GM SRS-Airbag System Operation 1993-2011 1993 to 1999 GM Cars Seat Belt and SRS Systems: Description and Operation CIRCUIT OPERATION: The purpose of the Passive Restraint Control Module is to allow the vehicle doors to be opened without restriction by the Seat Belts. Without this feature, the Retractor Lock Pendulums would lock the belts as they are pulled out of the retractors when a front door is opened for entry or for exit. This would make it difficult to open the door. To overcome this difficulty, the seat belt locking mechanism is controlled by the Passive Restraint Control Module which controls the operation of the Seat Belt Retractor Release Solenoids. The Passive Restraint Control Module receives power from the ELEC Fuse. When a door handle switch is operated or a door is open, the associated Door Latch Switch will close. The module will close its output switch and energize all the Seat Belt Solenoids for both doors. The door can then be opened without interference from the Seat Belts. When the door is closed, the Latch Switch opens again and the solenoids are de-energized after about three seconds. The belts are now ready to protect the wearer. To ensure that the solenoids are released, a vehicle speed signal is also supplied to the Module. The Module will release the solenoids any time the vehicle is moving, even if a Latch Switch is closed. If a Latch Switch fails to open and the speed signal causes the solenoids to release, the solenoids cannot be energized again until the switch opens to reset the Module. WARNING: A shorted Latch Switch will keep the Module from resetting, so both Seat Belts will interfere with door opening. An open Latch Switch will still allow the system to operate properly when the other door is used. AIR BAGS or SRS SYSTEM: On vehicles which have experienced an air bag system deployment, certain air bag system components must be replaced. To determine which air bag system components require replacement, refer to the specification replacent chart, from the manufacturer.All system components should be inspected for dents, cracks, exposure to excessive heat and other damage. All air bag system wiring should be checked for chafing and interference with other vehicle components. The instrument panel should also be inspected, as it plays a major role in SRS systems. During repairs, a vehicle system should be disarmed as described in the AB disabling section of this software. Also, when performing service procedures, do not expose components or wiring to heat guns, welding or spray guns. WARNING: Proper operation of sensors and SIR system requires that any repairs to the vehicle structure must return it to the original production configuration. Deployment requires, at a minimum, replacement of the sensors in the area of accident damage, inflator module and dimensional inspection of the steering column. Any damage to the SRS module mounting brackets requires replacement. Any visible damage to the sensors in the area of accident damage must be replaced and the steering column must be inspected. If any system components are damaged, they must be replaced. If SIR component mounting points are damaged, they must be repaired or replaced.1. Never use SIR parts from another vehicle. This does not include remanufactured parts available from GM and third party vendors.2. Do not attempt to service discriminating sensors, arming sensor, SRS computers, SIR coil assembly or inflator module. Service of these items is by replacement only.3. Verify part number of replacement detonator module. Some GM inflator modules look identical but contain different internal components.4. After deployment has occurred, SIR diagnostic trouble codes must be cleared using scan tool. This must be done to turn the air bag warning lamp off. On certain SRS modules, once an accident is stored the memory chip has to be replaced (SRS module rebuild). SRS DESCRIPTION: The Sensing and Diagnostic Module (SRS Computer) performs several system functions. These functions include back-up power supply, air bag deployment, malfunction detection, malfunction diagnosis, driver notification, frontal crash detection and frontal crash recording. OPERATION: The SDM contains a sensing device which converts vehicle velocity changes to an electrical signal. The electrical signal generated is processed by the SDM and then compared to a value stored in memory. When the generated signal exceeds the stored value, additional signals are compared to signals stored in memory. When two of the generated signals exceed the stored values or one of the generated signals exceeds the stored value and the forward discriminating sensor closes, the SDM will cause current to flow through the inflator modules deploying the air bags. DESCRIPTION: Discriminating Sensors and Arming Sensors. The discriminating and arming sensors are used by the SIR system to determine whether or not certain frontal crashes require deployment of the air bags. OPERATION: The sensor consists of a sensing element, normally open switch contacts and a diagnostic resistor. The sensing element closes the switch contacts when the vehicle velocity changes are severe enough to warrant air bag deployment. SRS TESTING: Use of a scan tool provides the following capabilities: Datastream readout, Read current and history diagnostic trouble codes. Clear the diagnostic trouble codes after a repair is completed. Make sure the scan tool contains an updated cartridge before attempting to communicate with the SIR system. To use the scan tool, connect it to the data link connector (DLC) and turn the ignition switch to the RUN position. The scan tool reads serial data sent from the inflatable restraint Sensing and Diagnostic Module (SDM) SERIAL DATA output terminal to the DLC terminal. However, a scan tool will not erase the crash memory, which is a permanent memory buffer used in insurance claims. To erase this memory, a memory chip must be replaced. 2000 - 2011 GM Cars Seat Belt and SRS Systems: Description and Operation Seat belts help to keep occupants inside the cabin and gradually reduce the impact forces during an accident. The SRS system protect during the following: Frontal impact type crashes, Rear impact type crashes, Side impact type crashes, Roll-over type crashes. All seat belt pre-tensioners have emergency locks. The pre-tensioners remain unlocked during normal operation and under normal driving conditions. The buckle locks remain unlocked during normal conditions in order to allow free movement of the upper body of each occupant. A pendulum locks the seat belt into position. The pendulum causes a locking pin to engage a cog on the spool of the pretensioner mechanism during an accident. A rapid extraction of the seat belt from the retractor, An abrupt change in the vehicle speed, An abrupt change in the vehicle direction, Operation of the vehicle on a steep upgrade, Operation of the vehicle on a steep downgrade. FRONT SEAT BELT SYSTEM The front seat belt system includes a driver and a passenger inflatable restraint seat belt retractor pretensioner and buckle. The driver seat belt system, includes a seat belt switch in the driver seat buckle which controls a reminder lamp and an alarm.When the driver seat belt is buckled, the driver's door is closed, and the ignition switch is turned ON, the tone alarm will be off. When the driver's seat belt is not buckled, the driver's door is closed, and the ignition switch is in ON position, the following condition will occur: The alarm will operate for 4-8 seconds and then go OFF. The reminder lamp will operate for 20 seconds, until the driver seat belt is buckled. REAR SEAT BELT SYSTEM The rear seat belt system includes the following components: The rear seat belt pretensioner is located under the rear trim panel and is attached to the rear body pillar. The center rear seat belt buckle and the outer seat belt buckle are located in the center of the seat cushion. CHILD SEAT RESTRAINT SYSTEM CAUTION: A child in a rear-facing child restraint can be seriously injured if the right-front passenger air bag inflates. This is because the back of a rear-facing child restraint would be very close to the inflating air bag. NEVER use a rear-facing child restraint in this vehicle. If a forward-facing child restraint is suitable for your child, ALWAYS move the front passenger seat as far back as it will go and then install the child restraint. Be sure the child restraint position does not conflict with any additional requirements provided by the manufacture. The child seat may only be used in a forward facing seating location. The child seat should be installed and secured according to the manufacturer's directions. If the child seat has a top strap, the seat will need to be anchored. Passengers should not be allowed to sit at locations where the seat belts are being used to secure the child seat.This seat belt system has an automatic locking feature, or cinch feature. The cinch feature is recommended for securing a child seat. The cinch feature is engaged by fully extending the seat belt from the retractor. Once engaged, the seat belt can retract, but cannot be extended again until the cinch feature is cancelled. The cinch feature is cancelled when the seat belt has fully retracted. If a child seat is to be used in the second seat position, a special dealer-installed anchor must be used in order to anchor the child seat top strap. (This only applies to the seats designed with the top strap provision and for the vehicles sold in Canada). In order to ensure the correct top strap angle, the child seat is only to be used at the seating position for which the top strap anchor is installed. SEAT BELT INDICATOR The IPC illuminates the fasten safety belt indicator when the following happens:The body control module (BCM) detects that the driver seat belt is unbuckled. The BCM sends a message to the IPC via the network requesting illumination. The IPC illuminates the indicator for 20 seconds and then flashes the indicator for 55 seconds. The BCM also sends the GMLAN or network message to the radio in order to activate an audible warning. The IPC performs the displays test at the start of each ignition cycle as commanded by the BCM. The indicator illuminates for approximately 3 seconds.The IPC detects a loss of SPI communications with the BCM. INFLATABLE RESTRAINT SENSING AND DIAGNOSTIC MODULE (SDM) The sensing and diagnostic module (SDM) or SRS computer is a microprocessor and the control center for the SIR system. This SDM has two fused power inputs: one fuse is for the battery voltage and the other fuse is for the ignition voltage. The SDM uses vehicle battery voltage as its main power input. The SDM then uses the vehicles GMLAN Serial Data Communication line and the ignition voltage logic input for enabling or disabling the SIR deployment loops. The SDM contains internal sensors along with several external sensors, mounted at strategic locations on the vehicle. In the event of a collision, the SDM compares the signals from the internal and external sensors to a value stored in memory. When the generated signals exceed the stored value, the SDM will cause current to flow through the appropriate deployment loops to deploy the air bags or seat belt pretensioners. If the force of the impact is not sufficient to warrant inflator module deployment, the SDM may still deploy the seat belt pretensioners. The SDM records the SIR system status when a deployment occurs and turns the AIR BAG indicator located in the IPC ON. As soon as three distinct deployment commands (representing different events) have been issued to any belt pretensioner or the SDM commands any front and side air bag to deploy once, the SDM shall be considered to not be reusable. The SDM performs continuous diagnostic monitoring of the SIR system electrical components and circuitry when the ignition is turned ON. If the SDM detects a malfunction, a DTC will be stored and the SDM will command the AIR BAG indicator ON, notifying the driver that a malfunction exists. In the event that ignition positive voltage is lost during a collision, the SDM maintains a 23-volt loop reserve (23 VLR) for deployment of the air bags.It is important when disabling the SIR system for servicing or rescue operations to allow the 23 VLR to dissipate, which could take up to 1 minute. CIRCUIT DESCRIPTION GM uses an SRS module called the SDM. The SDM module sense a serial data to the instrument cluster for the air bag indicator light. All functionality between the SRS light and the SDM module is achieved through the network. Most modern GM's are equipped with side inflatable airbags as well. The side impact airbags and their regular airbags work in conjunction in the event of an accident. Most GM's also have the side impact airbags talked away inside the front seat. The SDM module also employs a fuel pretension or connected or tied to the seat belt buckle. These pyrotechnic devices or pre-pensioners are deployed also in the event of an accident. The pre-pensioners actually explode and retract the seat belt during an accident. Old pyrotechnic devices including side impact airbags, regular airbags, and pyrotechnic pre-pensioners should not be OHM tested. In other words, a regular continuity tests cannot be employed to test these components. During SRS testing in airbag simulator should always be used to perform all tests. The SRS light is activated or turned on during them all function by the IPC module. The IPC module also called the instrument cluster module is the one that indirectly turns the SRS light on. The IPC module gets at the order to turn the SRS light on from the SDM module, which is the actual SRS module. General Motors employees either a class 2 network or a GM LAN network. All the commands between the IPC module, the body module and the SDM module are carried out across the network. In airbag squib or clock spring is also unemployed of all vehicles. The clock spring is the actual circuitry or connection whereby the steering wheel airbag is actually deployed. Most problems dealing with SRS systems arise from the fact that the clock spring breaks, pulsing in open circuit. Any important fact about SRS systems regardless off manufacturer is the use of shorting bars within the actual connectors. These shorting bars are imbedded inside the actual yellow connectors themselves. The idea behind using shorting bars is to prevent a technician from pinching the wire, inducing a voltage into the SRS circuit and deploying the air back accidentally. Always be mindful of this shorting bar whenever testing SRS circuits. The body-control module is also oftentimes integrated to the SRS system. It is important to understand that in many cases the SRS on the body module has to be reinitialized or reprogrammed in order for the system to work properly. This is usually done after an extensive repair or a module replacement. Always remember the old SRS wires are color-coded in yellow. So again, whenever you see a yellow wire pertaining to the SRS system do not pinch on it with a digital multimeter or test light. This action may actually deploy the airbag. As a final note most of the airbags found on the newer systems are of 2 stages. Stage one is deployed in the event of a hard accident and stage to is deployed in the event of a less traumatic accident. This two-stage configuration is typical of most of the modern SRS systems found on the market today. The use of a two-stage system is evident from the 4 wires going to the airbag unit. In an older single stage you in it or the two wires are going to the airbag unit. It is also for this reason why in airbag simulator capable all diagnosing tool stage airbag systems is important. The left inflatable restraint system also called the citing tight airbag works much the same way as a regular airbag. The difference is that the citing tight airbag is position oftentimes inside the seat and it is in charge of protecting a passenger in case of a side impact. This seat belt buckle switches are in charge both pending the SDM module or the SRS module that the seat belt is actually connected. This seat belt buckle switch is used in combination with a door switches to determine whether the occupant has this seat belt on bulb. This in turn is used to trigger the alarm to one the occupant that this seat built has not being employed. The final sensor used in these types of newer SRS systems is the seat switch. This seat or occupancy sensor tells the SDM module that there is in actual passenger seated on the seat. This sensor also tells the SDM module that there is a child or smaller person seated. The SDM module in turns makes the appropriate decision as to which stage of the airbag it should deploy in the event of an accident. All these sensors and components have to work in conjunction for the SRS system to work properly. SRS TERMINOLOGY AIR BAG -An inflatable cloth cushion designed to deploy in certain frontal crashes. It supplements the protection offered by the seat belts by distributing the impact load more evenly over the vehicle occupant's head and torso. ASYNCHRONOUS -The event can occur at any time. There is no defined time or interval for the event to occur within. B+ -This represents the battery voltage available at the time of the indicated measurement. With the ignition switch in the RUN position and the engine not running, the voltage will likely be between 11.5 and 12.5 volts. At engine idle, the voltage may be 14 to 16 volts. The voltage could be as low as to volts during engine cranking. BULB TEST -The inflatable restraint Sensing and Diagnostic Module (SDM) will cause the AIR BAG warning lamp to flash seven times. Under normal operation this occurs when the ignition switch transitions from the OFF position to the RUN position. The bulb test will also occur if IGNITION 1 voltage exceeds 17.1 volts and then returns to the normal operating voltage range. The presence of a malfunction could prevent the SDM from flashing the warning lamp. CONTINUOUS MONITORING TEST -Tests performed by the inflatable restraint Sensing and Diagnostic Module (SDM) on the SIR system during each 100 millisecond interval. The IGNITION I voltage at the SDM must be within the normal operating voltage range for continuous monitoring to occur. These tests follow the POWER-ON tests. DATA LINK CONNECTOR (DLC) -The DLC connects to multiple on-board computers using wires. The DLC allows communication with an off-board computer. such as a scan tool. DATUM LINE -A base line parallel to the plane of the underbody or frame of the vehicle. All vertical measurements originate from this base line. DEPLOY- To inflate the air bag. DEPLOYMENT LOOP CONTINUITY TEST -Tests performed by the inflatable restraint Sensing and Diagnostic Module (SDM) that check voltage differences between the DRIVER SIDE HIGH and LOW and PASSENGER SIDE HIGH and LOW circuits. First the SDM measures IGNITION I and 23 VLR in order to verify them to be within their respective normal voltage ranges. Then the SDM performs the DEPLOYMENT LOOP CONTINUITY test. Detection of a malfunction during these tests may prevent the RESISTANCE MEASUREMENT-I- 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 current to the inflator modules to deploy the air bags. DIAGNOSTIC TROUBLE CODE (DTC) - An alphanumerical designator used by the inflatable restraint sensing and Diagnostic Module (SDM) 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 its contents. HIGHER PRIORITY FAULT -Each diagnostic trouble code has an assigned priority based on its detectability with other DTCs present. The priority corresponds to the detectability of the malfunction ONLY. This does NOT relate to the criticality of the malfunction with respect to deployment or non-deployment under any given condition. IGNITION CYCLE -An event caused by the operation of the ignition switch. First the inflatable restraint Sensing and Diagnostic Module (SDM) must sense IGNITION I input greater than 8.2 volts with the ignition switch in the RUN position. Then this IGNITION I input must remain above 8.2 volts for at least 10 seconds before turning the ignition switch to the OFF position. IGNITION 1 -A battery voltage (B+) circuit receiving power with the ignition switch in the RUN, or START positions. INFLATABLE RESTRAINT I/P MODULE -An assembly located in the right side of the I/P consisting of an inflatable bag, an inflator and an initiator.INFLATABLE RESTRAINT SENSING AND DIAGNOSTIC MODULE -The Sensing and Diagnostic Module (SDM) that provides reserve energy to the deployment loops, deploys the air bags when required and performs diagnostic monitoring of all the 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 -The electrical component inside the inflator module. This component sets off the chemical reaction that inflates the air bag. This chemical reaction occurs when sufficient current flows through this component. NORMAL OPERATING VOLTAGE RANGE -The voltage measured between the inflatable restraint Sensing and Diagnostic Module (SDM) IGNITION I terminal and Ground terminal is between 8.2 and 17.1 volts. 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 I/P module circuit. PASSENGER CURRENT SOURCE -An output of the inflatable restraint Sensing and Diagnostic Module (SDM) that supplies current into the inflatable restraint 1/P module circuit. POW ER-ON TEST -Tests that the inflatable restraint Sensing and Diagnostic Module (SDM) performs on the SIR system once during each ignition cycle. These tests occur immediately after the SDM receives IGNITION I voltage. These tests occur before the CONTINUOUS MONITORING tests. RESISTANCE MEASUREMENT TEST -Test performed by the inflatable restraint Sensing and Diagnostic Module (SDM) once during each ignition cycle to measure the resistance of the deployment loops. First the SDM measures IGNITION l, 23 VLR, and the deployment loop voltages in order to verify them to be within their respective normal voltage ranges. Then the SDM sources a constant current into the deploy meat loop. The SDM then measures the voltage drop across the deployment loop. The SDM converts the measured voltage value to a resistance value. This resistance value is within the range of 0-6.3 ohms. Upon the detection of an open circuit, the SDM aborts the tests. This will prevent the calculation of the resistances until the next ignition cycle. This test also checks for proper operation of the driver and 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 used to read diagnostic information from on-board computers using the data link connector. SERIAL DATA -The information communicated to an off-board computer using the data link connector. Some of this information 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. This is an energy supply internal to the inflatable restraint Sensing and Diagnostic Module (SDM).