To Serve and ProtectHaving a thorough understanding of restraint systems
can ensure your customers’ safetyPassenger safety has become the focus of many car ads. Consumers are increasingly making their buying decisions based on their perception of a vehicle’s safety. Our customers want to be assured that when we work on their cars, it will be as safe when they pick it up as it was before the accident. They are not only concerned about the frame, but also about the air bags—and they depend on our knowledge to keep their families safe. With this high level of responsibility placed upon us, it’s imperative that we understand how various systems protect passengers so we can properly restore them during the repair process. Human Dynamics
Passenger safety is complex because several components and systems work together to protect occupants from injury. In an accident, a certain amount of energy is present. The speed and weight of the vehicle, or vehicles, will result in a certain amount of force, M x V = F, where M is mass, V is velocity or speed, and F is force. Injuries are largely determined by how much force is present and the length of time the force is exerted. The human body typically experiences the force of gravity as 1 G. Fighter pilots and racecar drivers may experience many times more than 1 G. Two or 3 Gs will seem uncomfortable to most. Sustained Gs of 5 or more will cause distorted vision and unconsciousness. G-forces in double digits can cause injuries. That’s where collision energy management and the use of test dummies come into play. The goal is to keep all G-forces as low as possible for the passengers. Engineers have designed cars to collapse in predictable ways to absorb as much energy as possible during a collision. Seat belts are designed to keep passengers in place so they slow down with the car instead of hurling into the steering wheel, dash and windshield. Air bags are there as a last measure to soften the blow in severe accidents.History
Safety was not a primary concern with early cars. After Henry Ford was cut on the front “windscreen” during a minor accident, he worked with a glass supplier and the laminated glass now required in all vehicles for front windshields was developed. Research revealed that many drivers were injured or killed when they hit the steering wheel. Mercedes introduced steering columns that collapsed early in the last century to protect drivers. Seat belts became standard first in open cockpit racing cars. Early road racing cars carried a mechanic as well as a driver. Seat belts protected the driver in case of an accident, but also kept the mechanic safely in the second seat. Slowly, seat belts, then shoulder harnesses, became standard equipment in passenger cars. Today, seat belts are not only required as equipment, but in most states by law require them to be worn at all times. The physics is simple; if the occupants are firmly attached to the vehicle by seat belts, they slow down at the same speed as the vehicle. All the damage the vehicle sustains uses energy and time, allowing the humans to be exposed to much smaller G-forces. Air bags are there to dissipate more energy and soften any contact with the vehicle. Supplemental Restraint Systems
Supplemental Restraint Systems (SRS) are designed to work with seat belts and shoulder harnesses. Some European car manufacturers are demonstrating two shoulder belt systems at car shows to gauge consumer acceptance. Even when worn tightly, belts stretch a small amount. Air bags inflate to provide a soft cushion preventing the head and chest from hitting the steering wheel, dash, A-pillar, side glass or side pillar. In addition to front and side air bags, there are knee-level bags in some vehicles. And experiments have been done with front bumper or cowl vent panel air bags to protect pedestrians. In some countries, pedestrian accidents are a big problem. Driver’s side air bags were an expensive option at first, and there was some debate about how effective they would be. Throughout time, the systems have proved valuable in severe accidents. Today, we have multiple bag systems in vehicles, designed to protect in case of frontal impact, side impact or a rollover.How Do They Work?
Air bags are inflated by a gas, which is generated by sodium azide that burns rapidly and is released from a storage canister. In most systems, this happens when an electrical signal fires an igniter. Some early air bag designs used mechanical devices to fire the igniter. Collision forces are detected by one or more sensors located on the car. Some of these sensors are located in the control module, which is typically mounted near the center of the passenger compartment. Additional sensors may be used at the front of the vehicle, typically mounted on the front frame rails or core support. If the forces are high enough, the sensors close a circuit, which results in the control module sending an electrical current to the igniter. In about 15 milliseconds, the control module sends the current to the igniter. At about 40 milliseconds, the bags are fully inflated. They are deflated within two seconds.Many control modules have a safing sensor to prevent unwanted deployments. The safing sensor closes at a lower G-force than the discriminating sensors, but because it is located within the passenger compartment, it is not subject to local spikes. In some early air bag designs, minor collisions, such as hitting the curb, could deploy air bags. The impact from a frame rail-mounted tie-down hook striking a concrete curb can cause a very localized force of tens of Gs. A sensor mounted on the frame rail right above the tie-down hook could close. Because this force will not travel through the vehicle, the safing sensor would not close, so the control module would not fire the air bags.Electrical conductivity must be absolutely maintained, so all wiring is routed to be protected at least in the early stages of a collision. It is also clearly marked with brightly colored jacketing. The clock spring was developed so that a hard-wired connection exists between the steering wheel bag and the control circuits. Those sliding conductors used for horn circuits in the last century just aren’t reliable enough to be sure the bags fire every time they should.Safety Lecture
Working with air bag modules that are designed to catch the weight of a heavy man’s torso, times several Gs, in a few thousandths of a second should be done cautiously. Anyone who has seen an air bag blast into the air when it deploys face down develops a healthy respect for the force they can generate. (This practice is NOT approved by vehicle manufacturers, who specify deploying bags clamped to a vise.) In an accident, wires can be cut. In floods, circuits can be compromised. Many newer vehicles have two-stage air bags, some of which could fire a second time. So, restraint systems should always be disabled before any parts are changed out. Also, turning on the ignition of a damaged vehicle should be done while sitting in the seat, not while leaning in the window. Injuries have been sustained by estimators looking to get the mileage off the electric odometer. Such things are rare, but they have happened. Especially in dry conditions, grounding straps should be worn by technicians to prevent damage from static electricity to microcircuits.Disabling the System
Removing electricity from the circuit should disable any air bag system. The difficulty is that we don’t always know all the sources of electricity. All systems have capacitors to store enough energy to fire the system for at least several seconds. Generally, disconnecting the battery and waiting 20 to 30 minutes will suffice. One manufacturer, however, suggests waiting overnight. Some cars have two batteries, and more will in the future. Some manufacturers require disconnecting fuses and/or wiring connectors as well as both battery terminals. The bottom line is that you must check the manufacturer's recommendations for each vehicle by year, make and model to be sure of what must be done to disable the system and make it safe.What Parts to Replace?
Of course, all deployed air bags and damaged components must be replaced. Many manufacturers also require other parts to be replaced. Control modules, sensors and clock springs are commonly replaced items. The only way to know what should be replaced is to look it up in something called an Air Bag Matrix. Vehicle manufacturers provide this information to the publishers of crash books. So, on paper copies or within the computer estimating software we have pages indicating what must be replaced and what should be inspected for damage. One of these is also available on the I-CAR Web site at www.i-car.com. However, the matrices all carry disclaimers stating that the information may not be completely up to date. This is because from the time the information is finalized at the manufacturer to the time it is published, it is at least weeks and maybe months. Then there is more time before an update is published. Often, the best information we have in the field may be out of date.Some parts will require new fasteners and special tools. Because the function is critical, care must to exercised to put parts back exactly as designed. Torque wrenches should be used to tighten fasteners precisely. Wire Repair
The only part of a restraint system that may be repaired by the manufacturer is the wiring. Some vehicle manufacturers allow wires to be repaired using special crimp connectors. Electrical conductivity must be assured so the repair is reliable. These connectors have redundant systems to make this possible. The center, metal part is crimped onto the wires using a special tool designed to apply just enough force to hold the wire without damaging it. Heat shrink tubing grips the insulation. When the heat is applied, a glue is activated inside the tubing to seal against the wire insulation. Additionally, a low-melt solder flows to create a second electrical connection. Check with vehicle manufacturers before repairing wiring, and acquire the proper connectors and tools.Checking the Codes
After all parts have been replaced, the battery is reconnected and, while safely seated in the driver’s seat, the ignition is turned on. The indicator lights on the dash will all come on, indicating only that they are not burned out. One by one, they will go out as the processors confirm that all systems are normal. If the SRS or air bag light goes out, all is well. If not, the system must be checked. Sublet is the first word many think about at this point. A restraint system repair manual will explain exactly what must be done to clear the codes. In some systems, all that is needed are common electrical tools. For other systems, specific testing tools will be required. There are vehicle-specific computer tools and generic tools. Not all tools will work on all cars. For some vehicles, the only option is to take the vehicle to a dealership or have a mobile service with the proper testing equipment come to the shop. Restoring restraint systems is, for the most part, a matter of replacing parts. Special equipment and vehicle-specific knowledge are required, and there is no room for compromise. Recycled air bags are not approved for use because of liability concerns. It is not yet possible to be certain of the reliability of such components. Repairing plastic air bag covers is similarly not approved, because any repair might alter the way the cover tears when the bag is deployed.
Passenger safety is complex because several components and systems work together to protect occupants from injury. In an accident, a certain amount of energy is present. The speed and weight of the vehicle, or vehicles, will result in a certain amount of force, M x V = F, where M is mass, V is velocity or speed, and F is force. Injuries are largely determined by how much force is present and the length of time the force is exerted. The human body typically experiences the force of gravity as 1 G. Fighter pilots and racecar drivers may experience many times more than 1 G. Two or 3 Gs will seem uncomfortable to most. Sustained Gs of 5 or more will cause distorted vision and unconsciousness. G-forces in double digits can cause injuries. That’s where collision energy management and the use of test dummies come into play. The goal is to keep all G-forces as low as possible for the passengers. Engineers have designed cars to collapse in predictable ways to absorb as much energy as possible during a collision. Seat belts are designed to keep passengers in place so they slow down with the car instead of hurling into the steering wheel, dash and windshield. Air bags are there as a last measure to soften the blow in severe accidents.History
Safety was not a primary concern with early cars. After Henry Ford was cut on the front “windscreen” during a minor accident, he worked with a glass supplier and the laminated glass now required in all vehicles for front windshields was developed. Research revealed that many drivers were injured or killed when they hit the steering wheel. Mercedes introduced steering columns that collapsed early in the last century to protect drivers. Seat belts became standard first in open cockpit racing cars. Early road racing cars carried a mechanic as well as a driver. Seat belts protected the driver in case of an accident, but also kept the mechanic safely in the second seat. Slowly, seat belts, then shoulder harnesses, became standard equipment in passenger cars. Today, seat belts are not only required as equipment, but in most states by law require them to be worn at all times. The physics is simple; if the occupants are firmly attached to the vehicle by seat belts, they slow down at the same speed as the vehicle. All the damage the vehicle sustains uses energy and time, allowing the humans to be exposed to much smaller G-forces. Air bags are there to dissipate more energy and soften any contact with the vehicle. Supplemental Restraint Systems
Supplemental Restraint Systems (SRS) are designed to work with seat belts and shoulder harnesses. Some European car manufacturers are demonstrating two shoulder belt systems at car shows to gauge consumer acceptance. Even when worn tightly, belts stretch a small amount. Air bags inflate to provide a soft cushion preventing the head and chest from hitting the steering wheel, dash, A-pillar, side glass or side pillar. In addition to front and side air bags, there are knee-level bags in some vehicles. And experiments have been done with front bumper or cowl vent panel air bags to protect pedestrians. In some countries, pedestrian accidents are a big problem. Driver’s side air bags were an expensive option at first, and there was some debate about how effective they would be. Throughout time, the systems have proved valuable in severe accidents. Today, we have multiple bag systems in vehicles, designed to protect in case of frontal impact, side impact or a rollover.How Do They Work?
Air bags are inflated by a gas, which is generated by sodium azide that burns rapidly and is released from a storage canister. In most systems, this happens when an electrical signal fires an igniter. Some early air bag designs used mechanical devices to fire the igniter. Collision forces are detected by one or more sensors located on the car. Some of these sensors are located in the control module, which is typically mounted near the center of the passenger compartment. Additional sensors may be used at the front of the vehicle, typically mounted on the front frame rails or core support. If the forces are high enough, the sensors close a circuit, which results in the control module sending an electrical current to the igniter. In about 15 milliseconds, the control module sends the current to the igniter. At about 40 milliseconds, the bags are fully inflated. They are deflated within two seconds.Many control modules have a safing sensor to prevent unwanted deployments. The safing sensor closes at a lower G-force than the discriminating sensors, but because it is located within the passenger compartment, it is not subject to local spikes. In some early air bag designs, minor collisions, such as hitting the curb, could deploy air bags. The impact from a frame rail-mounted tie-down hook striking a concrete curb can cause a very localized force of tens of Gs. A sensor mounted on the frame rail right above the tie-down hook could close. Because this force will not travel through the vehicle, the safing sensor would not close, so the control module would not fire the air bags.Electrical conductivity must be absolutely maintained, so all wiring is routed to be protected at least in the early stages of a collision. It is also clearly marked with brightly colored jacketing. The clock spring was developed so that a hard-wired connection exists between the steering wheel bag and the control circuits. Those sliding conductors used for horn circuits in the last century just aren’t reliable enough to be sure the bags fire every time they should.Safety Lecture
Working with air bag modules that are designed to catch the weight of a heavy man’s torso, times several Gs, in a few thousandths of a second should be done cautiously. Anyone who has seen an air bag blast into the air when it deploys face down develops a healthy respect for the force they can generate. (This practice is NOT approved by vehicle manufacturers, who specify deploying bags clamped to a vise.) In an accident, wires can be cut. In floods, circuits can be compromised. Many newer vehicles have two-stage air bags, some of which could fire a second time. So, restraint systems should always be disabled before any parts are changed out. Also, turning on the ignition of a damaged vehicle should be done while sitting in the seat, not while leaning in the window. Injuries have been sustained by estimators looking to get the mileage off the electric odometer. Such things are rare, but they have happened. Especially in dry conditions, grounding straps should be worn by technicians to prevent damage from static electricity to microcircuits.Disabling the System
Removing electricity from the circuit should disable any air bag system. The difficulty is that we don’t always know all the sources of electricity. All systems have capacitors to store enough energy to fire the system for at least several seconds. Generally, disconnecting the battery and waiting 20 to 30 minutes will suffice. One manufacturer, however, suggests waiting overnight. Some cars have two batteries, and more will in the future. Some manufacturers require disconnecting fuses and/or wiring connectors as well as both battery terminals. The bottom line is that you must check the manufacturer's recommendations for each vehicle by year, make and model to be sure of what must be done to disable the system and make it safe.What Parts to Replace?
Of course, all deployed air bags and damaged components must be replaced. Many manufacturers also require other parts to be replaced. Control modules, sensors and clock springs are commonly replaced items. The only way to know what should be replaced is to look it up in something called an Air Bag Matrix. Vehicle manufacturers provide this information to the publishers of crash books. So, on paper copies or within the computer estimating software we have pages indicating what must be replaced and what should be inspected for damage. One of these is also available on the I-CAR Web site at www.i-car.com. However, the matrices all carry disclaimers stating that the information may not be completely up to date. This is because from the time the information is finalized at the manufacturer to the time it is published, it is at least weeks and maybe months. Then there is more time before an update is published. Often, the best information we have in the field may be out of date.Some parts will require new fasteners and special tools. Because the function is critical, care must to exercised to put parts back exactly as designed. Torque wrenches should be used to tighten fasteners precisely. Wire Repair
The only part of a restraint system that may be repaired by the manufacturer is the wiring. Some vehicle manufacturers allow wires to be repaired using special crimp connectors. Electrical conductivity must be assured so the repair is reliable. These connectors have redundant systems to make this possible. The center, metal part is crimped onto the wires using a special tool designed to apply just enough force to hold the wire without damaging it. Heat shrink tubing grips the insulation. When the heat is applied, a glue is activated inside the tubing to seal against the wire insulation. Additionally, a low-melt solder flows to create a second electrical connection. Check with vehicle manufacturers before repairing wiring, and acquire the proper connectors and tools.Checking the Codes
After all parts have been replaced, the battery is reconnected and, while safely seated in the driver’s seat, the ignition is turned on. The indicator lights on the dash will all come on, indicating only that they are not burned out. One by one, they will go out as the processors confirm that all systems are normal. If the SRS or air bag light goes out, all is well. If not, the system must be checked. Sublet is the first word many think about at this point. A restraint system repair manual will explain exactly what must be done to clear the codes. In some systems, all that is needed are common electrical tools. For other systems, specific testing tools will be required. There are vehicle-specific computer tools and generic tools. Not all tools will work on all cars. For some vehicles, the only option is to take the vehicle to a dealership or have a mobile service with the proper testing equipment come to the shop. Restoring restraint systems is, for the most part, a matter of replacing parts. Special equipment and vehicle-specific knowledge are required, and there is no room for compromise. Recycled air bags are not approved for use because of liability concerns. It is not yet possible to be certain of the reliability of such components. Repairing plastic air bag covers is similarly not approved, because any repair might alter the way the cover tears when the bag is deployed.
