Restraint Systems (SRS)
Restraint system fundamentals
The supplemental restraint system includes airbags, seat-belt pretensioners, crash sensors, the sensing and diagnostic module, clockspring, and wiring. The module monitors circuits, stores codes, and commands deployment. Pretensioners retract the belt to remove slack. Side airbags, curtain airbags, and occupant classification systems vary by vehicle.
Traps: never probe an airbag or pretensioner circuit with an ohmmeter; the meter current can trigger deployment. Always disable the system per OEM procedure, disconnect the battery, and wait the specified time, often three minutes or more, for the backup capacitor to discharge. Verify zero voltage before working. Store modules face up, away from heat, and do not drop them. Do not install aftermarket components unless approved. A deployed module and sensors may require replacement; follow the OEM post-collision inspection.
Rule of thumb: after battery disconnect, wait at least the OEM-specified time, then verify no voltage at the module connector before touching any yellow SRS connector.
Worked example: The SRS lamp is on after steering-column repair. The horn and cruise control also do not work. The likely cause is a damaged or misaligned clockspring, which carries the driver airbag squib, horn, and cruise circuits. Disable SRS, replace the clockspring, center the steering, clear codes, and verify.