12.1 Supplemental Restraint System: Safe Handling & Diagnostics

What the Competency Standard Requires

Repair Supplemental Restraint System (SRS) requires you to check the SRS using a scanner, identify faulty components including the spiral cable, seat belt, SRS unit, control module and sensors and replace them, and inspect continuity of the wiring harness and repair or replace it as required.

Note the wording carefully. The scan tool is named as the diagnostic instrument. Nowhere does the standard invite you to put a multimeter on a squib circuit, and doing so can deploy an airbag into your face.

SRS Architecture

Design features that exist purely for safety

• Yellow harness and connectors. Every SRS circuit is coloured yellow specifically so a technician can recognise it instantly and avoid cutting, splicing or probing it.
• Shorting bars. SRS connectors contain a metal bar that automatically links the two squib wires together the moment the connector is unplugged. This prevents static or induced voltage from building a potential across the initiator. Forcing a probe into such a connector can lift the shorting bar and remove the protection.
• Twisted pair wiring to cancel induced electromagnetic interference.
• Redundant sensing. The module normally requires agreement between the internal accelerometer and a remote impact sensor before it will deploy, so a single sensor fault does not cause an accidental deployment.

The Rules That Keep You Alive

1. Switch the ignition off, disconnect the battery negative terminal, and wait the manufacturer's specified stand-down period. This is commonly between one and ten minutes depending on the vehicle, and it exists so the module's backup capacitor discharges. Look up the figure for the specific vehicle rather than assuming.
2. Never measure a squib circuit with an ohmmeter, a test light or a continuity buzzer. An initiator can fire on a few hundred milliamperes, and a typical multimeter on the ohms range or a test light can supply that. The control module measures squib resistance internally through a current-limited circuit, and the scan tool reports the result.
3. Never apply battery voltage to an airbag connector for any reason.
4. Carry a live airbag module with the trim cover facing away from your body, so that an unexpected deployment moves the module away from you rather than into you.
5. Store a live module face up on a flat, clear surface. Face down, a deployment turns the module into a projectile.
6. Do not expose airbag modules to temperatures above the manufacturer's limit, commonly around 93 degrees Celsius, and keep them clear of paint ovens, heat guns and welding.
7. Never strike, drop or subject the module to impact, and never attempt to open, repair or test a module.
8. Do not fit aftermarket seat covers over seats containing side airbags, and do not place accessories on the dash above a passenger airbag.
9. Disconnect the module before any welding, following the pre-welding isolation protocol in section 8.3.
10. Treat a deployed module as hazardous waste and dispose of both deployed and undeployed units through the approved route. An undeployed module removed from a scrapped vehicle is an explosive device.

Diagnosing SRS Faults

The method is short because the tool list is short.

1. Observe the prove-out. The SRS lamp should illuminate at key-on for a few seconds and then extinguish. A lamp that stays on indicates a stored fault. A lamp that never illuminates is just as serious, because it means the driver has no warning capability at all and someone may have removed the bulb to hide a fault.
2. Read the codes with a scan tool, recording whether each is current or historic and how many ignition cycles ago it was set.
3. Interpret the code type:

1. Clear the code and re-test. A code that clears and stays clear through several ignition cycles and a road test was likely a poor connection. A code that returns immediately identifies a hard fault.
2. Inspect the physical circuit only with the system de-energised, the stand-down period observed and the connectors unplugged so the shorting bars are engaged.

The clockspring

The clockspring (spiral cable) is the single most common SRS fault and also appears in the cruise control and horn competencies because it carries those circuits too.

• Symptoms include an intermittent driver airbag code, a horn that works only at certain steering positions, dead steering wheel switches and, on some vehicles, a steering angle sensor fault.
• The ribbon breaks from repeated flexing, especially if the wheel has been rotated while the clockspring was disconnected.
• Centring is critical. Before removing a clockspring, set the road wheels straight ahead and lock the steering. A replacement clockspring arrives with a locking tab that must stay in place until the wheel is fitted. Fitting an off-centre clockspring tears the ribbon at one extreme of lock, often days later.

After a Deployment

Repairing a deployed system is not a matter of fitting a new bag.

1. Read and record the crash data before replacing anything.
2. Replace every deployed component - airbag modules, pretensioners and any single-use bracket.
3. Replace the control module where the manufacturer requires it. Many modules lock permanently once crash data is written and cannot be reset.
4. Inspect and replace the impact sensors that recorded the event, plus any harness that has been heat damaged or stretched.
5. Inspect seat belts thoroughly. A belt that carried load in a collision must be replaced, including the retractor, even where it looks undamaged.
6. Replace, do not repair, the wiring. SRS harness splicing is prohibited by most manufacturers; the correct repair is a new harness section or connector kit.
7. Programme and configure the new module to the vehicle and then verify with a full scan and a lamp prove-out.

Seat Belts and Pretensioners

The competency standard names the seat belt among SRS components, and for good reason: the belt is the primary restraint and the airbag is supplemental to it.

A belt that has restrained an occupant in a collision, or whose pretensioner has fired, must be replaced as an assembly. Never attempt to reset, re-wind or reload a fired pretensioner.

Customer Communication

SRS work carries an obligation to explain clearly:

• If the SRS lamp is on, the system may not deploy in a collision. Say that plainly rather than describing it as a warning light problem.
• Explain that the airbag supplements the seat belt and does not replace it.
• Record the SRS lamp state on the job card at check-in and at hand-back. This protects both the customer and the workshop.
• Never suggest disconnecting a bag, removing a bulb or fitting a resistor to extinguish a lamp. All three defeat a safety system and expose the workshop to serious liability.

Faults the Practical Assessment Likes to Present

• Intermittent driver airbag code plus an erratic horn - clockspring.
• Passenger airbag code after seat removal - occupant classification calibration not performed.
• Side airbag code after a trim repair - pinched harness under the seat rail.
• SRS lamp that never lights at key-on - removed bulb or open lamp circuit hiding a real fault.
• Multiple SRS codes after a jump-start - poor connection or a voltage event; check supply before condemning the module.