Restraint System Review

Jan. 1, 2020
When technicians talk about restraint systems, they probably overuse, maybe misuse, the term "airbags." In today's vehicles, effective restraint systems consist of much more than driver and passenger airbags. Modern restraint systems include side cur

Fixing restraint systems involves much more than simply replacing airbags

When technicians talk about restraint systems, they probably overuse, maybe misuse, the term "airbags." In today's vehicles, effective restraint systems consist of much more than driver and passenger airbags. Modern restraint systems include side curtain bags, knee bolster bags, seatbelt tensioners, door and seatback inflatable restraints, rollover protection, mechanical and passive headrest protection, and energy-absorbing seats.

Because restraint systems become more sophisticated with each model year, technicians who repair these systems must "chase" this technology. That is, they must continually seek out the training and expertise needed to restore these systems to their pre-collision operating conditions. To do so, technicians must know what an "occupant classification system" is, how it works, how to diagnose it, how to repair it and how to test the system following repairs.

Technicians also need to know how to use diagnostic tools such as airbag simulators, along with scan tools that retrieve and erase diagnostic test codes (DTCs). They must be able to evaluate a vehicle whose malfunction indicator lamp (MIL) continues to indicate the system is not functioning correctly after repairs have been made. Finally, they must be able to find and repair, when recommended by the manufacturer, the wiring harnesses.

Service manuals for each vehicle are mandatory tools for repair because each vehicle has its own specific needs and requirements for repair. Some vehicles even come with a separate restraint service manual dedicated to the diagnosis and service of specific restraint systems. Service manuals identify all the components that should be replaced following a deployment. Restoring the safe operation of a restraint system is complex and the recommendations for their repair are exacting, but with the proper training tools and the use of service manuals, technicians can complete these tasks with confidence.

Front airbags

Front airbag systems use sensors, although not every system uses a "front airbag" sensor. All systems use sensors that deploy the airbag under specific conditions. Front airbags generally deploy when a vehicle is involved in a collision that is head-on or up to 30 degrees right or left of head-on. These sensors may operate by sensing rapid deceleration or body position change. There may be multipoint sensors or a single sensor that decides when to deploy the airbags. Deployment happens within 15 milliseconds of an impact.

These sensors send signals to the airbag or airbags through wiring harnesses, to the airbag module, then on to the clock spring in the driver's steering wheel and directly to the passenger's airbag. Sodium azide beads rapidly turn into a gas, inflating the airbags. After the airbag deploys, it deflates. The white powder and dust that is scattered during deployment is either cornstarch or talcum powder used during manufacturing of an airbag to prevent it from sticking to itself during long-term storage.

Along with front airbags, seatbelts play a significant role in collision protection by holding occupants in position. The front airbags and seatbelts are designed to work in concert. When used together properly, these safety devices provide a higher possibility of survival and reduced injuries for motorists during a frontal collision. On some vehicles, knee bolster bags also deploy at the same time as the front airbags, protecting the passenger's lower legs while helping maintain proper positioning of both seatbelts and upper airbags.

Properly restoring a system that deploys upper airbags, seatbelt tensioners and knee bolster bags is a complex operation. Varying component replacement and testing is required. Each vehicle model's individual service manual must be consulted and its recommendations followed completely.

Seatbelts

Depending on vehicles and manufacturers, seatbelts can either be manual (belts that must be put on by the passengers) or passive (motorized automatic attaching) restraints. Some seatbelts keep occupants from moving forward in a collision by quickly locking in one position. Other belts rapidly retract when a sensor detects a collision. Both of these types of seatbelts need to be inspected following a collision and replaced if needed, following the manufacturer's recommendations.

Some manufacturers recommend that if a belt is in use during an impact of 40 mph or greater, the belt must be replaced, even if no visible damage is found. Others recommend that the belt be thoroughly inspected for damage and replaced if damage is found. To meet the varying model-specific recommendations, a technician must read, understand and follow the service manual recommendations for that particular vehicle.

Side-Impact airbags

As mentioned earlier, front airbags are designed to deploy during a front-end collision. If the collision is from the side, the front bags do not deploy. Side curtain bags do — if the vehicle is equipped. These bags are designed to protect passengers from injuries resulting from side impact or rollover collisions. Side-impact bags, often called side-curtain bags, deploy from the roof. These airbags also can deploy from the door or the seat back, depending on the make or model of vehicle.

Though front airbags and side-impact protection are designed to deploy under different conditions and may not deploy simultaneously, they might both deploy during the same collision. If a vehicle is involved in a front-impact collision, causing the front airbag system to deploy, and then is struck again before it comes to a stop (or rolls over), the side-impact system also may deploy. Engineers must anticipate that a vehicle may be involved in secondary collisions, which means restraint systems must be able to operate following a primary collision. Restraint systems therefore may remain active even after vehicle power has been severed in a collision. This becomes critical when deactivating a vehicle for repairs. After a vehicle's power is disconnected, a period of time must expire before the system can be considered completely disarmed. Recommended times can be found in the vehicle service manual.

Other restraints and protection energy

Current protection systems also include items such as headrest restraints – headrests that come forward to keep the passenger's head from hyper-extending during an impact. These restraints sometimes are designed to work with the seatbelt tensioners.

Another development is pop-up roll bars that are triggered by cylinders of compressed gas or compressed springs, which activate when sensors detect a rollover condition. Some vehicles are being equipped with energy-absorbing seats that move on impact, absorbing collision energy that would otherwise be transferred to the passengers.

Other systems such as occupant classification systems (OCS) calculate the occupant's weight and distance from the airbag, so that the system can determine if it should deploy the system at full speed and intensity or at lower speed and intensity levels due to a passenger's stature or position. These two-stage systems can deploy only one stage of the system, with the secondary system still active even after the airbag has been deployed. A technician servicing the system must proceed with caution so as not to accidentally deploy the second stage while removing what looks like a fully deployed airbag.

Resetting restraints

Most supplemental restraint systems are single-use. Following their deployment, they are not repairable and must be replaced. Some estimating systems may list the recommended replacement parts called for by manufacturers following a deployment. However, collision repair shops typically are not equipped to test and reset diagnostic codes that were set when the restraint system activated. Without the proper testing equipment such as an airbag simulator and scan tool, proper testing is not possible.

When restraint systems are deployed, an in-dash check-engine light or malfunction indicator light (MIL) activates. The vehicle's computer records the deployment, an action commonly called "setting a code." On some vehicles, when new equipment is installed and the vehicle's internal diagnostic checks determine the newly installed equipment is working properly, the MIL light will go out. On other vehicles, even though new equipment is installed and working properly, the code remains, and the "check engine" light remains on as well.

To remove these codes, the service technician must test the system for proper operation before installing the new airbag. Once the system, with an airbag simulator installed, passes the entire recommended test, the new restraint can be installed without fear of accidental deployment when power is restored. With the system working properly and all tests completed, the scan tool can turn off the MIL.

Prior to onboard diagnostic equipment being standardized in January 1996, each make of vehicle and even certain models required a dedicated scan tool or module used in a scan tool for the specific vehicle for diagnostic code reading. In fact, even the connecting cables used to attach the scan tool may have been vehicle-specific. When first introduced, the cost of scan tools and the complexity of their use were overwhelming and not thought to be economical for occasional use to reset supplemental restraint systems.

Today, with On Board Diagnostic (OBD) II laws, most of this specialization has been eliminated, and though each vehicle manufacturer has a specific recommended scan tool, aftermarket scan tools are readily available. Their costs have significantly come down. In the past, collision repair technicians often did not see the value of learning how to use scan tools. If supplemental restraint system (SRS) resetting is to be done in-house, shops will find it's worth obtaining both these tools and the training to properly use them. Technicians now can learn to use these tools relatively quickly, making them more practical for SRS reset, even if they're not used every day.

Conclusion

With all of the different types of restraint systems in modern vehicles, the varying recommendations for each system and constant system updates, technicians can't simply memorize repair information for these systems. Add to that the fact that these systems pose serious liability issues for shops. Techs need to turn to vehicle-specific service manuals to restore restraint systems and keep up to date with their training.

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