ignition LEXUS SC300 1991 Service Repair Manual

WIRING DIAGRAM
SRS Warning Light System (Always Lit Up, when ignition switch in
LOCK position)
CIRCUIT DESCRIPTION
The SRS warning light is located on the combination meter.
When the supplemental restraint system is normal, the SRS warning light ligh\
ts up for approx. 6 seconds after
the ignition switch is turned from LOCK position to ACC or ON position, and\
then turns off automatically.
If there is a malfunction in the supplemental restraint system, the SRS wa\
rning light lights up to inform the driver
of the abnormality.
When terminals Tc and E
1 of the DLC2 are connected, the diagnostic trouble code is displayed by \
the blinking
of the SRS warning light.
DIAGNOSTIC CHARTDIAGNOSTIC CHART
HINT: If the SRS warning light is always lit up by a DTC check procedure, per\
form Tc T erminal circuit check proce-
dure (See page RS±121).
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SUPPLEMENTAL RESTRAINT SYSTEM TROUBLESHOOTINGRS±119
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YESNO
INSPECTION PROCEDURE
1Does SRS warning light turn off?
C
Hint
P1. Turn ignition switch LOCK.
2. Disconnect negative (±) terminal cable from battery
and wait at least 90 seconds.
3. Remove steering wheel pad (See page RS±19).
4. Disconnect connectors of front passenger airbag assembly (See page RS±32).
5. Disconnect center airbag sensor assembly con- nector.
6. Connect negative (±) terminal cable to battery.
Check operation of SRS warning light.
Ignition Switch is LOCK position.
Check SRS warning light circuit or terminal AB circuit of
DCL1 or DLC2.
Replace center airbag sensor assembly.
RS±120±
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NGOK
NGOK
INSPECTION PROCEDURE
HINT: If the diagnostic trouble code is not displayed, perform the following \
troubleshooting:
1Check voltage between terminals Tc and E1 of DLC1 or DLC2.
C
OK
PTurn ignition switch ACC or ON.
Measure voltage between terminals Tc and E
1 of DLC1
or DLC2.
Voltage: Battery positive voltage
Go to step
.
2Check voltage between terminal Tc of DLC1 or DLC2 and body ground.
C
OK
Measure voltage between terminal Tc of DLC1 or DLC2
and body ground.
Voltage: Battery positive voltage
Check harness between terminal E1 of DLC1 or DLC2
and body ground.
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SUPPLEMENTAL RESTRAINT SYSTEM TROUBLESHOOTINGRS±123
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NGOK
3Check center airbag sensor assembly.
C
OK
P1. Turn ignition switch LOCK.
2. Disconnect battery negative (±) terminal cable, andwait at least 90 seconds.
3. Remove steering wheel pad (See page RS±19).
4. Disconnect connectors of front passenger airbag assembly (See page RS±32).
Check operation of SRS warning light.
SRS warning light comes on.
Check harness between center airbag sensor assembly
and DLC1 or DLC2.
Replace center airbag sensor assembly.
RS±124±
SUPPLEMENTAL RESTRAINT SYSTEM TROUBLESHOOTING
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NGOK
HINT: If the diagnostic trouble code is continuously displayed, perform the f\
ollowing troubleshooting.
1Check resistance between terminal Tc of center airbag sensor assembly and body ground.
C
OK
P1. Turn ignition switch LOCK.
2. Disconnect center airbag sensor assembly con-nector.
3. Disconnect battery negative (±) terminal cable, and wait at least 90 seconds.
4. Remove steering wheel pad (See page RS±19).
5. Disconnect connectors of front passenger airbag assembly (See page RS±32).
Check resistance between terminal Tc of center airbag
sensor assembly and body ground.
Resistance: 1 M
or higher
Replace center airbag sensor assembly.
Repair or replace harness or connector.
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SUPPLEMENTAL RESTRAINT SYSTEM TROUBLESHOOTINGRS±125
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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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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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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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When the vehicle is involved in a frontal collision in the
hatched area (Fig. 1) and the shock is larger than a predeter-
mined level, the SRS is activated automatically. A safing sen-
sor is designed to go on at a smaller deceleration rate than
the front and center airbag sensors. As illustrated in Fig. 2,
ignition is caused when current flows to the squib, which hap-
pens when a safing sensor and a front airbag sensor and/or
the center airbag sensor go on simultaneously. When a de-
celeration force acts on the sensors, two squibs in the driver
airbag and front passenger airbag ignite and generate gas.
The gas discharging into the driver and front passenger air-
bags rapidly increases the pressure inside the bags breaking
open the steering wheel pad and instrument panel door.
Bag inflation then ends, and the bags deflate as the gas is
discharged through discharge holes at the bag's rear or side.
RS±14±
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