What is an SRS control unit?
What is an SRS control unit?
The SRS control unit (Supplemental Restraint System) is the central control unit of the airbag system in the vehicle. It evaluates the signals from all crash and acceleration sensors and decides within 10 milliseconds whether and which airbags and belt tensioners must be deployed. In the event of a defect, the entire restraint system is deactivated.
The SRS control unit – also called airbag control unit or airbag ECU – is the brain of the passive safety system. It permanently monitors all connected sensors, stores crash data after an accident, and communicates with other vehicle systems via the CAN bus. Without a functioning SRS control unit, neither airbags nor belt tensioners are operational.
Structure and Components of the SRS Control Unit
A modern SRS control unit consists of a microprocessor, internal acceleration sensors, energy reserve capacitors, and an EEPROM memory for crash data. It can control up to 48 restraint systems and has interfaces to external satellite sensors throughout the vehicle.
| Component | Function | Special Feature |
|---|---|---|
| Microprocessor | Processing of sensor data and deployment decision | Decision in under 10 ms |
| Internal Acceleration Sensors | Detection of frontal and rear collisions | High-g sensors for strong decelerations |
| Energy Reserve (Capacitors) | Power supply even in case of battery failure | Ensures deployment with destroyed wiring |
| EEPROM Memory | Storage of crash data and fault codes | Non-volatile memory, retains data without power |
| Firing Loops | Electrical deployment of gas generators | Up to 48 individual firing loops possible |
| CAN Bus Interface | Communication with other control units | Data bus for vehicle network |
External Sensors in the SRS System
In addition to the internal sensors, the SRS control unit uses numerous external sensors distributed throughout the vehicle. For the detection of side collisions, pressure sensors are installed in the doors – these report a rapid pressure increase even before a major acceleration affects the vehicle. Additional sensors include:
- Satellite Crash Sensors: In front and side areas for faster impact detection
- Seat Occupancy Mats: Detection of whether the passenger seat is occupied
- Seat Position Sensors: Adjustment of deployment strength to seat position
- Seat Belt Buckle Contact Switches: Information on whether the belt is fastened
- Rollover Sensors: Detection of rollover situations
How Does the SRS Control Unit Work?
The SRS control unit permanently compares incoming sensor data with stored threshold values. In the event of an impact, the algorithm detects the type and severity of the collision within 10 milliseconds and specifically deploys the appropriate restraint systems – from frontal airbags to side airbags to belt tensioners.
The deployment algorithm distinguishes between different scenarios: hard braking, curb impact, parking bump, or severe collision. Deployment only occurs in an actual accident. Modern control units use multi-stage gas generators for this purpose – the first stage contains approximately 70 percent of the propellant charge, the second stage is additionally ignited only in severe collisions.
Sequence of an Airbag Deployment
- Crash Detection: Sensors report acceleration peak to the control unit
- Algorithm Decision: Within 10 ms, the control unit determines crash type and severity
- Ignition: Electrical impulse activates the gas generators of the relevant airbags
- Deployment: Airbag deploys completely within 20–50 ms
- Crash Data: Control unit stores deployment data in EEPROM
Which Manufacturers Produce SRS Control Units?
The main manufacturers of SRS control units are Bosch, Continental, Autoliv, Denso, and ZF/TRW. Bosch has produced over 250 million airbag control units since 1980 and is already in its twelfth generation. Each vehicle manufacturer uses specific control units that are tailored to the respective model.
| Manufacturer | Typical Vehicle Brands | Special Feature |
|---|---|---|
| Bosch | VW, Audi, Mercedes, BMW, Opel | Market leader, 250 million+ produced since 1980 |
| Continental | VW, Ford, Hyundai, Volvo | SAFE platform, up to 32 firing loops |
| Autoliv | Renault, PSA, Volvo, BMW | World's largest supplier for passive safety |
| Denso | Toyota, Honda, Mazda | Japanese market leader |
| ZF/TRW | Ford, GM, Fiat, various | Broad portfolio for all vehicle classes |
Important: SRS control units are not simply interchangeable. The deployment parameters stored in the control unit are tailored to the respective vehicle model. Externally identical control units can have different programming – an incorrect control unit can lead to false deployments or non-deployment.
How Do I Recognize a Defective SRS Control Unit?
The clearest symptom of a defective SRS control unit is a permanently illuminated airbag warning light in the instrument cluster. In the event of an internal defect, the control unit reports itself as faulty and deactivates the entire restraint system – airbags and belt tensioners are then no longer functional.
| Symptom | Possible Cause | Action Required |
|---|---|---|
| SRS light illuminated permanently | Internal defect or stored crash | Diagnosis at specialist workshop |
| Error Message in Display | Control unit reports itself as defective | Control unit repair or replacement |
| Fault Code Not Erasable | Hardware defect or crash data | EEPROM programming necessary |
| Multiple SRS Faults Simultaneously | Control unit failure affects all circuits | Have control unit checked |
| Sporadic Error Messages | Contact problems or electronic aging | Check connector and control unit |
Common Causes of Control Unit Defects
- Crash Data After Accident: Control unit locks itself after airbag deployment
- Water Damage: Moisture destroys the electronics
- Overvoltage: Short circuit or jump-start error
- Aging: Electronic components wear out after 15–20 years
- Corrosion: Oxidized connectors and contacts
SRS Control Unit: Repair Instead of New Purchase
A defective SRS control unit does not necessarily have to be replaced with an expensive new part. Specialist workshops like Airbag24 repair defective control units, delete crash data in the EEPROM, and reprogram the device – TÜV-certified and up to 50% cheaper than purchasing new from the authorized dealer.
| Option | Advantages | Disadvantages |
|---|---|---|
| New Control Unit (Authorized Workshop) | Original replacement part, manufacturer warranty | Very expensive (often 500–1,500 €), coding required |
| Used Control Unit | Cheaper | Must be reprogrammed, unknown history |
| Repair at Specialist Workshop | Up to 50% cheaper, own device, no coding required | Shipping required (3–4 working days) |
Airbag24 repairs SRS control units of all brands and models – from VW and Audi to BMW and Mercedes to Opel, Ford, Seat, and Skoda. The repair includes deleting crash data, replacing defective components, and completely testing the control unit. Exact prices for your vehicle model can be found in our shop.
Frequently Asked Questions About the SRS Control Unit
Here you will find answers to the most important questions about the SRS control unit and its function.
What Does SRS Mean Exactly?
SRS stands for Supplemental Restraint System. This means that airbags and belt tensioners supplement the seat belt but do not replace it. Without a fastened seat belt, the SRS cannot develop its full protective effect.
Where Is the SRS Control Unit Located in the Vehicle?
The SRS control unit is typically located in the center of the vehicle – often under the center console, under the driver or passenger seat, or behind the dashboard. The central position protects it in the event of an impact and enables short cable runs to the sensors.
Can You Install an SRS Control Unit From Another Vehicle?
No, this is not recommended. Even if control units look externally identical, the stored deployment parameters are tailored to the respective vehicle. An incorrect control unit can lead to false deployments or non-deployment of the airbags. Professional repair of your own device is safer and cheaper.
Does the Airbag Still Work When the SRS Light Is Illuminated?
No. When the SRS warning light is permanently illuminated, the control unit has deactivated the restraint system. Airbags and belt tensioners will not deploy in the event of an accident. Immediate diagnosis at a specialist workshop is urgently recommended to restore safety.