Inflatable Restraint Sensing and Diagnostic Module (SDM)

The sensing and diagnostic module (SDM) performs the following functions within the SIR system:

Energy Reserve: The SDM maintains a 23 Volt Loop Reserve (23 VLR) energy supplies in order to provide deployment energy. If ignition voltage is lost during a frontal crash, the 23 VLR supply deploys the air bags.

Frontal Crash Detection: The SDM monitors the vehicle velocity changes in order to detect frontal crashes that are severe enough to warrant deployment.

Air Bag Deployment: When a frontal crash of sufficient force is detected, the SDM causes enough current to flow through the inflator modules in order to deploy the air bags.

Frontal Crash Recording: The SDM records information regarding the SIR system status during a frontal crash.

Malfunction Detection: The SDM performs diagnostic monitoring of the SIR system electrical components. The SDM sets a diagnostic trouble code when the SDM detects a malfunction.

Malfunction Diagnosis: The SDM displays SIR diagnostic trouble codes and system status information through the use of a scan tool.

Driver Notification: The SDM warns the vehicle driver of SIR system malfunctions by controlling the AIR BAG warning lamp.

The SDM connects to the SIR wiring harness using a 12-way connector. The SDM harness connector uses a shorting bar across certain terminals in the contact area. If you remove part of the SDM connector position assurance (CPA) or the harness connector, the AIR BAG warning lamp connects to ground through the shorting bar. The AIR BAG warning lamp will turn ON steady with the power applied to the SDM when either 1 of the following 2 conditions exists:

The SDM receives power whenever the ignition switch is at the ON or the START positions.

AIR BAG Warning Lamp

The ignition switch applies ignition voltage to the AIR BAG warning lamp. The AIR BAG warning lamp receives power whenever the ignition switch is at the ON or the START positions. The inflatable restraint SDM controls the lamp by providing ground with a lamp driver. The SIR system uses the AIR BAG warning lamp in order to perform the following actions:

The AIR BAG warning lamp notifies the driver of SIR system malfunctions. For proper lamp operation, refer to SIR Diagnostic System Check .

Inflatable Restraint Steering Wheel Module Coil

The inflatable restraint steering wheel module coil consists of 2 or more current-carrying coils. These coils are attached to the steering column. Two of the current-carrying coils allow rotation of the steering wheel while maintaining continuous contact of the driver deployment loop to the inflatable restraint steering wheel module.

A shorting bar is placed on the yellow 2-way connector near the base of the steering column. This connector joins the SIR coil to the SIR wiring harness. The shorting bar shorts the circuits to the SIR coil and the inflatable restraint steering wheel module whenever you disconnect the yellow 2-way connector. The circuit to the inflatable restraint steering wheel module is shorted in this way in order to help prevent an unwanted deployment of the air bag when servicing the steering column or other SIR system components.

Inflator Modules

The inflator modules consist of an inflatable bag and an inflator (a canister of gas-generating material and an initiating device). The initiator is part of the deployment loop.

When the vehicle is in a frontal crash of sufficient force, the inflatable restraint SDM causes a current to flow through the deployment loops. Current passing through the initiator ignites the material in the inflator module. The gas produced from this reaction rapidly inflates the air bag.

There is a shorting bar on the inflatable restraint steering wheel module side of the upper steering column connector that connects the inflatable restraint steering wheel module coil to the inflatable restraint steering wheel module. The shorting bar shorts across the inflatable restraint steering wheel module circuits when you disconnect the upper steering column connector. The circuit to the inflatable restraint steering wheel module is shorted in this way in order to help prevent unwanted deployment of the air bag while you are servicing the inflatable restraint steering wheel module, the steering column, or the other SIR system components.

A shorting bar is placed on the inflatable restraint IP module connector that connects to the SIR wiring harness. This shorting bar shorts across the inflatable restraint IP module circuits when the inflatable restraint IP module connector disconnects. The circuit to the inflatable restraint IP module is shorted in this manner in order to help prevent unwanted deployment of the air bag while you are servicing the inflatable restraint IP module, the instrument panel, or the other SIR system components.

Inflatable Restraint Front End Discriminating Sensor

The presence of an inflatable restraint front end discriminating sensor enhances the SIR system performance. Use of the inflatable restraint front end discriminating sensor provides timely deployment of the air bags during some crashes. Although this is a mechanical sensor, the sensor is not part of the deployment loop but instead only provides an input signal to the inflatable restraint sensing and diagnostic module (SDM).

Steering Column

The steering column is energy absorbing. The steering column is designed to compress in a frontal crash, decreasing the chance of injury to the driver.

Knee Bolsters

The knee bolsters absorb energy. These bolsters limit leg movement by controlling the forward movement of the vehicle's front seat occupants during a frontal crash.

Definitions

Air Bag: An inflatable cloth cushion designed in order to deploy in certain frontal crashes. The air bags distribute the impact load more evenly over the occupant's head and torso in order to supplement the safety belt protection.

Asynchronous: An event that may occur at any time without a warning and without falling within a defined time period.

B+: The battery voltage available at the time of the indicated measurement. With the ignition in RUN and the engine stopped, the voltage is usually 11.5 volts to 12.5 volts. With the engine at idle, the voltage may be 14.0 volts to 16.0 volts. During engine cranking, the voltage may be as low as 10.0 volts.

Bulb Test: The inflatable restraint sensing and diagnostic module (SDM) will cause the air bag warning indicator to flash 7 times. Under normal operation, this occurs when the ignition is turned from OFF to RUN. The bulb test will also occur if Ignition Positive Voltage exceeds 17.1 volts and then returns within the normal operating voltage range. A malfunction may prevent the module from flashing the indicator.

Continuous Monitoring Test: The inflatable restraint sensing and diagnostic module (SDM) continuously monitors the SIR system during each 100 millisecond interval. The IGNITION POSITIVE VOLTAGE at the module must be with the normal operating voltage range for the continuous monitoring to occur. These tests follow the power-on tests.

Data Link Connector (DLC): The DLC electrically connects to many on-board computers and allows communication with an off-board computer such as a scan tool.

Datum Line: A base line parallel to the plane of the vehicle underbody or the frame. All vertical measurements originate from this base line.

Deploy: To inflate the air bag

Deployment Loop Continuity Test: The inflatable restraint sensing and diagnostic module (SDM) performs this test in order to inspect for voltage variations in the Driver High/Low and the Passenger High/Low circuitry. The module first measures the Ignition Positive Voltage and the 23 VLR in order to verify that the voltage is within specifications. The module then performs the deployment loop continuity test. Detection of a malfunction during these tests may prevent the resistance measurement test from occurring until the next ignition cycle. This test is part of the power-on tests and occurs before the continuous monitoring tests.

Deployment Loops: The circuits that supply the current to the inflator modules for air bag deployment

Diagnostic Trouble Code (DTC): An alphanumeric designator used by the inflatable restraint sensing and diagnostic module (SDM) in order to indicate specific SIR system malfunctions.

Driver Current Sink: An output of the inflatable restraint sensing and diagnostic module (SDM) that supplies a low resistance path to ground for the inflatable restraint steering wheel module circuit.

Driver Current Source: An output of the inflatable restraint sensing and diagnostic module (SDM) that supplies current into the inflatable restraint steering wheel module circuit.

EEPROM (Electrically Erasable Programmable Read Only Memory) : Memory that does not require power to the inflatable restraint sensing and diagnostic module (SDM) in order to retain the contents.

Higher Priority Fault: Each DTC has an assigned priority based on the detectability with other DTCs present. The priority corresponds to the detectability of the malfunction only. This does not relate to the seriousness of the malfunction with respect to deployment or non deployment under any given condition.

Ignition Cycle: The operation of the ignition switch causes this cycle to occur. The inflatable restraint sensing and diagnostic module (SDM) must first sense Ignition Positive Voltage input greater than 8.2 volts with the ignition switch in RUN. The Ignition Positive Voltage input voltage must remain above 8.2 volts for at least 10 seconds before turning the ignition switch to OFF.

Ignition Positive Voltage: A B+ circuit receiving power with the ignition in RUN or START.

Inflatable Restraint Front End Discriminating Sensor: A sensor mounted on the front of the vehicle that supplies an input signal to the inflatable restraint sensing and diagnostic module (SDM).

Inflatable Restraint IP Module: An assembly located on the right side of the instrument panel. The assembly consists of an inflatable air bag, an inflator, and an initiator.

Inflatable Restraint Sensing and Diagnostic Module: The SDM provides reserve energy to the deployment loops, deploys the air bags when required, and performs diagnostic monitoring of all SIR system components.

Inflatable Restraint Steering Wheel Module: An assembly located in the steering wheel hub consisting of an inflatable bag, an inflator, and an initiator.

Inflatable Restraint Steering Wheel Module Coil: An assembly of 2 or more current carrying coils mounted within the steering column hub. The assembly allows the rotation of the steering wheel while maintaining continuous electrical circuit contact. Two of the coils provide continuous contact of the driver deployment loop to the inflatable restraint steering wheel module.

Initiator: Located in the inflator module, the initiator initiates the chemical reaction that inflates the air bag when sufficient current flows through the component.

Normal Operating Voltage Range: The range is 8.2 volts to 17.1 volts when measuring between the inflatable restraint sensing and diagnostic module (SDM) Ignition Positive Voltage terminal and ground.

Passenger Current Sink: An output of the inflatable restraint sensing and diagnostic module (SDM) that supplies a low resistance path to ground for the inflatable restraint IP module circuit.

Passenger Current Source: An output of the inflatable restraint sensing and diagnostic module (SDM) that supplies current into the inflatable restraint IP module circuit.

Power-on Test: Verifies that the inflatable restraint sensing and diagnostic module (SDM) performs on the SIR system once per ignition cycle. These tests occur immediately after the module receives Ignition Positive Voltage, and before the continuous monitoring tests.

Resistance Measurement Test : A test that the inflatable restraint sensing and diagnostic module (SDM) performs once per ignition cycle in order to measure the resistance of the deployment loops. The module first measures Ignition Positive Voltage, 23  VLR, and the deployment loop voltages in order to verify readings within specifications. The module then sources a constant current into the loop. The module measures the voltage drop across the loop and converts the value to a resistance value. This resistance value is within the range of 0.0 - 6.3 ohms. Upon the detection of an open circuit, the module aborts the tests in order to prevent the calculation of the resistances until the next ignition cycle. This test also tests for proper operation of the driver/passenger current sources. This test is part of the power-on tests and occurs before the continuous monitoring tests.

Scan Tool: An off-board computer that reads on-board computer diagnostic information through the use of a DLC.

Serial Data: The information communicated to an off-board computer using the DLC. Some of this data represents the status of the SIR system.

SIR: Supplemental inflatable restraint.

SIR Wiring Harness: The wires and connectors that electrically connect the components in the SIR system.

23 VLR: The 23 volt loop reserve. The energy supply that is internal to the inflatable restraint sensing and diagnostic module (SDM).