembly
before starting work.
6. When disposing of a vehicle or the front passenger airbag assembly alone\
, the airbag should be deployed using an SST before disposal (See page RS±34). Perform the operation in a place away
from electrical noise.
RS±4
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SUPPLEMENTAL RESTRAINT SYSTEM GENERAL DESCRIPTION
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FRONT AIRBAG SENSOR
RS00N±07
1. Never reuse the front airbag sensor involved in a collision when the airbag h\
as deployed. (Replace boththe left and right airbag sensors.)
2. Install the front airbag sensor with the arrow on the sensor facing towa\
rd 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 sensors is equipped with an electrical connection check me\
chanism. 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 RS±12).
CENTER AIRBAG SENSOR ASSEMBLY
RSOOR±0A
1. Never reuse the center airbag sensor assembly involved in a collision when the airbag has
deployed.
2. Work must be started after 90 seconds from the time the ignition switch is t\
urned to LOCK position and the negative (±) terminal cable is disconnected from the batter\
y even just loosing the set bolts
of center airbag sensor assembly.
WIRE HARNESS AND CONNECTOR
RSO41±01
The SRS wire harness is integrated with the cowl wire harness assembly. The wires for the SRS wire har-
ness are encased in a yellow corrugated tube. All the connectors for the sys\
tem are also a standard yellow
color. If the SRS wire harness becomes disconnected or the connector becomes broken du\
e to an accident
etc., repair or replace it as shown on page RS±48.
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SUPPLEMENTAL RESTRAINT SYSTEM GENERAL DESCRIPTIONRS±5
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DESCRIPTION
RS00S±02
The SRS (Supplemental Restraint System), together with the seat belt, is designed to help protect the driv-
er. Frontal inspect, the airbag sensor detect the shock, and if the front±\
to±rear shock is greater than a spe-
cified value, an airbag stored in the steering wheel pad is inflated ins\
tantaneously to help reduce the shock
to the driver.
LOCATION OF COMPONENTS
RS00T±02
RS±6±
SUPPLEMENTAL RESTRAINT SYSTEM DESCRIPTION
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CENTER AIRBAG SENSOR ASSEMBLY CONNECTOR
RS00V±0B
 No. Symbol Terminal Name A ± Electrical Connection Check Mechanism B ± Electrical Connection Check Mechanism 1 P± Squib (Passenger) 2 P+ Squib
(Passenger) 3 D+ Squib
(Driver) 4 D± Squib  (Driver) 5 E1 Ground 6 E2 Ground 7 Tc Diagnosis 8 +SR Front Airbag Sensor RH
 9 ±SR Front Airbag Sensor RH  10 ±SL Front Airbag Sensor LH  11 +SL Front Airbag Sensor LH
 12 LA SRS Warning Light 13 IG2 Power Source (IGN Fuse) 14 ACC Power Source (CIG Fuse)
RS±8±
SUPPLEMENTAL RESTRAINT SYSTEM DESCRIPTION
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OPERATION
FUNCTION OF COMPONENTS
RS02M±04
1. STEERING WHEEL PAD (with AIRBAG)The inflater and bag of the supplemental restraint system are
stored in the steering wheel pad and cannot be disas-
sembled. The inflater contains a squib, igniter charge, gas
generant, etc., and inflates the bag in case of frontal collision.
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. FRONT PASSENGER AIRBAG ASSEMBLY The inflater and bag of the supplemental restraint system are
stored in the front passenger airbag assembly 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. SRS WARNING LIGHT The SRS 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.
In normal operating conditions when the ignition switch is
turned to the ACC or ON position, the light goes on for about
6 seconds and then goes off.
5. FRONT AIRBAG SENSOR A front airbag sensor is mounted inside each of the front fend-
ers. The sensor unit is a mechanical type. When the sensor
detects deceleration force above a predetermined limit, con-
tacts is m ade in the sensor, sending a signal to the center air-
bag sensor assembly. The sensor cannot be disassembled.
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SUPPLEMENTAL RESTRAINT SYSTEM OPERATIONRS±9
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6. CENTER AIRBAG SENSOR ASSEMBLYThe 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 sensor, diagnosis
circuit and ignition control, drive circuit, etc.
It receives signals from the airbag sensors, judges whether
the SRS must be activated or not.
7. SRS CONNECTORS
 No. Item Application
 (1) Terminal Twin±Lock Mechanism Connectors
,  ,  ,  ,  ,  , 
 (2) Airbag Activation Prevention Mechanism Connectors
,  ,  , 
 (3) Electrical Connection Check Mechanism Connectors
,  ,  (4) Connector Twin±Lock Mechanism Connectors  ,  , 
All connectors in the supplemental restraint system are col-
ored yellow to distinguish them from other connectors. Con-
nectors having special functions and specifically designed
for SRS are used in the locations shown below to ensure high
reliability. These connectors use durable gold±plated termi-
nals.
RS±10
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(1) Terminal Twin±Lock MechanismEach connector has a two±piece construction consisting of
a housing and a spacer. This design secures the locking of
the terminal by two locking devices (the spacer and the lance)
to prevent terminals from coming out.
(2) Airbag Activation Prevention Mechanism Each connector contains a short spring plate. When the con-
nector is disconnected, the short spring plate automatically
connects the power source and grounding terminals of the
squib.
HINT: The illustration shows connectors ,  and  . Con-
nector

has short spring plates on the female terminal side.
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SUPPLEMENTAL RESTRAINT SYSTEM OPERATIONRS±11
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(3) Electrical Connection Check MechanismThis mechanism is designed to electrically check if connec-
tors are connected correctly and completely. The electrical
connection check mechanism is designed so that the con-
nection detection pin connects with the diagnosis terminals
when the connector housing lock is in the locked condition.
HINT: The illustration shows connectors  and  Connector,
also has the same operating principle.
RS±12
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SUPPLEMENTAL RESTRAINT SYSTEM OPERATION
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