Research Bulletin 1994-6
Research Bulletin 1994-6 |
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Over the past twenty years, the advent and implementation of equipment required by the federally mandated safety standards has necessitated changes in passenger car design and development. The need to supplement the existing restraint system during frontal collisions has led to the development and increasing wide spread use of inflatable restraint systems as standard equipment on many vehicle makes and models. These passive, inflatable restraint systems are commonly referred to as "Air Bags." They may be a single or dual air bag system and are designed as supplemental protection for the driver or both front seat positions, respectively. Air bags are a supplement to seat belts, to provide additional protection, therefore seat belts MUST be worn at all times in an air bag equipped vehicle. The reason for seat belt utilization in conjunction with air bags is; the seat belt holds the person in position, and the air bag will not activate (or deploy) during a side, rear end or roll-over impact accident.
Both domestic and foreign auto manufacturers offer air bag systems as standard and optional equipment. These systems have common components because of inherent vehicle characteristics and system function, but component design varies between manufacturers. The goal of this document is to provide general information on air bag systems and identify the components that must be replaced after an air bag system deploys. Also included is the "Passive Restraint System Component Replacement Matrix", which contains the automobile manufacturer's current recommended component replacement guidelines for their passive restraint systems.
This bulletin is not intended to be all encompassing concerning air bag deployment, diagnosis, and repair. The information contained in this document is based on data provided by the various named manufacturers. Tech-Cor has not conducted independent testing of the procedures discussed, nor does it necessarily endorse or attest to the accuracy of the information provided within. Refer to the appropriate manufacturer's repair manual for specific model servicing information.
A typical air bag system is composed of sensors, a diagnostic/control module, a clock spring, and an air bag module. Upon impact of sufficient force the sensors close. For the air bag to deploy at least two sensors must close simultaneously. One sensor is normally located in the engine compartment with the other located in the passenger compartment. The closing of the passenger compartment sensor, in addition to the engine compartment sensor, ensures that the particular collision is severe enough to require air bag deployment, the system is activated.
New air bag systems are being developed which contain a single impact sensor mechanism and the electronic control module all in one unit. There are no engine compartment sensors, so the system is activated from the single electronic control unit. A very limited number of vehicle models currently use a single sensor system, but we anticipate that the usage of this type of system may increase on future vehicle models.
Once the system activates, current is applied to the igniter which starts a chemical chain reaction. Heat given off by this reaction causes the generant to produce nitrogen gas which inflates the air bag. The inflated air bag cushions the occupant's head and chest protecting them from injuries which may have occurred if allowed to contact the steering wheel, instrument panel or even the windshield. Upon full inflation, the bag will begin to deflate by venting the gas through holes on the back of the bag assisted by the occupants forward motion. Most systems are designed to deploy at a threshold of 10-15 mph when impacting a fixed object.
Air Bag Module - Composed of the air bag, inflator assembly, and other supporting components, It inflates to protect the occupant once current is applied to its igniter. After deployment this component cannot be serviced and must be replaced.
Inflator Assembly - Integral part of the air bag module which contains the igniter and generant.
Igniter - The component that converts the electrical current into a thermal reaction to ignite the generant.
Generant - Chemical pellets, normally Sodium Azide, that produce nitrogen gas which inflates the air bag.
Clock Spring (a.k.a. Coil Assembly, Cable Reel Assembly, Coil Spring Unit, Contact Reel) - Unit located between the column and the steering wheel that allows steering wheel rotation while maintaining electrical contact between the air bag module and the system.
Diagnostic / Control Module - The unit which continuously monitors the readiness of the air bag electrical system. If a problem arises an indicator light on the instrument panel will light and stay on, service should be performed as soon as possible. The module is frequently located in the passenger compartment under the instrument panel or center console.
Impact Sensor - A switch calibrated to make electrical contact when a frontal collision occurs where the angle of impact is within 30 degrees right or left of vehicle centerline and the speed of impact is at least 10-15 mph barrier equivalent velocity (The speed of the vehicle hitting a fixed immovable barrier). See Figure 1.
A typical sensor is composed of a gold-plated ball held in place by a magnet at the end of a close fitted hollow tube. See Figure 2.
At the point of sufficient force described above, the deceleration of the car produces an acceleration of the ball forward. The acceleration allows the ball to travel and make contact with the other electrical contacts to complete the circuit. After impact the ball will return to its proper position. The importance of the ball returning or the sensor resetting is that most manufacturers allow their sensors to be reused, if not physically damaged by the collision.
Knee Bolster - A pad which keeps the occupant from submarining or sliding under the air bag.
Wiring Harness - The wires and connectors that electronically connect the air bag system components.
Pig Tail - The wire between the component and its connector.
Determining Which Air Bag Components Must Be Replaced
Disabling the Air Bag System
Before inspecting a non-driveable vehicle with an air bag either deployed or not, make sure to disable the system. This is done by removing the negative battery cable. Taping over the negative lead will prevent further possible contact with the negative battery terminal. Some manufacturers also recommend removal of the air bag fuse, and disconnection of the inflator module at the inflator or base of the steering column. Once the system is disabled let the vehicle stand 5 minutes before proceeding. This is done to prevent accidental deployment of the air bag as many systems contain a back-up power source which can deploy the air bag for a short time after the battery is disconnected or discharged.
Component replacement will vary depending on accident severity and the vehicle make, model & year. Attached, as stated earlier, is the Passive Restraint System Component Replacement Matrix which contains specific make and model information from each vehicle manufacturer. This matrix was developed to aid in estimating system damage. Minimum component replacement requirements for a deployed air bag system are listed, as specified by each vehicle manufacturer. The air bag module will always be listed. Aside from the required replacement components, the remaining components in the system should be inspected for damage and proper operation.
Note: All air bag system components should still be inspected on a collision damaged vehicle, even if the air bag(s) did NOT deploy.
To inspect components they must first be located. The first place to start is the vehicle's owners manual. Some manufacturers have included sections on air bag system operation which include general component location. Some components can easily be identified by the color of their pig tail and connectors, usually yellow or orange. The service manual or a dealer service department could also provide assistance.
Physical damage is apparent: dents, cracks, deformation. Its pig tail or connector is damaged: burnt, cut, cracked. Diagnostics identifies a component faulty.
Make sure air bag system is disabled.
Return all structure to proper dimensions. This includes all air bag component mounting areas. If components are not replaced in their factory position the system may not deploy as designed.
Use caution when welding or working around system components. Heat can damage components, remove them if necessary.
Do NOT repair air bag sensor mounting brackets, replace with a new part only.
Read and follow all safety precautions written on the replacement component(s) and contained in the service manual.
Never attempt to repair the inflator module, sensors, diagnostic/control module, or clock spring. Replacement of damaged or inoperable components is necessary.
System wiring can normally be repaired; make sure to consult the manufacturer's repair manual for proper instructions. If a component's pig tail wire is damaged, it must be replaced. Attempting to repair the pig tail wire could damage the component.
Salvage parts should not be used. No method is presently known to determine the reliability of salvaged air bag components at this time.
Use the new mounting fasteners when provided by the manufacturer.
When replacing sensors, make sure it is the correct replacement part for that vehicle's make, model, year and mounting location. Sensors are calibrated for each specific location and application.
Reinstall sensors with their arrows pointing forward.
Always carry the live air bag module with the trim cover facing away from your body. Always place the module with trim cover facing upward. These precautions will minimize the chance of injury should accidental deployment occur.
When handling a deployed module make sure to wear gloves and eye protection. A small amount of sodium hydroxide may be left on the air bag after deployment. Sodium hydroxide causes skin irritation, if left on the skin for a extended period of time.
Most all air bag systems have a self diagnostics and system operational check which starts with the air bag system readiness indicator light located on the instrument panel. Under normal operation the air bag readiness light will come on after ignition and will light either steady or by flashing for 6 to 10 seconds. The light turning off signals that a self check showed the system to be completely operable. This will vary between manufacturers so check the owner or service manual for correct lamp sequence. If the lamp does not light at ignition or if the lamp stays on after the designated time period, system service should be performed.
Two common ways to conduct system diagnosis are: using an interactive diagnostic tool, or reading flash codes given off by the system readiness lamp. Most manufacturers electronic control modules have the ability to perform diagnosis and display flash codes. For specific vehicle or system information consult the manufacturer's vehicle service manual for detailed information on system diagnosis and repair procedures.
Another option is service manual publishers, such as Mitchell and Motor, which both have an "Air Bag Service and Repair" manual specifically on air bag systems which covers most all vehicle makes and models. If the air bag system still show a fault after replacement of the damaged components and minor trouble shooting, have the vehicle serviced at the dealer to correct any problems.
The
information provided in this bulletin is for educational purposes only. Although
every effort has been made to ensure the accuracy of the information contained
in this bulletin, Tech-Cor assumes no responsibility or liability for
any repairs performed using information from any publication issued by Tech-Cor.
Any person performing repairs must determine whether any suggested or recommended
procedures or repairs are suitable or appropriate for the particular vehicle
being repaired. The repairer remains solely responsible for such determination,
as well as for the proper completion of the repairs.
Reproduction of this bulletin is not permitted without the written approval
of Tech-Cor.
Tech-Cor, LLC, Wheeling, IL
© 1994 Allstate Insurance Company
All rights reserved.
