Principles of Operation
Air Bag and Safety Belt Pretensioner Supplemental Restraint System (SRS)
Principles of Operation
NOTE: Some postal vehicles are NOT equipped with the following components:
- Front passenger seat
- Occupant Classification System (OCS) and module
- Passenger air bag deactivation indicator
- Passenger dual-stage air bag module
- Passenger side air bag
- Passenger side impact sensor
- Passenger safety belt buckle and buckle switch
- Passenger safety belt retractor pretensioner
- Passenger safety belt tension sensor
The Restraints Control Module (RCM) continuously receives/monitors all inputs from the following Supplemental Restraint System (SRS) components:
- LH and RH front impact severity sensor
- Driver and passenger side impact sensor
- Safety belt buckle switches
- Driver seat track position sensor
- Occupant Classification System Module (OCSM)
If the RCM (Restraints Control Module) detects sudden vehicle deceleration and/or lateral deceleration based on the information received from sensors and switches and determines that deployment is required, the RCM (Restraints Control Module) sends voltage and current to deploy the appropriate SRS (Supplemental Restraint System) components.
During a frontal or side crash, the RCM (Restraints Control Module) may deploy the following SRS (Supplemental Restraint System) components, based on crash severity and sensor input.
- Safety belt retractor pretensioner(s)
- Seat side air bag(s)
- Driver/passenger dual-stage air bag (one or both stages)
The fact that the pretensioners or air bags did not activate for both front seat occupants in a crash does not mean that something is wrong with the system.
The RCM (Restraints Control Module) performs a self-test of the complete SRS (Supplemental Restraint System) during each startup. If a SRS (Supplemental Restraint System) fault exists, the air bag warning indicator will illuminate and remain illuminated for the rest of the ignition cycle. In addition to the self-test at start up, the RCM (Restraints Control Module) continuously monitors all SRS (Supplemental Restraint System) components and circuitry for correct operation.
Air Bag Warning Indicator
The air bag warning indicator:
- located in the Instrument Cluster (IC) module, will prove out by lighting for 6 seconds and then turn off.
- will flash and/or illuminate based on the message the IC (Instrument Cluster) module receives from the RCM (Restraints Control Module).
- will illuminate if the IC (Instrument Cluster) module does not receive a message from the RCM (Restraints Control Module).
Air Bag Module Second Stage Deployment Check
Because the driver and passenger front air bags each have 2 deployment stages, it is possible that stage 1 has deployed and stage 2 has not.
If a front air bag module has deployed, it is mandatory that the front air bag module be remotely deployed using the appropriate air bag disposal procedure.
- For information on driver air bag module and/or passenger air bag module remote deployment, refer to Pyrotechnic Device Disposal Service and Repair.
Clockspring
The clockspring allows for continuous electrical connections between the driver air bag module and the RCM (Restraints Control Module) when the steering wheel is turned.
Driver Air Bag Module
The driver air bag module is a dual-stage air bag, upon receiving a flow of current from the RCM (Restraints Control Module), deploying at 1 of 2 different rates depending upon vehicle impact severity and sensor input.
Medium Speed Controller Area Network (MS-CAN)
This vehicle utilizes a communication system called a Medium Speed Controller Area Network (MS-CAN). The RCM (Restraints Control Module) communicates with various modules for required information about the vehicle. for additional information about the MS-CAN (Medium Speed Controller Area Network).
Occupant Classification System (OCS) System
The OCS (Occupant Classification System) system is found only on the front passenger seat. The OCS (Occupant Classification System) system is comprised of a silicone gel-filled bladder mounted between the seat cushion foam and half pan, a pressure sensor and OCSM (Occupant Classification System Module) that are mounted to the cushion half pan. Pressure is applied to the OCS (Occupant Classification System) system bladder when weight of any occupant or object in the front passenger seat is present. The pressure is then transferred through a tube, sensed by the OCS (Occupant Classification System) system pressure sensor, then electronically communicated to the OCSM (Occupant Classification System Module).
The OCS (Occupant Classification System) system is also used for operation of the passenger Belt-Minder(R). For information on the passenger Belt-Minder(R) feature, refer to Seat Belt Systems. To deactivate or reactivate the passenger Belt-Minder(R) feature, refer to Instrument Panel, Gauges and Warning Indicators or the Owner's Literature.
Belt Tension Sensor (BTS)
The Belt Tension Sensor (BTS) :
- is a 3-wire Hall-effect sensor located at the safety belt anchor point and is part of the front passenger safety belt and retractor assembly.
- is used in conjunction with the OCS (Occupant Classification System) system.
The BTS (Belt Tension Sensor) is used by the OCS (Occupant Classification System) system to identify the presence of a child safety seat on the front passenger seat. The BTS (Belt Tension Sensor) senses the tension on the safety belt assembly then provides an output to the OCSM (Occupant Classification System Module), indicating that the safety belt assembly is cinched. After sensing the weight applied to the seat by the occupant or using the safety BTS (Belt Tension Sensor) input, the OCSM (Occupant Classification System Module) determines how the occupant should be classified and communicates this information to the RCM (Restraints Control Module). If the occupant is classified as a child, the RCM (Restraints Control Module) will then automatically deactivate the passenger air bag module and illuminate the Passenger Air Bag Deactivation (PAD) indicator.
Passenger Air Bag Deactivation (PAD) Indicator
The RCM (Restraints Control Module) controls the state of the PAD (Passenger Air Bag Deactivation) indicator, based on information provided by the OCSM (Occupant Classification System Module) system. The PAD (Passenger Air Bag Deactivation) indicator is illuminated to indicate the passenger air bag module is disabled. An exemption to this is when the front passenger seat is determined to be empty, and therefore indication of a deactivated passenger air bag module is not necessary. In all other cases, the PAD (Passenger Air Bag Deactivation) indicator is off when the passenger air bag module is enabled.
The RCM (Restraints Control Module) briefly activates the PAD (Passenger Air Bag Deactivation) indicator to prove-out the indicator function and verify to the front occupants correct function operation of the PAD (Passenger Air Bag Deactivation) indicator.
The following table indicates the passenger air bag status and the PAD (Passenger Air Bag Deactivation) indicator status based on the size of the front passenger occupant.
Passenger Air Bag and Passenger Air Bag Deactivation (PAD) Indicator Status
Passenger Air Bag Module
The passenger air bag module is a dual-stage air bag, deploying at 1 of 2 different rates depending upon vehicle impact severity and sensor input.
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.
The RCM (Restraints Control Module) carries out the following functions:
- Deploys the air bag(s) in the event of a deployable collision
- Activates the driver and passenger safety belt retractor pretensioner(s) to control the tension of the driver and passenger safety belts in the event of a deployable collision
- Monitors the SRS (Supplemental Restraint System) for faults
- Sends a message to the IC (Instrument Cluster) module to illuminate the air bag warning indicator if a fault is detected
The RCM (Restraints Control Module) monitors the SRS (Supplemental Restraint System) for possible faults. If a fault is detected, the RCM (Restraints Control Module) will request the IC (Instrument Cluster) module to illuminate the air bag warning indicator. When the ignition is turned OFF and then ON, the IC (Instrument Cluster) module will prove out the air bag warning indicator by lighting for 6 seconds. If no faults are detected by the RCM (Restraints Control Module), the IC (Instrument Cluster) module will turn the air bag warning indicator off and it will remain off. If the RCM (Restraints Control Module) detects a fault, it will send a message to the IC (Instrument Cluster) module to turn the air bag warning indicator on and it will remain on for the rest of the ignition cycle. If the RCM (Restraints Control Module) requests illumination of the air bag warning indicator and the air bag warning indicator does not function, the IC (Instrument Cluster) module will automatically activate an audible chime.
The RCM (Restraints Control Module) includes a backup power supply. This feature provides sufficient backup power to deploy the air bags 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 (Restraints Control Module).
Safety Belt Buckle Switches
The safety belt buckle switches indicate to the RCM (Restraints Control Module) whether the safety belts are buckled or unbuckled.
The RCM (Restraints Control Module) also communicates the driver safety belt buckle switch status to the IC (Instrument Cluster) module, which monitors the information to control the safety belt warning indicator. For information on the safety belt warning indicator, refer to Instrument Panel, Gauges and Warning Indicators.
Safety Belt Retractor Pretensioner
The safety belt retractor pretensioners remove excess slack from the safety belt webbing. The pretensioners are activated by the RCM (Restraints Control Module) when the module detects a crash event force exceeding a programmed limit.
Seat Track Position Sensor
The seat track position sensor indicates to the RCM (Restraints Control Module) the position of the seat, forward or rearward. The RCM (Restraints Control Module) uses this information in determining the deployment rate of the dual-stage driver/passenger air bag module.
Secondary Air Bag Warning (Chime)
The secondary air bag warning chime is an audible chime located in and controlled by the IC (Instrument Cluster) module. If a fault is detected with the air bag warning indicator, a DTC will be stored in memory of the IC (Instrument Cluster) module. After receiving the message from the RCM (Restraints Control Module) that a SRS (Supplemental Restraint System) fault has been detected, the IC (Instrument Cluster) module will sound the secondary air bag warning chime in a pattern of 5 sets of 5 beeps.
Impact Sensors
For this vehicle line, the SRS (Supplemental Restraint System) uses 4 satellite sensors in addition to the RCM (Restraints Control Module). The RCM (Restraints Control Module) is mounted to the center tunnel beneath the console. All vehicles have 2 front impact severity sensors located in the front of the vehicle. The side impact sensors are mounted at the base of each B-pillar. Mounting orientation is critical for correct operation of all impact sensors.
Seat Side Air Bag Module
The seat side air bag module:
- will deploy upon receiving a flow of current from the RCM (Restraints Control Module).
- is mounted in the seat backrest.