SIR System Component Description and Definitions. Chevrolet Metro 1999

For 1990-2009 cars

### 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 in order to provide deployment energy. If ignition voltage is lost during a frontal crash, the 23-volt loop reserve causes deployment of the air bags.

• | Frontal Crash Detection -- The SDM monitors the changes in the vehicle's speed in order to detect frontal crashes which are severe enough to cause deployment.

• | Air Bag Deployment -- If a frontal crash of sufficient force occurs, the SDM causes deployment of 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 if 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 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 SDM Connector Position Assurance (CPA) or the harness connector, the AIR BAG warning lamp will connect to ground through the shorting bar. The AIR BAG warning lamp will illuminate with the power to the SDM if either one of the following two conditions exists:

• | You remove the SDM CPA

• | You disconnect the SDM harness connector

The SDM receives power if the ignition switch is at the ON or at 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 when the ignition switch is at the ON or at 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 functions:

• | Verifies lamp operation and SDM operation by flashing the lamp 7 times when the igniton switch is in the ON position.

• | Warn the vehicle driver of any SIR electrical system malfunctions which could potentially affect the operation of the SIR system. These malfunctions could result in non-deployment during a frontal crash or in an accidental deployment.

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

### Inflatable Restraint Steering Wheel Module Coil

The inflatable restraint steering wheel module coil (1) consists of two or more coils that carry current. These coils are attached to the steering column. Two of the current-carrying coils allow rotation of the steering wheel and maintain continuous contact of the driver deployment loop to the inflatable restraint steering wheel module.

A shorting bar (5) is placed on the yellow 2-way connector (4) 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 to the inflatable restraint steering wheel module when you disconnect the yellow 2-way connector. The circuits to the inflatable restraint steering wheel module short in order to prevent an unwanted deployment of the air bag when servicing the steering column or the SIR components.

### Inflatable Restraint Forward Discriminating Sensor

The inflatable restraint forward discriminating sensor is used by the SDM in order to determine if a frontal crash requires deployment of the air bags. The sensor consists of the following components:

• | A sensing element

• | Normally open switch contacts

• | A diagnostic resistor

If the changes in vehicle speed are severe enough, the sensing element closes the switch contacts in order to cause deployment. The diagnostic resistor is connected in parallel with the switch contacts inside the sensor. The diagnostic resistor and the resistors in the SDM cause a voltage drop that allows the SDM to monitor the inflatable restraint forward discriminating sensor and the associated circuitry.

If the vehicle is in a frontal crash of sufficient force, the SDM allows current through the deployment loops. The current that passes through the initiator ignites the material in the inflator module (2). The gas from this reaction quickly inflates the air bag (1).

A shorting bar is placed on the connector of the inflatable restraint IP module (1) that connects to the SIR wiring harness. If the connector of the inflatable restraint IP module (1) is disconnected, the shorting bar shorts across the circuits of the inflatable restraint IP module (1). This prevents inadvertent deployment of the air bag during service to the module, to the instrument panel (2), or to the other components of the SIR system.

### Steering Column

The steering column is energy-absorbing. The wheel compresses in a frontal crash. This decreases the chance of injury to the driver. A steering column reinforcement is also used for additional support.

### Knee Bolsters

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

### Definitions

23-Volt Loop Reserve: The 23-volt loop reserve is an energy supply internal to the inflatable restraint SDM.
Air Bag: The air bag is an inflatable cloth cushion that inflates in certain frontal crashes. This cushion supplements the seat belt protection. The cushion evenly distributes the impact over the vehicle occupant's head and over the vehicle occupant's torso.
Asynchronous: An operation that occurs in a non-period manner, or at a time or at an interval that is not specified.
\x94B+\x95: The voltage available at the battery during the indicated measurement. With the key turned to the ON position and with the engine not running, the system voltage will probably be 12-12.5 volts. At idle the voltage may be 14-16 volts. The voltage could only be 10 volts during engine cranking.
Bulb Test: The instrument cluster will flash the AIR BAG warning lamp seven times when the ignition switch is turned from OFF to ON. The SDM commands the instrument cluster to turn the warning lamp OFF through serial data communication. This indicates that malfunctions are not present in the SIR system.
Continuous Monitoring Tests: When IGNITION 1 voltage is in the normal operating range at the SDM, these tests are performed by the inflatable restraint SDM on the SIR system every 100 milliseconds.
Data Link Connector (DLC): A connector which is connected by wires to multiple on-board computers allowing communication with an off-board computer, such as a scan tool.
Datum Line: A baseline parallel to the plane of the underbody or of the frame. All vertical measurements are based on this line.
Deploy: To inflate the air bag.
Deployment Loop Continuity: The SDM performs this test when IGNITION 1 voltage and the 23-volt loop reserve are within the correct ranges. This test checks the differences in voltages between the high-side circuits and the low-side circuits.
Deployment Loops: These circuits supply current to the inflator modules in order to deploy the air bags.
Diagnostic Trouble Code (DTC): An alphanumeric designator that is used by the SDM in order to indicate specific malfunctions in the SIR system.
Driver Current Sink: An output of the SDM that supplies a low-resistance path to ground for the circuit of the inflatable restraint steering wheel module
Driver Current Source: An output of the SDM which supplies current to the circuit of the inflatable restraint steering wheel module
EEPROM: Electronically erasable programmable read-only memory is memory that is retained if power is removed from the SDM.
Higher Priority Fault: Each diagnostic trouble code is assigned a priority. DTCs that are more detectable have a higher priority. The priority corresponds to how detectable the malfunction is. The priority does not relate to how important the malfunction is with respect to correct operation of the system.
Ignition Cycle: The voltage at the SDM input of IGNITION 1 is more than 8.2 volts for at least 10 seconds with the ignition switch in the ON position.
IGNITION 1: A battery voltage (B+) circuit which is only powered with the ignition switch in the ON position or in the START position.
Inflatable Restraint IP Module: An assembly in the right side of the instrument panel (IP) that includes an inflatable bag, an inflator, and an initiator.
Inflatable Restraint Steering Wheel Module Coil: An assembly of two current-carrying coils in the driver deployment loop allows rotation of the steering wheel and maintains the continuous contact of the driver deployment loop to the inflatable restraint steering wheel module.
Inflatable Restraint Steering Wheel Module: An assembly in the steering wheel hub that includes an inflatable bag, an inflator, and an initiator.
Initiator: The electrical component inside the inflator module. When sufficient current flows, this component sets off the chemical reaction that inflates the air bag.
Normal Operating Voltage Range: The voltage between the SDM's IGNITION 1 terminal and the ground terminal is 8.2-17.2 volts.
Passenger Current Sink: An output of the SDM which supplies a low resistance path to ground for the circuit of the inflatable restraint IP module.
Passenger Current Source: An output of the SDM which supplies current to the circuit of the inflatable restraint IP module.
Resistance Measurement Test: The SDM performs this test when the IGNITION 1 voltage, the 23-volt loop reserve, and the deployment loop voltage are within the correct ranges. This test checks for correct resistance of the deployment loops, and for correct operation of the driver current source and of the passenger current source. If an open is detected, this test stops before resistances are calculated. However, if the open clears, the test will be completed, and resistances will be checked.
Scan Tool: An off-board computer that reads diagnostic information from on-board computers via the data link connector.
Serial Data: Information that represents the status of the SIR system.
SIR: Supplemental Inflatable Restraint
SIR Wiring Harness: The wires and the connectors that connect the components in the SIR system.
Turn-on Tests: The inflatable restraint SDM performs these tests on the SIR system once during each ignition cycle. The tests are performed immediately after the IGNITION 1 voltage is applied to the SDM, and before the Continuous Monitoring Tests.