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Airbag Safety & Wiring Repairs

How to not get your head blown off...

by Julian Edgar

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At a glance...

  • Airbag types
  • Airbag locations
  • Safe handling and removal
  • Airbag wiring harness repairs
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This material first appeared in the I-CAR Advantage Online, which is published and distributed free of charge. I-CAR, the Inter-Industry Conference on Auto Collision Repair, is a not-for-profit international training organization that researches and develops quality technical education programs related to collision repair. To learn more about I-CAR, and to subscribe to the free publication, visit www.i-car.com.

Airbags

When working on a collision-damaged vehicle there is often a risk factor involving undeployed airbags. Simply disconnecting the battery does not eliminate that risk. Late model vehicles can have airbags in several locations.

  • Inflation

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When front driver airbags were first developed, the main inflator propellant used was sodium azide. With these types of inflators, an electrical signal causes the pellets to begin burning, creating nitrogen gas, which deploys and inflates the airbag. Sodium azide was also the propellant for the first front passenger airbags.

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Today, many airbags are deployed by compressed gas cylinders. A compressed gas inflator operates by sending an electrical signal from the restraint system control module to a small disk (performing a function similar to a soda bottle cap) on the gas inflator. The disk burns and the escaping compressed gas inflates the airbag. Compressed gas cylinders may be stored in a compressed state upwards of 435 kPa (3,000 psi). Puncturing the cylinder will cause a high burst of high-pressure gas that could result in flying metal, plastic, or glass. This can occur even if the airbag system is disabled.

One of the reasons for choosing compressed gas as a propellant is the space restriction where these airbags are located. The compressed gas inflators are the shape of a long cylinder. With this design, the inflator can be positioned inside the instrument panel, seat back, door panel, or along any rail or pillar of the vehicle. Another concern with sodium azide is heat. When sodium azide burns, it generates a great amount of heat. If located in the soft trim areas of a vehicle, fires may be caused by airbag deployment. Compressed gas allows airbag deployment temperatures to be kept lower.

  • Location

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Front airbags are located in the steering wheel for the driver and the passenger-side instrument panel for the passenger. Side airbags are located either in the doors, seat backs, or pillars. Side curtain airbags are dropped down from the headliner along the roof rail. Knee airbags deploy from the knee bolster.

  • Removal and Storage

If it is necessary to remove an undeployed airbag, it should be deactivated. To deactivate and remove an airbag module and inflator, the 12-volt electrical system of the vehicle must be de-energized by following the vehicle maker’s specific procedures. Not all passive restraint systems de-energize the same. Depending on the discharge rate of the capacitors used within the system, they may discharge as fast as 30 seconds or it make take a number of minutes before airbag removal can safely occur.

Deactivating an airbag assembly or even disconnecting the airbag connectors does not eliminate the risk of deployment. Since only a small amount of current is required to activate an airbag circuit, static electricity could still deploy an airbag. The best way to avoid static electricity build-up is to handle the airbag like a computer module. Ground yourself with an anti-static discharge strap, attaching the strap to vehicle ground. Once the airbag is out of the vehicle, remove the wrist strap and carry the airbag so that it would deploy away from your body.

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When removing and installing a seat, the technician or the seat should be grounded to prevent static electricity from deploying the airbag. Do not hold the seat with your hands over the undeployed side airbag, and carry the seat so that the undeployed airbag is away from your body.

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Hold an undeployed side curtain airbag so that if deployed, it would fold out or downward, not toward you.

The undeployed airbag, or seat, should be stored in a safe location where it is not subjected to conditions that could cause an airbag module to deploy, such as stray voltage or static electricity.

  • Single-Stage or Dual-Stage

The driver and passenger airbag have the potential to be a dual-stage design. This means that even though the airbag is deployed, there may still be a live charge inside the airbag module that could be deployed.

For technicians, two ways to identify if an airbag is a two-stage design is to refer to the vehicle maker service manual or when the airbag is removed, look for two wire connectors attached to the airbag module. All dual-stage airbags have two connectors, and single stage airbags have one connector going to the airbag module assembly.

Restraint System Wiring Repairs

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Confusion exists concerning the repair of some damaged wiring and wiring connectors for restraint systems. The perception is that restraint system wiring should never be repaired. While it is true that some vehicle makers do not recommend the repair of wiring and connectors for the restraints system, others have very detailed printed procedures and even offer repair kits expressly for the repair of restraint system wiring. Repairing damaged restraint system wiring, when applicable and the parts and procedures are available, can save unnecessary replacement of the main body and instrument panel wiring harnesses.

As an example, we will focus on the repair of restraint system wiring on US market General Motors (GM) vehicles. Whenever doing restraint system wiring repairs, always follow the procedures and use the parts and equipment specified by the vehicle maker for the vehicle being repaired. This information can be found typically in the “BODY” section of the service information under “WIRING REPAIR” for GM vehicles.

  • Self Diagnostics

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Let’s begin with some background on how the self-diagnostic capabilities of modern restraint systems function. When a problem exists in the electrical circuit of a restraint system, a diagnostic trouble code (DTC) is stored in the restraint system control module and a malfunction indicator lamp (MIL), or airbag warning lamp, on the instrument panel is turned on to alert the driver.

The computer is able to detect problems by monitoring system voltage, amperage, and resistance readings, and comparing them to specified values. If a monitored value falls outside the programmed parameters, a DTC is set and the MIL is turned on. Wiring repairs done improperly can increase the resistance in a circuit to the point that the control module senses a fault and sets a DTC. Repairs that do not completely seal the splice joint from moisture can allow corrosion of the connection. This corrosion will also increase circuit resistance and eventually lead to a DTC and MIL.

  • Restraint System Requirements

What makes a restraint system wiring repair different from any other wiring repair? While there are a couple of recommendations that may be specific to restraint systems, the reality is that restraint system wiring repair demands the same best practices that should be observed with any other wiring repair. The first, and maybe only real difference, is that while GM recommends repairs rather than the replacement of a wiring harness, short pigtails that are attached to parts such as sensors and inflator modules are not repairable. If a pigtail connected to a part is damaged, the part should be replaced.

Restraint system wiring repairs vary from a simple splice of a cleanly cut wire or replacement of sections of damaged wire, to replacement of terminals or even entire connectors. All of these operations may involve splicing wires together.

  • Splice Sleeves

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To help address the issue of making low resistance and moisture-proof, or air-tight splices, special “DuraSeal” crimp and seal splice sleeves were designed (pictured). These connectors differ from the conventional butt connectors in a couple of important ways.

First, they have a special cross-hatched (knurled) core crimp barrel that provides the necessary contact integrity to make a low resistance splice. Secondly, they have a special heat shrink sleeve that contains a sealing adhesive inside. When the connectors are heated, they shrink over the wire and make an air-tight seal that protects the connection from the environment. Although these connectors were initially designed for repairing restraint system wiring, because of their superior performance, they are now the recommended splice by GM for all wiring repairs. The only limitation these connectors have is that they can only be used to splice two wires together. For repairs to original equipment splices of three or more wires, special splice clips are available from GM. These types of connections are typically limited to grounds and their application to restraint system wiring is not common. The splice clips have detailed instructions included for applications and usage.

The splice sleeves are available in different sizes and are colour coded. Green coloured splices (88988379) are used for 22-24 gauge, salmon (#12089189) for 18-20 gauge, blue (12089190) for 14-16 gauge, and yellow (12089191) for 10-12 gauge. To ensure circuit integrity the correct splice sleeve for the wire size must be used.

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The tools and procedures used when making wire splices is as important as the type of splice connector. If the crimping tool used on the connector damages the insulation tubing over the metal core crimp barrel, the connection may not be sealed sufficiently from moisture, resulting in corrosion. Because of this, GM recommends a special splice crimping tool (J-38125-8, GM P/N 12085115) be used (pictured).

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The crimping tool has three nest positions. Position 1 is used for 18-24 gauge wire, or the green and salmon coloured splice sleeves. Position 2 is for 14-16 gauge wire, or the blue splice sleeves, and position 3 is for 10-12 gauge wire, or the yellow splice sleeves. The nest positions on the crimp tool are colour-coded for easy reference. The crimping tool also has a locking ratchet mechanism in the handle that keeps the tool from being reopened until the proper amount of pressure has been applied to the splice sleeve. After the splice sleeve has been put into the proper nest, and the wire inserted into the metal core crimp, the handles on the crimping tool are closed and squeezed until they open when released. This ensures that just the right amount of pressure is applied to the splice sleeve.

To repair damage to GM restraint system wiring, follow this procedure.

  1. Open the wiring harness by removing any tape. Do not damage the wiring insulation when cutting the harness open. A sewing ripper works well for this.

  1. Cut as little wire off the harness as possible. Damaged sections of wire must be removed and other sections may need to be cut away to change the locations of splices. Splices should be located at least 40 mm (1.5 in) away from harness branches, connectors, or other splices.

  1. Strip the insulation from the splice locations. You must obtain a clean strip with all of the wire strands intact. If you are unsure of the wire size, and have no way to accurately measure it, begin with the largest opening on the wire stripper and work down until a clean strip of the insulation is achieved. Strip about 7.5 mm (5/16 in) of insulation from each wire to be spliced. After stripping the wire, inspect the wire strands for nicks or cut strands. If any damage is found to the wire, cut the damaged portion off and repeat the stripping procedure.

  1. Select the proper size splice sleeve for the wire size.

  1. Obtain the correct splice crimp tool to crimp the splice sleeve.

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  1. Place the splice sleeve in the correct nest of the crimping tool. Ensure that the crimp made falls midway between the end of the metal crimp barrel and the wire stop. The wire stop is in the centre of the barrel. Close the crimping tool handles slightly in order to firmly hold the splice sleeve in the crimp tool nest.

  1. Insert the wire into the splice sleeve barrel until the wire hits the stop.

  1. Tightly close the crimping tool handles until they open when released.

  1. Repeat steps 6-8 for the opposite end of the splice sleeve.

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  1. Using a heat torch (J-38125-5 or equivalent), apply heat to the crimped area of the barrel. Gradually move the heat to the open end of the splice sleeve tubing. The shrink tubing will shrink down tightly against the wire as the heat is applied, and a small amount of sealant will come out of the open end of the tubing. The heat torch used should have a heating barrel to distribute the heat evenly around the entire splice sleeve.

  • Replacing Wiring Harness Parts

Damaged connector terminals present other considerations. The terminals are gold plated to ensure the contact integrity of the sensitive low energy restraint system circuit. Damaged terminals in the sensing and diagnostic module (SDM) harness connector can be replaced, but must only be replaced with the terminated leads in the SIR/SRS connector repair assembly pack. Do not substitute any other terminals for those in the SIR/SRS assembly packs. These terminals are crimped onto short sections of wire that can be spliced into the harness with the splice sleeves after they are inserted into the connector body. Damaged terminals in other restraint system connectors are repaired by splicing a new connector assembly into the wiring harness. The wiring harness side connectors are available as service parts.

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When sections of wire require replacement, ensure that the wire used is the same wire size and type as that being replaced. Other parts that are available for restraint system wiring repairs include connector position assurance (CPA) inserts and terminal position assurance (TPA) inserts (pictured). The CPAs are inserted through locking tabs on the connectors to ensure that the connector halves cannot vibrate apart, and the TPAs keep the terminal pins seated securely in the connector body. Both of these inserts must be undamaged and in place to ensure good contact between the mating terminals of the connection.

All of these parts are available separately or in an SIR Repair Kit tray. The SIR tray is a part of a bigger Terminal Repair Kit (J-38125) that includes all the tools, connectors, splice sleeves, and other parts to repair any part of a damaged wiring harness. Also included in the SIR Repair Kit tray is shrink tubing that is used to cover splices and protect them from heat in areas where they are exposed to high temperatures. Yellow electrical tape is supplied so that colour coding of the wiring can be maintained for splices made in areas that have colour-coded wiring.

  • Conclusion

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Restraint system wiring repairs are allowed by several vehicle makers, including GM. While not difficult, always follow the vehicle maker’s recommendations as to the parts and procedures used. This will ensure that the integrity of the system is maintained and that repairs will have the durability necessary to maintain proper system functioning.

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