Toyota Previa 1991-1997 Repair Information

General Information

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See Figure 1

The air bag system used on Previa vans is referred to as the Supplemental Restraint System (SRS). The SRS provides additional protection for the driver (and front passenger on later models) if a forward collision of sufficient force is encountered. The SRS assists the normal seatbelt restraining system by deploying an air bag, via the steering column.



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Fig. Fig. 1: Supplemental Restraint System (SRS) component locations

The center air bag sensor is the heart of the SRS. It consists of safing sensors, ignition control and drive circuit, diagnosis circuit, etc. The center air bag receives signals from the air bag sensors and determines whether the air bag must be activated or not. The center air bag sensor is also used to diagnose system malfunctions.

The air bag warning light circuit is equipped with an electrical connection check mechanism which detects when the connector to the center air bag sensor assembly is not properly connected.

All connectors in the air bag system are colored yellow. These connectors use gold-plated terminals with twin-lock mechanism. This design assures positive locking; there-by, preventing the terminals from coming apart.

SYSTEM OPERATION



When the ignition switch is turn to the ON or ACC position, the air bag warning lamp will turned on for approximately 6 seconds. If no malfunctions are detected in the system, after the 6 second period have elapse, the warning light will go OFF .

The safing sensors are designed to trip at a lower deceleration rate than the front or center air bag sensor. When the vehicle is involved in a frontal collision, the shock is great enough to overcome the predetermine level of the front or center air bag sensor. When a safing sensor and a front air bag sensor and/or the center air bag sensor trip simultaneously, it causes the squib of the air bag to ignite and the air bag is deployed automatically. The inflated bag breaks open the steering wheel pad.

After air bag deployment have occurred, the gas is discharged through the discharge holes provided behind the bag. The bag become deflated as a result.

The connector of the air bag contains a short spring plate, which provides an activation prevention mechanism. When the connector is disconnected, the short spring plate automatically connects the power source and grounding terminals of the inflator module (squib).

SYSTEM COMPONENTS



Front Air bag Sensors

See Figure 2

A front air bag sensor is mounted inside each of the front fenders. The sensor unit is basically a mechanical switch. When the sensor detects a deceleration force above a predetermined level in a collision, the contacts in the sensor close, sending a signal to the center air bag sensor assembly. The sensor cannot be disassembled. If the front fenders on the vehicle are damaged in any way, do a visual check of the sensors even if the air bag is not deployed. Inspect for:



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Fig. Fig. 2: The air bag sensors are all labeled with CAUTION tags



Bracket deformation
 
Peeling of paint
 
Cracks, dents or chips in the case
 
Cracks, chipping or dents in the wiring connector
 
Peeling of the label or damage to the series number
 

Center Air Bag Sensor

See Figure 3

The center air bag sensor is mounted on the floor inside the console box. The air bag sensor determines whether or not the air bag should be deployed and is also used to diagnose system malfunction.



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Fig. Fig. 3: The center air bag sensor is mounted on the floor inside the console box

Spiral Cable

The spiral cable, part of the combination switch, is used as an electrical joint from the vehicle body to the steering wheel. The spiral cable is referred to as a clock spring.

Driver's Air Bag

See Figure 4

The driver's air bag, located in the steering wheel pad, contains a gas generant which will rapidly inflate the bag in a case of frontal collision.



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Fig. Fig. 4: The driver's air bag is stored in the steering wheel pad

Passenger's Air Bag

See Figure 5

The inflator and bag of the SRS are located in the front of the passenger's air bag assembly and can not be disassembled. The air bag will inflate only when the sensor instructs it to do so.



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Fig. Fig. 5: The passenger air bag is located in the top of the dash

SRS Warning Lamp

See Figure 6

The air bag SRS warning lamp, located on the combination meter, is used to alert the driver of any malfunctions within the air bag system. In normal operating conditions when the ignition switch is turned to the ON or ACC , the light goes on for about 6 seconds and then goes off.



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Fig. Fig. 6: The air bag warning lamp is located on the combination meter

SRS Connectors

See Figures 7 and 8

All connectors in the SRS are colored yellow to distinguish them from the other connectors. These connectors have special functions are specifically designed for the SRS. These connectors use durable gold-plated terminals.



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Fig. Fig. 7: Spring plate type SRS connectors



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Fig. Fig. 8: Twin locking type SRS connector

SERVICE PRECAUTIONS



  1. Work must be started after 90 seconds from the time the ignition switch is turned to the LOCK position and the negative battery cable has been disconnected. The SRS is equipped with a back-up power source so that if work is started within 90 seconds of disconnecting the negative battery cable, the SRS may deploy. When the negative terminal cable is disconnected from the battery, memory of the clock and radio will be canceled. Before you start working, make a note of the contents memorized by the audio memory system. When you have finished working, reset the audio systems and adjust the clock. Never use a back-up power supply from outside the vehicle.
  2.  
  3. In the event that of a minor frontal collision where the air bag does not deploy, the steering wheel pad, front air bag sensors and center air bag sensor assembly should be inspected.
  4.  
  5. Before repairs, remove the air bag sensors if shocks are likely to be applied to the sensors during repairs.
  6.  
  7. Never disassemble and repair the steering wheel pad, front air bag sensors or center air bag sensors.
  8.  
  9. Do not expose the steering wheel pad, front air bag sensors or center air bag sensor assembly directly to flames or hot air.
  10.  
  11. If the steering wheel pad, front air bag sensors or center air bag sensor assembly have been dropped, or there are cracks, dents or other defects in the case, bracket or connectors, have them replaced with new ones.
  12.  
  13. Information labels are attached to the periphery of the SRS components. Follow the instructions of the notices.
  14.  
  15. After arming the system, check for proper operation of the SRS warning light.
  16.  
  17. If the wiring harness in the SRS system is damaged, have the entire harness assembly replaced.
  18.  

DISARMING THE SYSTEM



Work must be started only after 90 seconds from the time the ignition switch is turned to the LOCK position and the negative battery cable has been disconnected. The SRS is equipped with a back-up power source so that if work is started within 90 seconds of disconnecting the negative battery cable, the SRS may deploy. When the negative terminal cable is disconnected from the battery, memory of the clock and radio will be canceled. Before you start working, make a note of the contents memorized by the audio memory system. When you have finished work, reset the audio systems as before and adjust the clock. To avoid erasing the memory of each system, never use a back-up power supply from outside the vehicle.

ARMING THE SYSTEM



Reconnect the negative battery cable and perform the airbag warning light check by turning to the ON or ACC position, the air bag warning lamp will turned ON for approximately 6 seconds. If no malfunctions are detected in the system, after the 6 second period have elapse, the warning light will go OFF .

 
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