Principles of Operation

PRINCIPLES OF OPERATION

The supplemental restraint system (SRS) consists of a driver and passenger air bag module, safety belt retractor pretensioners and safety canopies (containing an inflator/squib or gas generator and an air bag), impact sensors, a restraints control module (RCM), a clockspring, an air bag warning indicator, occupant classification sensor (OCS) system and a passenger air bag deactivation (PAD) indicator lamp. 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 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 the battery ground cable is disconnected. If a SRS fault exists, the air bag warning indicator will illuminate and remain illuminated for the rest of the key 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 sensors located in the front of the vehicle detect the sudden deceleration and send an electrical signal to the RCM. The RCM uses the information from the impact sensors and the OCS system in the 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)/safety canopies and/or activates the safety belt pretensioners to remove slack from the safety belt(s).

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 instrument cluster (IC) module, PCM and the occupant classification system module (OCSM).

Air Bag Warning Indicator
The air bag warning indicator:
- is located in the instrument cluster (IC) module.
- lamp and circuitry prove out is a function of the IC module. The IC module will prove out the air bag warning indicator by lighting the air bag warning indicator for 6 seconds and then turn off.
- will flash and/or illuminate 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 restraints control module (RCM) when the steering wheel is turned.

Driver Air Bag Module
The driver air bag module:
- is installed as an assembly.
- is mounted in the center of the steering wheel.
- cannot be interchanged between Edge and MKX vehicles.

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
- Transmission control module (TCM)
- Restraints control module (RCM)
- Occupant classification system module (OCSM)
- Air suspension (vehicle dynamics module [VDM]) (if equipped)
- 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 6 satellite sensors in addition to the restraints control module (RCM). The RCM is mounted to the center tunnel beneath the console. All vehicles have 2 front impact severity sensors located in the front-center area of the vehicle, behind the grille mounted on the hood latch bracket. The first row impact sensors are mounted behind the trim panel near the floor on the B-pillar, the second row sensors are located on each C-pillar. 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.

Occupant Classification Sensor (OCS) System
The occupant classification sensor (OCS) system is found only on the front passenger seat. The front passenger OCS system is comprised of the following: 4 OCS weight sensor bolts that are mounted to each corner of the seat track and an occupant classification system module (OCSM) which is mounted to the electrical bracket underneath the seat cushion pan. All the components that make up the OCS system are all serviced separately. The weight of any occupant or object on the front passenger seat is electronically communicated to the OCSM. The RCM uses this information in determining if the passenger air bag module or passenger seat side air bag module is to be deployed in the event of a deployable collision.

The OCSM will inform the restraints control module (RCM), via a high-speed controller area network (HS-CAN), of the weight of any occupant or object on the front passenger seat.

The OCSM monitors the OCS system for faults and communicates on-demand and continuous DTCs via the data link connector (DLC) with the use of a scan tool.

The OCS 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.

When the front passenger seat is removed for service, the Zero Seat Weight Test must be carried out after the installation of the front passenger seat. When an OCS system component is installed new, the System Reset must be carried out after the installation of the front passenger seat. The Zero Seat Weight Test and/or System Reset must be carried out only as instructed to do so in the Vehicle System. For information on the Zero Seat Weight Test and/or System Reset, refer to Occupant Classification Sensor (OCS) System Zero Seat Weight Test or Occupant Classification Sensor (OCS) System Reset. Occupant Classification Sensor (OCS) System Zero Seat Weight Test Programming and Relearning

In the event of a deployable crash, a new passenger seat track with OCS weight sensor bolts must be installed. Refer to Occupant Classification Sensor - Manual Seat Track or Occupant Classification Sensor - Power Seat Track.

Passenger Air Bag Deactivation (PAD) Indicator
The passenger air bag deactivation (PAD) indicator is a visual indicator used to inform the front seat occupants of the passenger air bag deactivation state. The PAD indicator is a stand-alone lamp installed into the vehicle instrument panel in a position visible to each front seat occupant.

The restraints control module (RCM) controls the state of the PAD indicator through a direct hardwire connection, based on information provided by the occupant classification sensor (OCS) system. The PAD indicator illuminates 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 the passenger safety belt is unbuckled, therefore indication of a deactivated passenger air bag module is not necessary. In all other cases, the PAD indicator is unlit when the passenger air bag module is enabled.

When the ignition switch is in the ON position, the PAD indicator prove-out period is initiated by the RCM. The RCM briefly activates the PAD indicator to prove-out the indicator function and verify to the front occupants correct functional operation of the PAD indicator.

When the OCSM detects a fault that causes DTC B2290 and/or B1013 to set on-demand in the OCS system, the OCSM sends a message to the RCM. Upon receiving the fault message from the OCSM, the RCM disables the passenger air bag module, illuminates the PAD indicator indicating the passenger air bag module is disabled and sends a message to the instrument cluster (IC) module to illuminate the air bag warning indicator. The passenger air bag module will remain disabled until the cause for the on-demand DTC has been corrected. For information on the OCS system, refer to Occupant Classification Sensor System.

The following table indicates the passenger air bag status and the PAD indicator status based the size of the front outboard passenger occupant.

Passenger Air Bag And PAD Indicator Status:

Passenger Air Bag Module
The passenger air bag module:
- is installed as an assembly.
- is mounted in the passenger side of the instrument panel.
- cannot be interchanged between Edge and MKX vehicles.

Restraints Control Module (RCM)

WARNING:
- Do not handle, move or change the original horizontal mounting position of the restraints control module (RCM) while the RCM is connected and the ignition switch is ON. Failure to follow this instruction may result in the accidental deployment of the safety canopy and cause serious personal injury or death.
- 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) module, 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). Failure to follow these instructions may result in incorrect operation of the supplemental restraint system (SRS) and may cause system failure.

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)/safety canopies in the event of a deployable collision.
- activates the safety belt retractor 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 or historical DTCs.

The RCM monitors the SRS for possible faults. If a fault is detected, the RCM will request illumination of the air bag warning indicator.

When the ignition is cycled (turned OFF and then ON), the IC module will prove out the air bag warning indicator by lighting for 6 seconds and then turn off. If a current SRS fault exists, the RCM requests illumination of the air bag warning indicator and will remain illuminated for the rest of the key cycle. During that same key cycle, some faults that recover (Loss of Communication with the PCM), the RCM will send a request to the IC module to turn the air bag warning indicator off IF no other fault is present. The RCM will also communicate the on-demand (current) and continuous (historical) DTCs through the DLC, to the scan tool. 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. 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.

Safety Belt Buckle Switches
As part of the supplemental restraint system (SRS), the driver and front passenger safety belt buckles are equipped with safety belt buckle switches. The safety belt buckle switches indicate to the restraints control module (RCM) whether the safety belts are connected or disconnected. The RCM uses this information in determining the deployment of the dual-stage driver and passenger air bag modules.

Safety Belt Pretensioners
As part of the supplemental restraint system (SRS), the driver and front passenger safety belt retractors are equipped with pretensioners. The safety belt retractor 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 exceeding a programmed limit.

Safety Canopy Module
Vehicles are equipped with safety canopies for protection during side impacts or rollovers.

Safety canopies require a specific headliner. The word AIRBAG will appear on the headliner where it meets each B-pillar trim panel.

The safety canopy module:
- is installed as an assembly.
- is mounted above the headliner.
- attaches from the A-pillar frame to the C-pillar frame.
- is standard equipment.
- cannot be interchanged from side-to-side.

Seat Track Position Sensor
The seat track position sensor is mounted to a bracket attached to the inboard side of the driver seat track. The seat track position sensor informs the restraints control module (RCM) of the driver seat position. The RCM uses this information in determining the deployment of the dual-stage driver air bag module.

Side Air Bag Module

NOTE: References to side air bag modules refer to the seat side air bag module, not to the steering wheel or instrument panel mounted air bag components of the supplemental restraint system (SRS).

A side air bag module provides protection of the thorax area (between the neck and abdomen) of the body, working in conjunction with the head protection provided by a safety canopy module.

The side air bag module:
- will deploy upon receiving a flow of current from the restraints control module (RCM), initiated by the side impact sensor and internal RCM circuitry.
- is installed as an assembly.
- is mounted in the driver or passenger seat backrest.
- is used in conjunction with a safety canopy module.

Secondary Air Bag Warning (Chime)
The secondary air bag warning, is an audible tone controlled by the instrument cluster (IC) module. If a fault is detected with the air bag warning indicator a DTC will be stored in memory of the IC module. Upon receiving the message from the restraints control module (RCM) that a supplemental restraint system (SRS) fault has been detected, the IC module will sound the secondary air bag warning chime in a pattern of 5 sets of 5 beeps.