Prepare for ASE Electrical/Electronic Systems (A6) certification test content area “H” (Accessories Diagnosis and Repair).
List the safety precautions for working around an airbag.
Describe the procedures to diagnose and troubleshoot airbag faults.
OBJECTIVES: After studying Chapter 46, the reader should be able to:
airbag • arming sensor
deceleration sensor • dual-stage airbags
event data recorder (EDR) • event file
occupant detection systems
KEY TERMS: Continued
passenger presence system (PPS) • pretensioners
squib • supplemental air restraints (SAR) • supplemental inflatable restraints (SIR) • supplemental restraint system (SRS)
SAFETY BELTS AND RETRACTORS
Safety Belts Used to keep the driver and passengers secured to the vehicle in the event of a collision. Most safety belts include three-point support and are constructed of nylon webbing.
Continued Collision 1 —The vehicle strikes another vehicle or object. Collision 2 —The driver and/or passengers hit objects inside the vehicle if unbelted. Collision 3 —The internal organs of the body hit other organs or bones, which causes internal injuries. The support points include two points on either side of the seat for the belt over the lap and one crossing over the upper torso, which is attached to the “B” pillar or seat back. Every crash has three types of collisions:
Figure 46–1 (a) Safety belts are the primary restraint system. (b) During a collision the stretching of the safety belt slows down the impact to help reduce bodily injury.
If a safety belt is being worn, the belt stretches, absorbing a lot of the impact, thereby preventing collision with other objects in the vehicle and reducing internal injuries.
Belt Retractors Safety belts are also equipped with one of the following types of retractors:
Nonlocking retractors, which are usually only used on recoiling lap belts
Emergency locking retractors, which lock the position of the safety belt in the event of a collision or rollover
Emergency and web speed-sensitive retractors, which allow freedom of movement for the driver and passenger but lock if the vehicle is accelerating too fast or if the vehicle is decelerating too fast.
Continued See Figure 46–2 for an example of an inertia-type seat belt locking mechanism. Safety Belt Lights and Chimes All late-model vehicles are equipped with a safety belt warning light on the dash and a chime that sounds if the belt is not fastened. See Figure 46–3. Some vehicles will intermittently flash the reminder light and sound a chime until the driver and sometimes the front passenger fasten their safety belts.
Figure 46–2 Most safety belts have an inertia-type mechanism that locks the belt in the event of rapid movement. Figure 46–3 A typical safety belt warning light.
A pretensioner is an explosive (pyrotechnic) device that tightens the seat belt as the airbag is being deployed. The purpose of the pretensioning device is to force the occupant back into position against the seat back and to remove any slack in the seat belt. See Figure 46–4. Pretensioners are explosive devices that could be ignited if voltage is applied to the terminals. Do not use a jumper wire or powered test light around the wiring near the seat belt latch wiring. Always follow the vehicle manufacturer’s recommended test procedures.
Figure 46–4 A small explosive charge forces the end of the seat belt down the tube, which removes any slack in the seat belt. CAUTION: The seat belt pretensioner assemblies must be replaced in the event of an airbag deployment. Always follow the vehicle manufacturer’s recommended service procedure. Pretensioners are explosive devices that could be ignited if voltage is applied to the terminals. Do not use a jumper wire or powered test light around the wiring near the seat belt latch wiring. Always follow the vehicle manufacturer’s recommended test procedures.
Airbag passive restraints are designed to cushion the driver (or passenger, if the passenger side is so equipped) during a frontal collision. Airbags may be known by many different names including the following:
Supplemental restraint system (SRS)
Supplemental inflatable restraints (SIR)
Supplemental air restraints (SAR)
Most airbags are designed to supplement the safety belts in the event of a collision, and front airbags are meant to be deployed only in the event of a frontal impact within 30 degrees of center.
Figure 46–5 A typical airbag system showing many of the components.
Operation Parts included in a typical airbag system include:
Airbag (inflator) module
Clockspring wire coil in the steering column
Wiring and connectors
Force required to deploy a typical airbag is equal to the force of a vehicle hitting a wall at over 10 miles per hour (16 km/hr). Continued
Figure 46–6 A simplified airbag deployment circuit. Note that both the arming sensor and at least one of the discriminating sensors must be activated at the same time. The arming sensor provides the power and either one of the discriminating sensors can provide the ground for the circuit.
To cause inflation, the arming sensor is required to close, to provide the power-side voltage to the inflator module.
Continued Before the airbag can inflate, the squib circuit also must have a ground. The ground is provided through the actuation of either the forward or the passenger discriminating sensor . Two sensors must be triggered at the same time before deployment.
Types of Airbag Inflators Two different types of inflators used:
The Solid fuel type uses sodium azide pellets and when ignited, generates a large quantity of nitrogen gas that quickly inflates the airbag. First type used; still common. The squib heating element ignites the gas-generating material. It takes about 2 amps to heat the element and ignite the inflator.
The Compressed gas type, commonly used in passenger-side airbags and roof-mounted systems, this system uses a canister filled with argon gas, plus a small percentage of helium at 3,000 psi(435 kPa). A small igniter ruptures a burst disc to release the gas when energized. The compressed gas inflators are long cylinders that can be installed inside the instrument panel, seat back, door panel, or along any rail or pillar of the vehicle. See Figures 46–7 and 46–8.
Figure 46–7 Lifting the squib from the airbag housing. The squib is the heating element that ignites the pyrotechnic gas generator that rapidly produces nitrogen gas to fill the airbag. Figure 46–8 This shows a deployed side-curtain airbag on a training vehicle. Continued
Once the inflator is ignited, the nylon bag quickly inflates (in about 30 milliseconds [ms] or 0.030 seconds) with nitrogen gas generated by the inflator. During an actual frontal collision accident, the driver is being thrown forward by the driver’s own momentum toward the steering wheel. The strong nylon bag inflates at the same time. Personal injury is reduced by the spreading of the stopping force over the entire upper-body region.
Continued Sensors All three sensors are switches that complete an electrical circuit when activated. The sensors are similar in construction and operation, and the location of the sensor determines its name. All airbag sensors are rigidly mounted to the vehicle and must be mounted with the arrow pointing toward the front of the vehicle to ensure that the sensor can detect rapid forward deceleration.
There are three basic styles (designs) of airbag sensors:
The Magnetically retained gold - plated ball sensor . This sensor uses a permanent magnet to hold a gold-plated steel ball away from two gold-plated electrical contacts. See Figure 46–9. If the vehicle (and the sensor) stops rapidly enough, the steel ball is released from the magnet and makes contact with the two gold-plated electrodes.
The Rolled up stainless - steel ribbon-type sensor . This sensor is housed in an airtight package with nitrogen gas inside to prevent harmful corrosion of the sensor parts. See Figure 46–10. If the vehicle (and the sensor) stops rapidly, the stainless-steel roll “unrolls” and contacts the two gold-plated contacts.
Figure 46–9 An airbag magnetic sensor. Continued Figure 46–10 Some vehicles use a ribbon-type crash sensor.
The Integral sensor . Some vehicles use electronic deceleration sensors built into the inflator module. These sensors measure the rate of deceleration and, through the computer logic, determine if the airbags should be deployed. See Figure 46–11 for a timeline that illustrates how quickly a typical airbag system can react and inflate.
Continued CAUTION: In the event of a collision that causes the airbag to deploy, some vehicle manufacturers require that all sensors be replaced along with the airbag assembly. The force of impact can cause unseen damage inside the sensor; the sensor may not work correctly if used again.
Figure 46–11 Notice that within 1/4 second of a collision, the sensors have closed, the airbag has deployed, and the airbag has deflated. Continued
Wiring By worldwide agreement, all electrical wiring for airbags is yellow. To ensure proper electrical connection to the inflator module in the steering wheel, a coil assembly is used in the steering column. This coil is a ribbon of copper wires that operates much like a window shade when the steering wheel is rotated. This coil, called an SIR coil or a clockspring , prevents the lack of continuity between the sensors and the inflator assembly that might result from a horn-ring type of sliding conductor. See Figure 46–12.
Figure 46–12 The airbag control module is linked to the power train control module (PCM) and the body control module (BCM) on this DaimlerChrysler system. Notice the airbag wire connecting the module to the airbag through the clockspring. Continued
Most airbag systems also contain a diagnostic unit that often includes an auxiliary power supply, which is used to provide the current to inflate the airbag if the battery is disconnected from the vehicle during a collision. This auxiliary power supply usually uses capacitors that are discharged through the squib of the inflation module. See Figures 46–13 and 46–14.
Continued Troubleshooting The electrical portion of most airbag systems is constantly checked by the circuits within the airbag-energizing power unit or through the vehicle’s computer system. If continuity exists, a small voltage drop will be measured by the testing circuits. If an open or short circuit occurs, a dash warning light is lighted and a possible diagnostic trouble code (DTC) is stored.
Figure 46–14 An airbag diagnostic tester. Included in the plastic box are electrical connectors and a load tool that substitutes for the inflator module during troubleshooting. Figure 46–13 An airbag being deployed as part of a demonstration in an automotive laboratory. Continued
AIRBAG DIAGNOSIS TOOLS AND EQUIPMENT
Diagnosis and service of airbag systems usually requires:
Digital multimeter (DMM) & Scan Tool
Airbag simulator, often called a load tool
Shorting bar or shorting connector(s)
A irbag system tester
Vehicle-specific test harness
Special wire repair tools or connectors, such as crimp-and-seal weatherproof connectors
CAUTION: Most vehicle manufacturers specify that the negative battery terminal be removed when testing or working around airbags. Be aware that a memory saver device used to keep the computer and radio memory alive can supply enough electrical power to deploy an airbag.
When replacing any steering gear such as a rack-and-pinion steering unit, be sure that no one accidentally turns the steering wheel. If the steering wheel is turned without being connected to the steering gear, the airbag wire coil (clock-spring) can become off center. This can cause the wiring to break when the steering wheel is rotated after the steering gear has been replaced. To help prevent this from occurring, simply remove the ignition key from the ignition and keep it in your pocket while servicing the steering gear. Pocket the Ignition Key to be Safe
Precautions Working with or around airbags requires precautions:
Always follow all precautions and warning stickers on vehicles equipped with airbags.
Maintain a safe working distance from all airbags to help prevent the possibility of personal injury in the unlikely event of an unintentional airbag deployment.
In the event of a collision in which the bag(s) were deployed, the inflator module and all sensors usually must be replaced to ensure proper future operation of the system.
Avoid using a self-powered test light around the yellow airbag wiring. Even though it is highly unlikely, a self-powered test light could provide the necessary current to accidentally set off the inflator module and cause an airbag deployment.
Use care when handling the inflator module section when it is removed from the steering wheel. Always hold the inflator away from your body.
If handling a deployed inflator module, always wear gloves and safety glasses to avoid the possibility of skin irritation from the sodium hydroxide dust, which is used as a lubricant on the bag(s) and that remains after deployment.
Never jar or strike a sensor. The contacts inside the sensor may be damaged, preventing the proper operation of the airbag system in the event of a collision.
When mounting a sensor in a vehicle, make certain that the arrow on the sensor is pointing toward the front of the vehicle. Also be certain that the sensor is securely mounted.
Many vehicles are equipped with dual-stage airbags (two-stage airbags) that actually contain two separate inflators; one for less severe crashes and one for higher-speed collisions. During some airbag deployment, both stages are deployed. If one stage is deployed, the other stage is still active and could be accidentally deployed. A service technician cannot tell by looking at the airbag whether both stages have deployed. Always handle a deployed airbag as if it has not been deployed and take all precautions necessary to keep any voltage source from getting close to the inflator module terminals. Dual-Stage Airbags
AIRBAG TESTING AND SERVICE
Airbag system components and their location in the vehicle vary according to system design, but the basic principles of testing are the same as for other electrical circuits. Some airbag systems require the use of special testers. The built-in safety circuits of such testers prevent accidental deployment of the airbag. If such a tester is not available, follow the recommended alternative test procedures specified by the manufacturer. Access the self-diagnostic system and check for diagnostic trouble code (DTC) records. The factory scan tool is needed to access the data stream on some systems.
An SRS has a diagnostic module that detects system electrical faults, disables the system, and notifies the driver through a system readiness indicator or airbag warning lamp in the instrument cluster.| The warning lamp should illuminate with the ignition key on and engine off as a bulb check. If not, the diagnostic module is likely disabling the system. If the airbag warning light remains on, the airbags may or may not be disabled, depending on the specific vehicle andthe fault detected. Some warning lamp circuits have a timer that extinguishes the lamp after a few seconds. The SRS generally does not require service unless there is a failed component.
DRIVER-SIDE AIRBAG MODULE REPLACEMENT
For the specific model being serviced, carefully follow the procedures provided by the vehicle manufacturer to disable and remove the airbag module. Failure to do so may result in serious injury and extensive damage to the vehicle. Replacing a discharged airbag is costly. The following procedure reviews the basic steps for removing an airbag module. Do not substitute these general instructions for the specific procedure recommended by the manufacturer.
Turn the steering wheel until the front wheels are positioned straight ahead. Some components on the steering column are removed only when the front wheels are straight.
Switch the ignition off and disconnect the negative battery cable, which cuts power to the airbag module.
Once the battery is disconnected, wait as long as recommended by the manufacturer before continuing. When in doubt, wait at least 10 minutes to make sure the capacitor is completely discharged.
Loosen and remove the nuts or screws that hold the airbag module in place. On some vehicles, these fasteners are located on the back of the steering wheel. On other vehicles, they are located on each side of the steering wheel. The fasteners may be concealed with plastic finishing covers that must be pried off with a small screwdriver to access them.
Carefully lift the airbag module from the steering wheel and disconnect the electrical connector. Connector location varies. Some are below the steering wheel behind a plastic trim cover, while others are at the top of the column under the module. See Figures 46–15 and 46–16.
Store the module with the pad side up in a safe place where it will not be disturbed or damaged while the vehicle is being serviced. Do not attempt to disassemble the airbag module. If the airbag is defective, replace the entire assembly.
Figure 46–15 This figure shows the process of removing the airbag inflator module from the steering wheel and disconnecting the yellow airbag electrical connector. Figure 46–16 Shorting bars are used in most airbag connectors. These spring-loaded clips short across both terminals of an airbag connector to help prevent accidental deployment of the airbag. If electrical power was applied to the terminals, the shorting bars would simply provide a low-resistance path to the other terminal and not allow current to flow past the connector. The mating part of the connector has a tapered piece that spreads apart the shorting bars. Continued
When installing the airbag module, make sure the clockspring is correctly positioned to ensure module-to-steering column continuity. Always route wiring exactly as it was before removal. Make sure the module seats completely into the steering wheel. Secure the assembly using new fasteners, if specified.
Figure 46–17 A typical coil assembly (often called a clockspring) is used to electrically connect the inflation module with the airbag module that actually supplies the voltage at the time of a collision to deploy the airbag. It is critical that the column-mounted coil assembly be installed correctly. Continued
Airbag modules cannot be disposed of unless they are deployed. To prevent injury when manually deploying an airbag:
SAFETY WHEN MANUALLY DEPLOYING AIRBAGS Continued
W henever possible, deploy the airbag outside of the vehicle. Follow the vehicle manufacturer’s recommendations.
When deploying the airbag inside the vehicle, make sure personnel, tools, and equipment are removed from vehicle.
Follow the vehicle manufacturer’s procedures and equipment recommendations.
Wear the proper hearing and eye protection.
Deploy the airbag with the trim cover facing up.
Stay at least 20 feet (6 meters) from the airbag.
Allow the airbag module to cool.
OCCUPANT DETECTION SYSTEMS
United States Federal Motor Vehicle Safety Standard 208 specifies that the passenger side airbag be disabled or deployed with reduced force under the following conditions. This system is referred to as an occupant detection system, also called the passenger presence system ( PPS ).
When there is no weight on the seat and no seatbelt is fastened, the passenger side airbag will not deploy and the airbag disable light should be off.
The passenger side airbag will be disabled and the disabled- airbag light on if only 10 to 37 pounds (4.5 to 17 kg) are on the passenger seat, which would represent a seated child.
If 38 to 99 pounds (17 to 45 kg) is detected on the passenger seat, which represents a child or small adult, the airbag will deploy at a decreased force.
If 99 pounds (45 kg) or more is detected on the passenger seat, the airbag will deploy at full force, depending on the severity of the crash, speed of the vehicle, and other factors which may result in the airbag deploying at a reduced force.
The occupant detection systems use one of three types of sensors.
Figure 46–18 A bladder-type occupant detection sensor showing the pressure sensor and wiring. Continued
Gel-filled bladder sensor This sensor uses a silicone-filled bag with a pressure sensor attached. The weight of the passenger is measured by the pressure sensor, which sends a voltage signal to the module controlling the airbag deployment.
The module uses information from both the bladder and the seat belt sensor to determine if a tightened belt may be used to restrain a child seat.
Force-sensing resistor sensors This type of occupant sensor uses resistors, which change their resistance based on the stress that is applied. These resistors are part of the seat structure, and the module can determine the weight of the occupant based on the change in the resistance of the sensors. See Figure 46-19.
Capacitive strip sensors This type of occupant sensor uses several flexible conductive metal strips under the seat cushion. These sensor strips transmit and receive a low-level electric field, which changes due to the weight of the front passenger seat occupant. The module determines the weight of the occupant.
Figure 46–19 A resistor-type occupant detection sensor. The weight of the passenger strains these resistors, which are attached to the seat, thereby signaling to the module the weight of the occupant. CAUTION: Because resistors are part of the seat structure, it is very important that all seat fasteners be torqued to factory specs to ensure proper operation of the occupant detection system. A seat track position sensor is used by the airbag controller to determine the seat position. If the seat is too close to the airbag, the controller may disable the airbag. Continued
Diagnosing Occupant Detection Systems A fault in the system may cause the passenger-side airbag light to turn on when there is no weight on the seat.
Figure 46–20 A test weight is used to calibrate the occupant detection system on a Chrysler vehicle. A scan tool is often used to check or calibrate the seat, which must be empty, by commanding the module to rezero the seat sensor. Some systems, such as on DaimlerChrysler vehicles, use a unit that has various weights along with a scan tool to calibrate and diagnose the occupant detection system.
SEAT AND SIDE-CURTAIN AIRBAGS
Side-curtain airbags use a variety of sensors to determine if they need to be deployed. Side airbags are mounted in one of two general locations:
In the side bolster of the seat
In the door panel
Figure 46–21 A typical seat (side) airbag that deploys from the side of the seat.
Most side-airbag sensors use an electronic accelerometer to detect when to deploy the airbags, which are usually mounted to the bottom of the left and right B-pillars (where the front doors latch) behind a trim panel on the inside of the vehicle. Side-curtain airbags are usually deployed by the side-curtain airbag module based on input from many different sensors, including a lateral acceleration sensor and wheel speed sensors.
CAUTION: Avoid using a lockout tool (for example, a “slim jim”) in vehicles equipped with side airbags to help prevent damage to the components and wiring in the system.
EVENT DATA RECORDERS
Parts and Operation The purpose of the event data recorder ( EDR ) is to record parameters just before and slightly after an airbag deployment. They are part of the airbag controller on many vehicles. The parameters recorded include:
Seat belt fastened
G-forces as measured by the accelerometer
Unlike an airplane event data recorder, a vehicle unit is not a separate unit and does not record voice conversations or include all crash parameters. Additional crash data, such as skid marks and physical evidence at the crash site will still be needed.
The data is constantly being stored in a memory buffer and not recorded into the EPROM unless an airbag deployment has been commanded. The data is known as an event file . The airbag is commanded based on input from the accelerometer sensor, usually built into the airbag controller, located inside the vehicle. The accelerometer calculates the rate of change of the speed of the vehicle. The airbags will be deployed if the threshold g-value is exceeded. The passenger-side airbag will also be deployed unless it is suppressed because of either of the following:
No passenger is detected
The passenger-side airbag switch is off
Data Extraction Data can only be achieved using a piece of equipment known as the Crash Data Retrieval System manufactured by Vetronics Corporation. This is the only authorized method for retrieving event files and only certain organizations are allowed access to the data. These groups or organizations include:
Original equipment manufacturer’s representatives
National Highway Traffic Safety Administration
Law enforcement agencies
Accident reconstruction companies
Crash data retrieval must done by a person who has been trained as a crash data retrieval (CDR) technician or analyst, which requires specialized training and the passing of an examination. An analyst must attend additional training beyond that of a technician to achieve CDR analyst certification
If a vehicle equipped with the OnStar system is being driven aggressively and the electronic stability control system has to intercede to keep the vehicle under control, OnStar may call the vehicle to see if there has been an accident. The need for a call from OnStar usually will be determined if the accelerometer registers slightly over one-g of force, which could be achieved while driving on a race track. Aggressive Driving and OnStar ® ® ® ®
Airbags use a sensor(s) to determine if the rate of deceleration is enough to cause bodily harm. All airbag wiring is yellow. When working around an airbag, disconnect the wiring connectors to help prevent accidental deployment.
Frontal airbags only operate within 30 degrees from center and do not deploy in the event of a rollover, side, or rear collision.
Two sensors must be triggered at the same time for an airbag deployment to occur.
All electrical terminals are gold-plated to protect against corrosion.
Pretensioners are explosive (pyrotechnic) devices that remove the slack from the seat belt and help position the occupant.
Occupant detection systems use sensors in the seat to determine whether the airbag will be deployed and with full or reduced force.