Air Bag Systems: Description and Operation
DESCRIPTIONA dual front airbag system is standard factory-installed safety equipment on this model. The primary passenger restraints in this vehicle are the standard equipment factory-installed seat belts, which require active use by the vehicle occupants. The airbag system is a supplemental passive restraint that was designed and is intended to enhance the protection for the front seat occupants of the vehicle only when used in conjunction with the seat belts. See the owner's manual in the vehicle glove box for more information on the features, use and operation of all of the factory-installed passenger restraints, including the airbag system.
The dual front airbag system consists of the following components:
- Airbag Control Module (ACM)
- Airbag indicator lamp
- Clockspring
- Driver and passenger side airbag modules (including the airbag inflators)
- Driver and passenger side knee blockers
- Passenger side airbag on/off switch (only on certain models without rear passenger seating)
- Wire harness and connections.
This information provides complete service information for the ACM, both airbag modules, the clockspring, and the passenger side airbag on/off switch. Complete service information for the other airbag system components can be located as follows:
- Refer to Instrument Cluster in the Instrument Panel Systems for complete service information for the airbag indicator lamp.
- Refer to Knee Blocker in the Replacement of Instrument Panel Systems for complete service information on the driver side knee blocker.
- Refer to Glove Box in the Replacement of Instrument Panel Systems for complete service information on the passenger side knee blocker.
See the proper Diagnostic Procedures to test or diagnose a problem with any component of the airbag system.
OPERATION
The airbag system electrical circuits are continuously monitored and controlled by a microprocessor and software contained within the Airbag Control Module (ACM). The ACM also contains an impact sensor and a safing sensor, which are monitored by the ACM to determine when an impact occurs that is severe enough to require airbag system protection.
When a frontal impact is severe enough, the ACM signals the inflator units of both airbag modules to deploy the airbags.
An airbag indicator lamp in the instrument cluster lights for about seven seconds as a bulb test, each time the ignition switch is turned to the On or Start positions. Following the bulb test, the airbag indicator lamp is turned on or off by the ACM to indicate the status of the airbag system. If the airbag indicator lamp comes on at any time other than during the bulb test, it indicates that there is a problem in the airbag system circuits. Such a problem may cause the airbags not to deploy when required, or to deploy when not required.
The driver side airbag module includes an inflatable airbag and an inflator unit behind a trim cover in the hub area of the steering wheel. The passenger side airbag module includes a second inflatable airbag and an inflator unit behind an airbag door in the instrument panel above the glove box.
During a frontal vehicle impact, the knee blockers work in concert with properly adjusted seat belts to restrain the driver and front seat passenger in the proper position for an airbag deployment. The knee blockers also work to absorb and distribute the crash energy from the driver and front seat passenger to the structure of the instrument panel. The drier side knee blocker is a stamped and welded meal unit mounted to the steering column and pedal support bracket and located directly behind the instrument panel steering column opening cover. The passenger side knee blocker is integral to the glove box door.
Following are general descriptions of the major components in the airbag system.
DRIVER SIDE AIRBAG MODULE
DESCRIPTION
The driver side airbag module protective trim cover is the most visible part of the driver side airbag system. The driver side airbag module is mounted directly to the steering wheel. Located under the airbag module trim cover are the horn switch, the folded airbag cushion, and the airbag cushion supporting components. The resistive membrane-type horn switch is secured with heat stakes to the inside surface of the airbag module trim cover, between the trim cover and the folded airbag cushion.
The driver side airbag module cannot be repaired, and must be replaced if deployed or in any way damaged. The driver side airbag module trim cover and the horn switch are available as a unit for service replacement.
OPERATION
The driver side airbag module includes a stamped metal housing to which the cushion and an inflator unit are attached and sealed. The conventional pyrotechnic-type inflator assembly is mounted to studs on the back of the airbag module housing. The inflator seals the hole in the airbag cushion so it can discharge the gas it produces directly into the cushion when supplied with the proper electrical signal. Following an airbag deployment, the airbag cushion quickly deflates by venting this gas towards the instrument panel through the porous fabric material used on the steering wheel side of the airbag cushion.
The protective trim cover is fitted to the front of the airbag module and forms a decorative cover in the center of the steering wheel. The inside of the trim cover has locking blocks molded into it that engage a lip on the airbag module metal housing. Two stamped metal retainers then fit over the inflator mounting studs on the back of the airbag module housing and are engaged in slots on the inside of the securely locking the trim cover into place. The trim cover will split at predetermined breakout lines, then fold back out of the way along with the horn switch upon airbag deployment.
PASSENGER SIDE AIRBAG MODULE
DESCRIPTION
The passenger side airbag door on the instrument panel above the glove box is the most visible part of the passenger side airbag system. Located under the airbag door are the passenger side airbag cushion and the airbag cushion supporting components.
The passenger side airbag module includes an extruded aluminum housing within which the cushion and inflator are mounted and sealed. The airbag housing extrusion also includes two flanges and two channels. The flanges, one at the front and one at the rear, serve as the mounting brackets for the module. The two channels, one near the upper rear of the housing and one near the lower rear, are used to secure the passenger side airbag door to the module. Two stamped metal brackets, one on each end of the housing, enclose the cushion and inflator and also serve to lock the passenger side airbag door flanges into the upper and lower channels of the module housing.
Following a passenger side airbag deployment, the passenger side airbag module and the airbag door must be replaced. The passenger side airbag module cannot be repaired, and must be replaced if deployed or in any way damaged. The passenger side airbag door is available as a separate service item.
OPERATION
The hybrid-type inflator assembly includes a small canister of highly compressed argon gas. The inflator seals the hole in the airbag cushion so it can discharge the gas it produces directly into the cushion when supplied with the proper electrical signal. Following an airbag deployment, the airbag cushion quickly deflates by venting this gas through the porous fabric material used on each end panel of the airbag cushion.
The passenger side airbag module front bracket is secured beneath the instrument panel top pad with screws to the instrument panel structural support. The rear bracket is secured with screws to the upper glove box opening reinforcement. The molded plastic airbag door is secured to the airbag module by beaded upper and lower mounting flanges and also to the instrument panel base trim with five molded snap features. The airbag door has predetermined breakout lines concealed beneath its decorative cover. Upon airbag deployment, the airbag door will split at the breakout lines and the door will pivot out of the way.
AIRBAG CONTROL MODULE
The Airbag Control Module (ACM) is secured to a bracket on the floor panel below the drivers seat.
The ACM contains a microprocessor, the impact sensor, and energy storage capacitor. The microprocessor contains the airbag system logic. The ACM system logic includes On-Board Diagnostics (OBD) capability, and communicates with the instrument cluster circuitry on the Chrysler Collision Detection (CCD) data bus to control the airbag indicator lamp.
The microprocessor in the ACM monitors the impact sensor signal and the airbag system electrical circuits to determine the system readiness. If the ACM detects a monitored system fault, it sends messages to the instrument cluster on the CCD data bus to turn on the airbag indicator lamp. A pre-programmed decision algorithm in the ACM microprocessor determines when the deceleration rate signaled by the impact sensor indicates an impact that is severe enough to require airbag system protection. When the programmed conditions are met, the ACM sends an electrical signal to deploy the airbag system components.
Only one impact sensor is used in this airbag system. The impact sensor is an accelerometer that senses the rate of vehicle deceleration, which provides verification of the direction and severity of an impact. The impact sensor is calibrated for the specific vehicle, and is only serviced as a unit with the ACM.
The ACM also contains an energy-storage capacitor. This capacitor stores enough electrical energy to deploy the airbags, for one second following a battery disconnect or failure during an impact. The purpose of the capacitor is to provide airbag system protection in a severe secondary impact if the initial impact has damaged or disconnected the battery, but was not severe enough to deploy the airbags.
THE AIRBAG SYSTEM IS A SENSITIVE, COMPLEX ELECTROMECHANICAL UNIT. BEFORE ATTEMPTING TO DIAGNOSE OR SERVICE ANY AIRBAG SYSTEM OR RELATED STEERING WHEEL, STEERING COLUMN, OR INSTRUMENT PANEL COMPONENTS YOU MUST FIRST DISCONNECT AND ISOLATE THE BATTERY NEGATIVE (GROUND) CABLE. THEN WAIT TWO MINUTES FOR THE SYSTEM CAPACITOR TO DISCHARGE BEFORE FURTHER SYSTEM SERVICE. THIS IS THE ONLY SURE WAY TO DISABLE THE AIRBAG SYSTEM. FAILURE TO DO THIS COULD RESULT IN ACCIDENTAL AIRBAG DEPLOYMENT AND POSSIBLE PERSONAL INJURY.
NEVER STRIKE OR KICK THE AIRBAG CONTROL MODULE, AS IT CAN DAMAGE THE IMPACT SENSOR OR AFFECT ITS CALIBRATION. IF AN AIRBAG CONTROL MODULE IS ACCIDENTALLY DROPPED DURING SERVICE, THE MODULE MUST BE SCRAPPED AND REPLACED WITH A NEW UNIT.
The airbag warning lamp is the only point at which the customer can observe "symptoms" of a system malfunction, Whenever the ignition key is turned to the "run" or "start" position, the MIC performs a lamp check by turning the AIRBAG warning lamp ON for 6-8 seconds. If the lamp remains OFF, it means that the ACM has checked the system and found it to be free of discernible malfunctions. If the lamp remains ON, there could be an active fault in the system or the MIC lamp circuit may be internally shorted to ground. If the lamp comes ON and stays ON for a period longer than 6-8 seconds then goes OFF, there is usually an intermittent problem in the system.
The MIC monitors the airbag warning lamp and seat belt warning lamp during the bulb check for a defective bulb or lamp driver circuit. The MIC tests the airbag lamp during the cluster bulb check and substitutes the seat belt lamp if the airbag lamp is defective. If the airbag lamp is defective the MIC will turn the seat belt light ON and OFF approximately 30 times to warn the driver that the airbag lamp is defective. The lamp status is sent to the ACM continuously via a CCD bus message. The seat belt lamp acts as a backup in the event that the AIRBAG lamp is defective. This information is sent to the ACM continuously via a CCD bus message.
Perform the WARNING LAMP BULB OR DRIVER procedure in this book to find the cause of any customer complaint regarding the AIRBAG warning lamp, such as:
- warning lamp does not illuminate
- warning lamp stays illuminated with no active DTC's
The ACM cannot be repaired or adjusted and, if damaged or faulty, it must be replaced.
Service and general information labels about the airbag system can be found on the driver's sun visor, the glove box door, and in the engine compartment.
WARNING: FAILURE TO HAVE THE AIRBAG SYSTEM PROPERLY SERVICED BY AN AUTHORIZED DEALER MAY LEAD TO POSSIBLE INJURY IN THE EVENT OF AN ACCIDENT.
AIRBAG DIAGNOSTIC TROUBLE CODES
Airbag diagnostic trouble codes consist of active and stored codes. If more than one code exists, diagnostic priority should be given to the active codes.
Each diagnostic trouble code is diagnosed by following a specific testing procedure. The diagnostic test procedures contain step-by-step instructions for determining the cause of the trouble codes. It is not necessary to perform all of the tests in this book to diagnose an individual code.
Always begin by reading the diagnostic trouble codes using the DRB III. This will direct you to the specific test(s) that must be performed
Active diagnostic trouble codes for the airbag system are not permanent and will change the moment the reason for the code is corrected. In certain test procedures within this , diagnostic trouble codes are used as a diagnostic tool.
Active Codes
The code becomes active as soon as the malfunction is detected or key-OFF, whichever occurs first. An active trouble code indicates an on-going malfunction. This means that the defect is currently there every time the airbag control module checks that circuit/function. It is impossible to erase an active code; active codes automatically erase by themselves when the reason for the code has been corrected.
With the exception of the warning lamp trouble codes or malfunctions, when a malfunction is detected, the airbag lamp remains lit for a minimum of 12 seconds or as long as the malfunction is present.
The seat belt lamp replaces the airbag lamp if the airbag lamp is defective. The seat belt lamp pulses ON and OFF 30 times if the airbag warning lamp is defective.
Stored Codes
Airbag codes are automatically stored in the ACM's memory as soon as the malfunction is detected. The exception is the "Loss of Ignition Run Only" code which is an active code only.
A "stored" code indicates there was an active code present sometime. However, the code currently may not be present as an active code, although another active code could be.
When a trouble code occurs, the airbag warning lamp illuminates for 12 seconds minimum (even if the problem existed for less than 12 seconds). The code is stored, along with the time in minutes it was active, and the number of times the ignition has been cycled since the problem was last detected.
The minimum time shown for any code will be one minute, even if the code was actually present for less than one minute. Thus, the time shown for a code that was present for two minutes 13 seconds, for example, would be three minutes.
If malfunction is detected a diagnostic trouble code is stored and will remain stored as long as the malfunction exists. When and if the malfunction ceases to exist, an ignition cycle count will be initiated for that code. If the ignition cycle count reaches 100 without a reoccurrence of the same malfunction, the diagnostic trouble code is erased and that ignition cycle counter is reset to zero. If the malfunction reoccurs before the count reaches 100, then the ignition cycle counter will be reset and the diagnostic trouble code will continue to be a stored code.
If a malfunction is not active while performing a diagnostic test procedure, the active code diagnostic test will not locate the source of the problem. In this case, the stored code can indicate an area to inspect.
If no obvious problems are found, erase stored codes, and with the ignition "ON" wiggle the wire harness and connectors, rotate the steering wheel from stop to stop. Recheck for codes periodically as you work through the system. This procedure may uncover a malfunction that is difficult to locate.
Airbag Trouble Codes
The Airbag Control Module may report any of the following diagnostic trouble codes. For the following codes, replace the Airbag Control Module (ACM) even if set intermittently:
- ACM ACCELEROMETER
- ACM OUTPUT DRIVER CIRCUIT OPEN
- ACM INTERNAL DIAGNOSTIC CODE # 1
- ACM INTERNAL DIAGNOSTIC CODE # 2
- ACM STORED ENERGY
- ACM OUTPUT DRIVER
For these codes, refer to the appropriate diagnostic procedure:
Cluster Message Mismatch
Driver Squib Circuit Open
Driver Squib Circuit Shorted
Driver Squib Circuit Shorted to Battery
Driver Squib Circuit Shorted to Ground
Loss of Ignition Run Only
Loss of Ignition Run/Start
NO Cluster CCD Message
No CCD Communication
Passenger Squib Circuit Open
Passenger Squib Circuit Shorted
Passenger Squib Circuit Shorted to Battery
Passenger Squib Circuit Shorted to Ground
Warning Lamp Open/Shorted