What is The Airbag System & How It Works

When it comes to vehicle safety, the airbag system plays a vital role in protecting drivers and passengers during collisions. Since their introduction in the 1970s, airbags have evolved into one of the most important features of a car’s Supplementary Restraint System (SRS). But what exactly is the airbag system, how does it work, and why is it so critical?

In this article, we’ll explore the purpose, components, functionality, and importance of the airbag system, along with tips for maintenance and what to do if your SRS warning light comes on.

1. What is the Airbag System?

The airbag system is a vehicle safety mechanism designed to inflate cushioning bags during a crash, helping reduce the impact force on occupants. It is part of the broader SRS (Supplementary Restraint System), working alongside seatbelts to offer layered protection in accidents.

Unlike seatbelts, airbags are passive safety devices—they don’t require any action from the occupant. When a severe impact is detected, the system automatically deploys one or more airbags in milliseconds.

2. Components of the Airbag System

A modern SRS system consists of several integrated components that work together in milliseconds during a crash:

2.1. Airbag Modules

Airbag modules are self-contained assemblies that play a vital role in modern vehicle safety systems. Each module is engineered to deploy rapidly during a collision, reducing the risk of serious injury to occupants. Here’s a comprehensive look at their structure, types, and functions:

Component of airbag modules:

• Airbag Cushion: A flexible, high-strength fabric bag (usually nylon) folded tightly within the module.
• Inflator : The device that rapidly fills the airbag with gas (often nitrogen or argon) upon receiving a signal from the airbag control unit.
• Housing: The rigid casing that holds the airbag and inflator together, designed to fit specific vehicle locations.
• Cover: A breakaway panel or seam in the steering wheel, dashboard, seat, or pillar trim that allows the airbag to deploy unobstructed

Types of airbag modules:

2.2. Airbag Control Unit

This is the brain of the system. It receives sensor signals and decides whether and when to deploy airbags.

The system is responsible for several key functions:

• Detecting when an accident occurs.

• Promptly recognizing signals sent from various sensors.

• Activating the required firing circuits at the correct time.

• Supplying power to the firing circuits using a capacitor, independently of the car’s main battery.

• Performing self-checks across the entire system.

• Logging any issues in the system’s fault memory.

• Illuminating the airbag warning light in the event of a malfunction.

• Communicating with other electronic control units via the CAN bus.

Modern control units also store data gathered from various crash tests. This information helps the system assess and categorize an accident based on its severity:

• Crash severity 0: Minor impact; airbags are not deployed.

• Crash severity 1: Moderate impact; airbags may deploy at the first stage.

• Crash severity 2: Severe impact; airbags deploy at the first stage.

• Crash severity 3: Very severe impact; airbags deploy at both first and second stages.

In addition to crash severity, the system also considers the direction of the collision (e.g., 0° or 30° angle of force) and the type of accident to determine how the airbags should deploy. It also factors in whether the passengers are wearing seat belts.

2.3. Airbag Sensors & Detection Systems

These detect rapid deceleration or impact force. Located in various parts of the vehicle (front, sides, pillars), they are crucial for accurate, timely deployment.

1. Impact Sensors

• Frontal Impact Sensors: Installed in the front bumper or radiator support area, these sensors detect head-on collisions using accelerometers or mechanical pendulums.

• Side Impact Sensors: Located in doors and B-pillars, they utilize pressure sensors to identify deformation caused by side collisions.

• Rear Impact Sensors: Found in the rear bumper or fender areas, these detect rear-end impacts.

2. Safing Sensors

• Act as a secondary safety layer to prevent unintentional airbag deployment.

• Require concurrent signals from primary impact sensors to initiate airbag activation.

3. Advanced Detection Sensors

• Acceleration Sensors (G-sensors): Measure vehicle deceleration using comb-tooth electrode technology to assess impact severity.

• Gyroscopic Sensors: Detect rotational movement and rollovers, prompting the deployment of curtain or side airbags.

• Occupancy Sensors: Use pressure mats or infrared technology to determine if a seat is occupied, helping disable airbags for empty seats or child seats.

2.4. Seatbelt Pre-Tensioners

These tighten the seatbelt slightly at the moment of impact, reducing slack and ensuring the occupant is firmly restrained.

Functions:

• Immediate Response : When crash sensors detect a significant impact or rapid deceleration, the pretensioner is activated within milliseconds
• Tightening Mechanism : Activation triggers a mechanism (often a gas charge or explosive) that drives a piston or rapidly spins a spool, instantly retracting and tightening the seat belt webbing against the occupant’s body
• Optimal Positioning : This process eliminates slack, ensuring occupants are securely positioned against the seat back, which is crucial for maximizing the effectiveness of airbags and reducing the risk of injury from contact with the vehicle’s interior.
• Prevention of Submarining : By pulling the belt snugly, pretensioners help prevent “submarining,” where an occupant slides under the lap belt during a crash

3. How the Airbag System Works

The SRS Airbag System operates on a sophisticated sequence of events designed to provide rapid and effective protection.

• Crash Sensors Detect an Impact

The process begins when crash or impact sensors (placed at the front, sides, and sometimes rear of the vehicle) detect a sudden deceleration or collision force. These sensors monitor for rapid changes in momentum that typically occur in crashes.

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• SRS Control Unit Analyzes Data

When a potential collision is detected, the sensors immediately send signals to the SRS control module (airbag ECU). This module determines:

The severity of the impact

Where the impact occurred (frontal, side, rear)

Which safety systems to deploy (front airbags, side airbags, seatbelt pre-tensioners, etc.)

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• Seatbelt Pre-Tensioners Engage

If deployment is necessary, seatbelt pre-tensioners activate first—within milliseconds. These devices retract some of the slack in the seatbelt to pull the occupant firmly into the seat, reducing movement before the airbags deploy.

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• Airbags Deploy

Airbags are triggered based on the location and intensity of the collision. The airbag inflator ignites a gas-generating charge, rapidly filling the airbag with non-toxic gas (usually nitrogen or argon) in 20 to 50 milliseconds—faster than the blink of an eye.

The goal is to cushion the impact for the head, chest, and other vital areas, significantly reducing the risk of fatal injuries.

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• Airbags Deflate Immediately After Impact

After absorbing the impact energy, airbags quickly deflate through built-in vents, allowing occupants to move and exit the vehicle more easily.

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• System Locks and Records the Event

Once an airbag has deployed, the system locks out to prevent reactivation and stores crash data in the SRS control module. This data can later be retrieved for diagnostics or insurance assessments.

• Wiring Harness

Specialized crash-resistant circuitry connecting all components

• Warning System

This system include SRS warning light for system diagnostics, passenger airbag ON/OFF indicator

4. Design of the airbag system

The modern airbag system is a complex integration of mechanical, electronic, and chemical components engineered to deploy within milliseconds and provide critical occupant protection during a collision. Each element of the design is precisely calibrated to respond effectively to various types of impacts, occupant positions, and vehicle dynamics.

4.1. Airbag Module

• Fabric Material: Typically made of high-strength polyamide (nylon), treated with talcum powder to prevent sticking and reduce skin friction.

• Volume: Varies by position—driver’s airbag (~67L), front passenger airbag (~140L), side airbags (smaller, faster-inflating).

• Folding Patterns:

  • Standard Folding: Uniform expansion.

  • Star-Shaped Folding: Minimizes protrusion—useful if occupants are not in an ideal seated position.

4.2. Gas Generator

The inflator is responsible for rapidly filling the airbag with gas. There are several types:

• Pyrotechnic Generators: Use solid propellants such as sodium azide (now less common due to toxicity) or newer azide-free compounds. These produce nitrogen, carbon dioxide, and water vapor when ignited.

• Hybrid Generators: Combine a chemical propellant with stored compressed gases (usually 96% argon, 4% helium) for controlled, cooler inflation.

• Two-Stage Generators: Fire in two sequences depending on crash severity, allowing flexible inflation rates.

Inflation temperatures can reach 700°C inside the combustion chamber, but the gas is cooled to around 80°C before entering the airbag to prevent burns.

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4.3. Airbag Control Unit (ACU)

The ACU is the system’s brain. It constantly monitors data from multiple sensors at over 1,000 calculations per second. Once crash thresholds are met, it triggers the gas generator to deploy the airbags.

4.4. Sensors

• Frontal Impact Sensors (e.g., accelerometers or mechanical pendulums): Detect head-on collisions.

• Side Impact Sensors (pressure-based): Installed in doors or B-pillars for side crash detection.

• Rear Sensors: For rear-end collision monitoring.

• Safing Sensors: Act as secondary checks to prevent false deployment.

• Gyroscopic Sensors: Identify rollover events and help activate curtain airbags.

• Occupancy Sensors: Use pressure mats or infrared to detect if a seat is occupied and how the occupant is positioned.

4.5. Volute Spring (Clock Spring)

Ensures continuous electrical contact between the rotating steering wheel and stationary ACU, even as the wheel turns. Must be properly aligned during maintenance to avoid breakage.

4.6. Airbag Cables and Connectors

These are color-coded (usually yellow) and contain safety jumpers to prevent accidental discharge during repairs due to static electricity.

4.7. Seat Belt Pretensioners

Activate simultaneously with airbags to remove slack from seat belts caused by loose clothing or relaxed seating. Systems may use gas-driven pistons, retraction balls, or even mini rotary engines.

4.8. Belt Force Limiters

These adaptive devices control how much force the seat belt applies during a crash, helping prevent chest injuries.

5. Types of Airbags in Modern Vehicles

Airbags come in various forms and placements, each designed to address specific types of crashes and occupant protection needs. Modern vehicles often contain a combination of multiple airbags to ensure comprehensive safety.

Airbags come in various forms and placements, each designed to address specific types of crashes and occupant protection needs. Modern vehicles often contain a combination of multiple airbags to ensure comprehensive safety.

• Front Airbags

These are the most common and among the earliest types of airbags introduced in automobiles. Front airbags are located in the steering wheel for the driver and in the dashboard for the front passenger. They are designed to inflate in moderate to severe frontal collisions, providing cushioning to reduce the risk of serious head, neck, and chest injuries.

• Side Airbags

Side airbags are installed either in the side of the seat or in the door panel. They are designed to protect the torso and pelvis during side-impact collisions, which can be particularly dangerous due to the limited crumple zone between the occupant and the point of impact. In some cases, vehicles may have rear-seat side airbags as well.

• Curtain Airbags (Side Curtain Airbags)

These airbags are deployed from the roof lining above the side windows and cover the entire side window area. Curtain airbags protect the heads of occupants in both the front and rear seats during a side collision or rollover accident. They help reduce the risk of traumatic brain injuries and also minimize the chance of occupants being ejected from the vehicle.

• Knee Airbags

Knee airbags are positioned below the steering column or dashboard to protect the lower limbs of the driver and sometimes the front passenger. By preventing the knees from hitting hard structures, these airbags help reduce leg and lower body injuries in frontal collisions.

• Rear Seat Airbags

Luxury vehicles and newer models are increasingly offering airbags for rear-seat passengers. These may include front-facing airbags located in the back of the front seats or curtain airbags that extend further to cover the rear windows. These additions enhance protection for all vehicle occupants, especially in multi-row SUVs and family sedans.

• Center Airbags

Center airbags are a more recent development and are usually installed in the inner side of the driver’s seat. They deploy during side impacts to prevent the driver and front passenger from colliding with each other. These airbags are especially beneficial in vehicles where the driver and front passenger sit closely together, such as compact cars.

• Pedestrian Airbags

In certain high-end or safety-focused vehicles, external airbags deploy from the hood area to protect pedestrians in the event of a collision. These airbags help reduce head injuries by cushioning the impact zone between the pedestrian and the windshield or A-pillars.

• Inflatable seat belts

In 2011, Ford introduced an inflatable seat belt designed to minimize injuries for rear-seat passengers during a collision. This innovation enhances protection for both adults and children using booster seats or standard seat belts by inflating the shoulder strap upon impact. When activated, the belt disperses crash forces more evenly across the chest and torso, reducing the risk of injury.

Today, these inflatable seat belts are offered as optional features for the outboard second-row seats in select Ford and Lincoln models. Meanwhile, the Mercedes-Benz S-Class includes inflatable seat belts as a standard safety feature.

• Active heel airbag

An Active Heel Airbag is an advanced automotive safety feature designed to protect the lower legs and feet, particularly the heel and ankle areas, during a collision. This type of airbag is still relatively rare and often found in concept vehicles or high-end models with enhanced occupant protection systems.

6. Common Airbag Systems Issues and Their Causes

Airbag systems are among the most crucial safety components in modern vehicles, designed to deploy within milliseconds during a crash to protect occupants from serious injuries. However, like any electronic-mechanical system, airbags can malfunction due to a variety of issues, both electrical and mechanical. Below is a detailed breakdown of the most common airbag system problems and the typical reasons they occur.

6.1. Airbag Warning Light (SRS Light) Stays On

One of the most noticeable issues is the airbag warning light remaining on the dashboard. This light is part of the Self-Restraint System (SRS) and indicates a fault within the airbag or its related components.

Causes:

• Faulty crash or impact sensors: These sensors detect collisions and signal the airbags to deploy. A malfunction, misalignment, or water damage can cause false errors.

• Control unit (ACU) malfunction: If the airbag control unit experiences a software glitch or power failure, it may illuminate the warning light.

• Disconnected or damaged wiring: Loose connectors or frayed wires, especially under seats or in the steering wheel, can interrupt communication in the airbag circuit.

• Previous deployment not reset: If airbags have deployed in the past and were not properly replaced or reset, the system may remain in error mode.

• Weak backup power source: A discharged capacitor in the ACU may trigger a fault.

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6.2. Faulty Crash or Impact Sensors

Impact sensors are responsible for detecting a collision and initiating the deployment process. If they fail, the airbag may not deploy, or it might deploy incorrectly.

Causes:

• Physical damage from prior accidents, even if minor.

• Corrosion or moisture ingress into the sensor housing.

• Loose mounting or vibration affecting calibration.

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6.3. Clock Spring (Volute Spring) Failure

The clock spring is a spiral-wound connector that maintains an electrical link between the airbag in the steering wheel and the control unit, even while turning.

Causes:

• Wear and tear from steering wheel rotations over time.

• Improper handling during repairs or steering column replacement.

• Internal wire breakage that causes signal loss.

A failed clock spring not only affects the driver’s airbag but can also impact horn function and steering wheel-mounted controls.

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6.4. Seat Occupancy Sensor Malfunctions

Modern airbag systems use seat occupancy sensors to detect if a seat is occupied and how the passenger is seated, adjusting airbag deployment accordingly.

Causes:

• Faulty pressure mat under the seat cushion.

• Improper detection due to lightweight objects or child seats.

• Damaged infrared or ultrasonic sensors in advanced systems.

• Wiring damage during seat movement or cleaning.

An incorrect reading may prevent the passenger airbag from deploying or cause unnecessary deployment.

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6.5. Airbag Wiring and Connector Issues

Wiring problems are common, especially in systems where connectors are subjected to regular movement (under seats, in steering columns, etc.).

Causes:

• Corroded or loose connectors, often identifiable by their bright yellow color.

• Rodent damage or wear over time.

• Static electricity during service without proper discharge precautions.

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6.6. Inflator Unit or Propellant Issues

The inflator is the component that produces the gas to fill the airbag rapidly.

Causes:

• Degraded propellant, especially in older or recalled airbags (e.g., Takata recall).

• Contaminated propellant, causing abnormal or explosive inflation.

• Corrosion in the inflator housing.

Improper inflation can lead to underperformance or violent rupture, posing serious injury risks.

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6.7. Control Unit (Airbag ECU) Failure

The ECU controls the deployment logic. If it fails, the airbag system becomes completely unreliable.

Causes:

• Water damage, often from clogged drains or floods.

• Short circuits or over-voltage, especially during jump-starting.

• Software issues or corrupted crash data that prevent reset.

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6.8. Battery Cut-Off and Power Supply Problems

In a crash, the system uses backup power to deploy airbags even if the main battery is disconnected.

Causes:

• Failed capacitor inside the ECU, which is supposed to provide power briefly after a crash.

• Weak vehicle battery affecting the capacitor’s charge cycle.

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6.9. Improper Repairs or Modifications

Airbags require precise alignment, component compatibility, and calibration. Poor modifications can render them ineffective.

Causes:

• Aftermarket seats, wheels, or steering systems not compatible with airbag components.

• Incorrect installation of airbags or sensors.

• Reused deployed airbags, which should never be reinstalled.

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6.10. Seat Belt Pretensioner or Force Limiter Failure

While not airbags themselves, these systems work in conjunction. Failure here can reduce the effectiveness of the entire restraint system.

Causes:

• Broken retractor mechanism or gas charge.

• Electrical or signal failure, often shared with the airbag circuit.

• Mechanical blockage or worn-out components.

Get Professional Help from AutoExplain

At AutoExplain, we specialize in diagnosing and fixing SRS and airbag system issues for various vehicle brands, including Mercedes-Benz, BMW, Toyota, Honda, and more.

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FAQs About Airbag Systems

Q1: What does the SRS warning light mean?
The SRS (Supplemental Restraint System) warning light indicates a malfunction in the airbag system, requiring immediate attention to ensure proper functionality in a collision. This light alerts you to potential issues within the system that need professional diagnosis and repair.

Q2: Can I drive with the SRS warning light on?
Driving with the SRS warning light illuminated is not recommended, as it indicates the airbag system may not function properly in a crash, increasing the risk of injury. It is best to get your car inspected by a qualified technician as soon as possible.

Q3: How often should I have my SRS airbag system checked?
Regular checks of your SRS airbag system are advisable every 1-2 years or as part of your routine vehicle maintenance to ensure all components are functioning correctly and safely. Regular inspections can catch minor issues before they escalate into major problems.

Q4: What are common issues that trigger the SRS warning light?
Common triggers for the SRS warning light include faulty crash sensors, wiring issues, a malfunctioning SDM (Sensing Diagnostic Module), or problems with the airbags or seatbelt pre-tensioners. These issues can affect the system’s ability to deploy properly.

Q5: Can I reset the SRS warning light myself?
While some basic OBD-II scanners can clear SRS codes, it’s essential to address the underlying issue causing the light. If the light returns, seek professional diagnosis and repair to ensure the system is functioning correctly.

Q6: What is the role of coding and programming in SRS airbag systems?
Coding and programming in SRS airbag systems allow for customization, software updates, and compatibility between components, ensuring optimal performance and functionality. These processes can fine-tune the system for specific vehicle needs.

Q7: How can AutoExplain help with my SRS airbag system issues?
AutoExplain provides expert remote diagnostics, programming, and software installation services to accurately diagnose and resolve SRS airbag system faults, ensuring your vehicle’s safety systems are functioning correctly. Our experienced technicians offer cost-effective solutions and expert support.

Q8: What safety precautions should I take when working with SRS airbag systems?
Always disconnect the battery, handle airbags carefully, use proper tools, and follow manufacturer’s instructions to prevent accidental deployment or injury when working with SRS airbag systems. Prioritize safety to avoid any accidents.

Q9: Are there industry standards and regulations for SRS airbag systems?
Yes, industry standards such as those from SAE and FMVSS set minimum safety requirements for SRS airbag systems, ensuring compliance and optimal performance. Adhering to these standards is critical for vehicle safety.

Q10: Why should I choose professional service for SRS airbag system repairs?
Professional service ensures expertise, specialized tools, safety, and compliance with industry standards, providing accurate diagnoses and reliable repairs for SRS airbag systems. Professional technicians are trained to handle these systems safely and effectively.

Conclusion

The SRS airbag system is a critical safety feature that requires proper maintenance and repair to ensure its reliability. By understanding the components, common causes of faults, and troubleshooting steps, you can effectively address SRS issues and keep your vehicle safe.

AutoExplain is your trusted partner for SRS airbag system solutions. Our expert remote diagnostics, programming, and software installation services provide accurate and cost-effective solutions to keep your vehicle’s safety systems functioning correctly.

Don’t compromise on safety. Contact AutoExplain today via Whatsapp at +1(936)2896695, email us at [email protected], or visit our website at autoexplain.com to learn more about our services and how we can help you keep your SRS airbag system in top condition. Our address is 1500 N Grant ST Sten Denver, Colorado, United States. Click the WhatsApp button below to learn more about SRS airbag system coding and programming solutions from AutoExplain and ensure your vehicle is protected.

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