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Principles of Operation

PRINCIPLES OF OPERATION

A supplemental restraint system (SRS) consists of a driver and passenger single stage air bag module (containing an inflator/squib or gas generator and an air bag), safety belt buckle pretensioners (containing an inflator/squib or gas generator), impact sensor, a restraints control module (RCM), a clockspring, an air bag warning indicator. These components are all interconnected by a wiring harness and powered by the vehicle's battery. The RCM includes a backup power supply. This feature provides sufficient backup power to deploy the SRS components in the event that the ignition circuit is lost or damaged during impact. The backup power supply will deplete its stored energy approximately one minute after power and/or ground has been removed from the RCM. If a SRS fault exists, the RCM sends a message to the instrument cluster (IC) module to flash a coded sequence which is called a LFC on the air bag warning indicator. Each LFC is flashed 3 times after which the air bag indictor will remain lit for the remainder of the key-on cycle. In addition to the self-test at start up, the RCM continuously monitors all of its external and internal circuitry for faults.

In a frontal collision, the impact sensor located in the front of the vehicle detects sudden deceleration and sends an electrical signal to the RCM. The RCM uses the information from the impact sensor in deployment determination. If the RCM determines that deployment is required, the RCM sends voltage and current to the squib(s) causing the solid chemical propellant to undergo a rapid chemical reaction. This controlled reaction produces harmless nitrogen gas that fills the air bag(s) and/or activates the safety belt pretensioners to remove slack from the safety belt(s).

The fact that the pretensioners or airbags did not activate for both front seat occupants in a collision does not mean that something is wrong with the system.

The RCM communicates through the data link connector (DLC) the current and historical DTCs on the high-speed controller area network (HS-CAN). The RCM also communicates over the HS-CAN to the IC module and PCM.

Air Bag Warning Indicator
The air bag warning indicator:
- is located in the instrument cluster (IC) module.
- is a function of the IC module. The IC module will prove out the air bag warning indicator by illuminating it for 6 seconds. If no faults are detected by the RCM the IC module will turn the air bag warning indicator off and it will remain off. If the RCM detects a fault it will send a message to the IC module to turn the air bag warning indicator on and it will remain on for the rest of the key cycle.
- will illuminate and/or flash based on the message the IC module receives from the RCM.
- will illuminate if the IC module does not receive a message from the RCM.

Clockspring
The clockspring:
- is mounted on the steering column, behind the steering wheel.
- allows for continuous electrical connections between the driver air bag module and the RCM when the steering wheel is turned.

Driver Air Bag Module
The driver air bag module:
- is installed as an assembly.
- is single stage.
- is mounted in the center of the steering wheel.

Electrical System
The electrical system that supports the air bag supplemental restraint system (SRS):
- is powered from the battery through the ignition circuit.
- provides the electrical path from the restraints control module (RCM) to the SRS components.
- provides a communication path from the RCM to the air bag warning indicator, located in the instrument cluster (IC) module, via the high-speed controller area network (HS-CAN).
- provides the electrical communication path from the RCM to the data link connector (DLC) and all other modules on the HS-CAN.

High-Speed Controller Area Network (HS-CAN)
This vehicle utilizes a communication system called a high-speed controller area network (HS-CAN). The HS-CAN consists of a twisted pair of wires connected to the following:
- ABS module
- Instrument cluster (IC) module
- PCM
- Restraints control module (RCM)
- Data link connector (DLC)

The HS-CAN circuits use a bias voltage of approximately 2.5 volts, one is a positive 2.5 volts while the other is a negative 2.5 volts. The HS-CAN also uses 2 terminating resistors, one contained within the PCM, the other in the IC module. The terminating resistors are not serviced separately. The terminating resistors have a value of 120 ohms each, for a normal operating system total of 60 ohms. The HS-CAN may operate with only one terminating resistor and may communicate some messages to some of the control modules with only one circuit functioning. Refer to Information Bus (Module Communications Network).

Impact Sensors

WARNING: If a vehicle has been in a crash, inspect the restraints control module (RCM) and the impact sensor (if equipped) mounting areas for deformation. If damaged, restore the mounting areas to the original production configuration. A new RCM and sensors must be installed whether or not the air bags have deployed. Failure to follow these instructions may result in serious personal injury or death in a crash.

For this vehicle line, the supplemental restraint system (SRS) uses one satellite sensor in addition to the restraints control module (RCM). The RCM is mounted beneath the driver seat. All vehicles have one front impact severity sensor located in the front-center area of the vehicle, behind the grille mounted on the lower radiator support. Mounting orientation is critical for correct operation of all impact sensors.

Loops/Squibs
All deployable devices contain an initiating device called a squib. The squib is part of the deployment loop. Air bag/safety canopy modules can contain more than one squib, some vehicles may have up to 4 squibs in one air bag module. Squibs are often referred to as loops during the diagnostic process.

Passenger Air Bag Module
The passenger air bag module:
- is installed as an assembly.
- is single stage.
- is mounted in the passenger side of the instrument panel.

Restraints Control Module (RCM)

WARNING: If a vehicle has been in a crash, inspect the restraints control module (RCM) and the impact sensor (if equipped) mounting areas for deformation. If damaged, restore the mounting areas to the original production configuration. A new RCM and sensors must be installed whether or not the air bags have deployed. Failure to follow these instructions may result in serious personal injury or death in a crash.

CAUTION: Prior to removal of the restraints control module (RCM), it is necessary to upload module configuration information to the scan tool. This information needs to be downloaded into the new RCM module once installed. Refer to Information Bus (Module Communications Network).

NOTE: When installing a new restraints control module (RCM), always make sure the correct RCM is being installed. If an incorrect RCM is installed, erroneous DTCs will result.

The RCM carries out the following functions:
- Deploys the air bag(s) in the event of a deployable collision
- Activates the safety belt buckle pretensioners to remove slack from the safety belt in the event of a deployable collision
- Monitors the supplemental restraint system (SRS) for faults
- Sends a message to the instrument cluster (IC) module to illuminate the air bag warning indicator if a fault is detected
- Communicates through the data link connector (DLC) the current and/or historical DTCs

The RCM monitors the SRS for possible faults. If a fault is detected, the RCM will request the IC module to illuminate the air bag warning indicator. When the ignition is turned OFF and then ON, the IC module will prove out the air bag warning indicator by lighting for 6 seconds. If no faults are detected by the RCM the IC module will turn the air bag warning indicator off and it will remain off. If the RCM detects a fault it will send a message to the IC module to turn the air bag warning indicator on and it will remain on for the rest of the key cycle. If the RCM requests illumination of the air bag warning indicator and the air bag warning indicator does not function, the IC module will automatically activate an audible chime. The chime is a series of 5 sets of 5 tone bursts. If the chime is heard, the SRS and the air bag warning indicator require repair.

The RCM includes a backup power supply. The backup power supply will deploy the SRS components in the event that the ignition circuit is lost or damaged during impact. The backup power supply will deplete its stored energy approximately one minute after power and/or ground has been removed from the RCM. The backup power supply will deplete its stored energy approximately one minute after the battery ground cable is disconnected.

The RCM will also communicate the on-demand (current) and continuous (historical) DTCs through the DLC, to the scan tool.

Safety Belt Buckle Switch
As part of the supplemental restraint system (SRS), the driver safety belt buckle is equipped with a safety belt buckle switch. The safety belt buckle switch indicates to the restraints control module (RCM) whether the driver safety belt is buckled or unbuckled.

Safety Belt Pretensioners
As part of the supplemental restraint system (SRS), the driver and front outboard passenger safety belt buckles are equipped with pretensioners. The safety belt buckle pretensioners remove excess slack from the safety belt webbing. The pretensioners are activated by the restraints control module (RCM) when the module detects a collision event force exceeding a programmed limit.
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