Air Bag Systems: Description and Operation
GENERAL DESCRIPTION RESTRAINT SYSTEMThis supplemental inflatable restraint (SIR) helps supplement the protection offered by the seat belts by deploying air bags during certain frontal crashes. The air bags deploy when the vehicles is involved in a frontal crash of sufficient force up to 30 degrees left or right of the centerline of the vehicle. To further absorb the crash energy there are knee bolsters located beneath the instrument panel for both the driver and passenger. Additionally, the steering column is collapsible.
SIR Warning Label:
SIR Wiring Schematic:
SIR SYSTEM DESCRIPTION
The SIR System consists of the sensing and diagnostic module (SDM), the driver inflator module, the SIR coil assembly, the passenger inflator module and the AIR BAG Warning Lamp in the instrument cluster. The SDM, SIR coil assembly (driver's side only), driver inflator module, passenger inflator module and connecting wires make up the deployment loops. The function of the deployment loops is to supply current through the inflator modules, which will cause deployment of the air bags in the event of a frontal crash of sufficient force, up to 30 degrees left or right of the center line of the vehicle. The inflator modules are only supplied enough current to deploy when the SDM detects vehicle velocity changes severe enough to warrant deployment.
The SDM contains a sensing device which converts the vehicles acceleration changes to an electrical signal. The electrical signal generated is processed by the SDM and then compared to values stored in electrically erasable programmable read only memory (EEPROM). When the proper thresholds are exceeded, the SDM will cause current to flow through the inflator modules deploying the air bags.
Component Location And Wiring View:
SENSING AND DIAGNOSTICS MODULE (SDM)
WARNING: During service procedures, be very careful when handling a sensing and diagnostic module (SDM). Never strike or jar the SDM. never power up the sir system when the SDM is not rigidly attached to the vehicle. All SDM and mounting bracket fasteners must be carefully torqued and the arrow must be pointing toward the front of the vehicle to ensure proper operation of the sir system. The SDM could deploy the AIR bags resulting in personal injury if powered and not rigidly attached to the vehicle. Be sure that the ignition is "off" and the SDM connector disconnected before mounting or dismounting the SDM.
NOTE: Do not open the ADM case for any reason. Touching the connector pins or soldered components may cause electrostatic discharge damage. Repair of a faulty SDM is by replacement only.
The sensing and diagnostic module (SDM) is designed to perform the following functions in the SIR system:
1. Frontal Crash Detection: The SDM monitors vehicle acceleration changes to detect frontal crashes which are severe enough to warrant deployment.
2. Air Bag Deployment: When a frontal crash requiring deployment is detected, the ADM provides enough current to deploy the air bags.
3. Energy Reserve: The SDM maintains a 36 volt loop energy reserve (36 VLR) energy supply to provide deployment current if ignition voltage is lost in a frontal crash.
4. Malfunction Detection: The SDM performs diagnostic monitoring of SIR system electrical components and sets diagnostic fault codes when a malfunction is detected.
5. Driver Notification: The SDM warns the driver of the SIR system malfunctions by controlling the "Air Bag" warning lamp.
6. Malfunction Diagnosis: The SDM displays SIR diagnostic fault codes and system status information through a scan tool.
7. Crash Recording: The SDM records information regarding the SIR system status when deployment has been commanded.
The SDM is connected to the SIR wiring harness by a 20-way connector. This connector has a shorting bar across two terminals which connects the AIR BAG warning lamp to ground when the SDM connector is disconnected. This will cause the AIR BAG telltale to come "On" steady whenever the ignition switch is at the RUN or START positions with the SDM disconnected.
AIR BAG WARNING LAMP
Ignition voltage is applied to the AIR BAG warning lamp when the ignition switch is at the RUN or START position. The SDM controls the lamp by providing ground with a lamp driver. The AIR BAG warning lamp is used in the SIR system to notify the operator of the following:
1. Correct system operation by flashing seven times when the ignition switch is first turned "On" and then extinguishing.
2. Warn the vehicle driver of SIR electrical system malfunctions which could potentially affect the operation of the SIR system.
The AIR BAG warning lamp is the key to driver notification of SIR system malfunctions. For proper lamp operation, refer to "Sir Diagnostic System Check."
SIR Coil Assembly:
SIR COIL ASSEMBLY
The SIR coil assembly consists of current carrying coils, two of which are required for air bag operation. They are attached to the lever combination switch and allow rotation of the steering wheel while maintaining continuous contact of the driver deployment loop to the inflator module.
There is a shorting bar on the yellow two-way connector near the base of the steering column which connects the SIR coil to the instrument panel wiring harness. The shorting bar shorts the circuits to the SIR coil and driver inflator module when the yellow two-way connector is disconnected. The circuit to the driver inflator module is shorted in this way to help prevent unwanted deployment of the air bag when servicing the steering column or other SIR system components.
1 SIR Coil Assembly
2 Driver Inflator Module Connector
3 Steering Column Pigtail and Connector
Inflator Module - Driver Side:
Inflator Module - Passenger Side:
INFLATOR MODULES
The inflator modules consist of an inflatable bag and an inflator (a canister of gas-generating material and an initiating device). When the vehicle is in a frontal crash requiring deployment, the SDM causes current to flow through the deployment loops. Current passing through the initiator ignites the material in the inflator. The gas produced from this reaction rapidly inflates the air bag.
There is a shorting bar on the driver inflator module. The shorting bar shorts across the driver inflator module circuits when the connector is disconnected. The circuit to the driver inflator module is shorted in this way to help prevent unwanted deployment of the air bag when servicing the driver inflator module, the steering column or other SIR system components.
There is a shorting bar on the passenger inflator module connector which connects the passenger inflator module to the instrument panel wiring harness. The shorting bar shorts across the passenger inflator module circuit when disconnected. The circuit to the passenger inflator module is shorted in this way to help prevent unwanted deployment of the air bag when servicing the passenger inflator module, the instrument panel or other SIR system components.
STEERING COLUMN
The steering column is energy absorbing and is designed to compress in a frontal crash to minimize injury to the driver.
KNEE BOLSTERS
The knee bolsters are used to absorb energy and control the forward movement of the driver and passenger during a frontal crash by limiting leg movement.
DEFINITIONS
ACTIVE FAULT - Fault currently detected.
AIR BAG - An inflatable cloth cushion designed to deploy in certain frontal crashes. It supplements the protection offered by the seat belt by distributing the impact load more evenly over the occupant's head and torso.
B+ - Battery voltage, the voltage available at the battery at the time of the indicated measurement. With the key "On" and the engine not running, the system voltage will likely be between 12 and 12.5 volts. At idle the voltage may be 14 to 16 volts. The voltage could be as low as 10 volts during engine cranking.
BULB CHECK - The ADM will cause the AIR BAG warning lamp to flash seven times and then go "Off" whenever the ignition switch transitions to the RUN or START positions from any other ignition switch position and no malfunctions are detected.
CONTINUOUS MONITORING - Tests performed by the SDM on the SIR system every 100 milliseconds while "Ignition 1" voltage is present at the SDM.
CURRENT SOURCE DIAGNOSTICS - Tests performed by the SDM after ignition 36 VLR (Voltage Loop Energy Reserve), and deployment loop voltages are all measured within their respective normal voltage ranges. This test checks for proper resistance of the deployment loops and for proper operation of the driver and passenger current sources.
DATA LINK CONNECTOR (DLC) (Formerly called the ALDL) - Connector which is connected by wires to multiple on-board computers allowing communication with an off-board computer such as a scan tool.
DEPLOY - To inflate the air bag.
DEPLOYMENT LOOP- The circuit which supplies current to the inflator module to deploy the air bag.
DIAGNOSTIC TROUBLE CODE (DTC) - A numerical designator used by the SDM to indicate specific SIR system malfunctions.
DRIVER CURRENT SOURCE - An output of the SDM which provides current to the driver inflator module.
DRIVER INFLATOR MODULE - An assembly located in the steering wheel hub consisting of an inflatable bag, an inflator and an initiator.
EEPROM - Electrically erasable programmable read only memory. Memory which retains its contents when power is removed from the SDM.
HIGHER PRIORITY FAULT - Each diagnostic fault code is assigned a priority based on the detectability with other codes present. The priority corresponds to the detectability of the malfunction only, and does not relate to the criticality of the malfunction with respect to deployment or non-deployment under any given condition.
HISTORY FAULT - Faults detected since last "CLEAR Codes" command after 125 fault free ignition cycles history codes are cleared.
IGNITION CYCLE - The voltage at the SDM "Ignition 1" input, with ignition switch "On", is greater than seven volts for at least ten seconds before turning ignition switch "Off."
IGNITION 1 - A battery voltage (B +) circuit which is only powered with the ignition switch in the RUN or START position.
INITIATOR - The electrical component inside the inflator module which, with sufficient current flow starts the chemical reaction that inflates the air bag.
NORMAL OPERATING VOLTAGE RANGE - The voltage measured between the SDM "Ignition 1" terminal and "GROUND" terminal is between 7 and 16 volts.
PASSENGER CURRENT SOURCE - An output of the SDM which provides current to the passenger inflator module.
PASSENGER INFLATOR MODULE - An assembly located in the right side of the instrument panel consisting of an inflatable bag, an inflator and an initiator.
PORTABLE DIAGNOSTIC TOOL (PDT) - An off-board computer used to read diagnostic information from on-board computers via data link connector.
SDM - Sensing and diagnostic module which provide reserve energy to the deployment loops, deploys the air bag when required and performs diagnostic monitoring of all system components.
SERIAL DATA - Information representing the status of the queried system.
SIR- Supplemental inflatable restraint.
SIR COIL - An assembly of current-carrying coils (two for SIR) in the driver deployment loop that allows the rotation of the steering wheel while maintaining the continuous contact of the driver deployment loop to the driver inflator module.
SIR WIRING HARNESS - The wires and connectors that electrically connect the components in the SIR system.
TURN-ON-TESTS - Tests which the SDM performs on the SIR system once during each ignition cycle immediately after Ignition 1 voltage is applied to the ADM and before "Continuous Monitoring."
36 VLR DELAY (volt loop reserve) - Charging of the 36 VLR power supply is inhibited briefly during "TURN-ON." This allows detection of certain malfunctions related to "Ignition 1" voltage which cannot be tested with 36 volts present.
DIAGNOSIS
NOTE: To avoid deployment when troubleshooting the sir system, do not use electrical test equipment such as a battery powered or AC powered voltmeter, ohmmeter, etc., or any other type of electrical equipment other than that specified. Instructions must be followed carefully, otherwise personal injury may result.