How Does a Crash Sensor in a Car Work?
How does a crash sensor work?
A crash sensor is an acceleration sensor that measures the sudden deceleration of the vehicle during an impact. If the measured acceleration exceeds a threshold value of 3 to 5 g, the sensor sends an electrical signal to the airbag control unit, which then decides whether to deploy the airbags.
Crash sensors are among the most important safety components in a vehicle. Without them, the airbag system could not respond in time. Modern vehicles have multiple crash sensors installed at different positions.
What types of crash sensors are there?
There are three main types of crash sensors: acceleration sensors for front and rear collisions, pressure sensors in the doors for side impacts, and structure-borne sound sensors as the latest generation. Each type uses a different physical measurement principle for accident detection.
| Sensor type | Measurement principle | Application |
|---|---|---|
| Acceleration sensor (MEMS) | Capacitance change through mass deflection | Front and rear impact |
| Pressure sensor | Pressure increase in door cavity | Side impact |
| Structure-borne sound sensor | Sound waves in deformed sheet metal | Rapid plausibility check |
MEMS acceleration sensor: The standard technology
Most modern crash sensors are based on MEMS technology (Micro-Electro-Mechanical Systems). A tiny silicon mass is suspended on spring-like supports. During an impact, the mass moves – the resulting capacitance change is measured as an electrical signal.
Advantages of MEMS technology:
- Extremely small and lightweight (a few millimeters)
- Highly reliable and durable
- Cost-effective in mass production
- Fast reaction time in the millisecond range
Pressure sensors in the doors
During a side impact, the door deforms and the pressure in the cavity rises suddenly. Pressure sensors detect this pressure change faster than acceleration sensors – a decisive advantage, since side collisions have less crumple zone available.
Structure-borne sound sensors: The latest generation
Structure-borne sound sensors detect an accident based on sound waves that propagate through the deforming body sheet metal. They enable particularly rapid plausibility checking of the crash signal and work in addition to the other sensor types.
Where are the crash sensors located in the car?
Crash sensors are installed at strategic positions in the vehicle: centrally in the control unit under the center console, as front sensors in the front end area, as pressure sensors in the doors, and sometimes as rear sensors in the rear area.
- Central sensor: In the airbag control unit, usually under the center console or in the footwell – firmly bolted to the floor pan
- Front sensors: In the front end, often on the longitudinal members or behind the bumper
- Side sensors: In the doors (pressure sensors) or on the B-pillar
- Rear sensors: In vehicles with rear airbags in the rear area
The central position of the main sensor is deliberately chosen: it is firmly bolted to the body to measure vehicle deceleration directly and without damping.
What distinguishes simple from intelligent crash sensors?
Simple crash sensors only detect whether an impact has occurred. Intelligent crash sensors can additionally determine the impact severity, direction, and type, and activate different protection systems with adjusted intensity depending on the situation.
| Feature | Simple sensor | Intelligent sensor |
|---|---|---|
| Detection | Impact yes/no | Severity, direction, type |
| Deployment | All or nothing | Gradually adjusted |
| Communication | Simple signal | Digital data transmission |
| Application | Older vehicles | Modern vehicles (from approx. 2005) |
How does the crash sensor work together with the control unit?
The crash sensor detects the impact signal and transmits it to the airbag control unit. The control unit performs a plausibility check – only if at least two independent sensors agree does the airbag deployment begin.
- Signal detection: Sensor measures acceleration/pressure
- Data transmission: Signal goes to the control unit
- Plausibility check: Control unit compares multiple sensors
- Algorithm: Calculation of impact severity and type
- Decision: Deployment of appropriate protection systems
This dual-sensor principle is an important safety measure: a single faulty sensor cannot cause unintended airbag deployment.
When is a crash sensor defective?
A defective crash sensor is indicated by a continuously illuminated airbag warning light. Common causes are corrosion on the connectors, mechanical damage from a minor accident, or cable breakage due to age.
Typical symptoms of a defective crash sensor:
- Airbag warning light illuminates continuously
- Error codes stored in the control unit
- Airbag system disables itself
- TÜV inspection is not passed
A defective crash sensor means that the airbag system will not deploy in an emergency. Immediate repair is therefore safety-critical and required by TÜV.
Frequently asked questions about crash sensors
Here you will find answers to the most important questions about crash sensors in cars.
Must a crash sensor be replaced after an accident?
Not always. If the sensor was not directly damaged and no error codes are present, it can continue to be used. After a severe accident with airbag deployment, however, it should be checked.
Can a crash sensor be tested?
Yes. With a diagnostic device, the workshop can read out the sensor and check whether it provides correct values. Error codes provide information about the nature of the problem.
How many crash sensors does a modern car have?
Depending on the vehicle, between 4 and 8 sensors: at least one central sensor in the control unit, two front sensors, and pressure sensors in the doors. Premium vehicles have additional sensors for rear and rollover detection.
What does it cost to replace a crash sensor?
The costs consist of the replacement part, installation, and diagnostics. You can find exact prices for your vehicle model in our shop. At Airbag24, you get suitable crash sensors for all brands – we are happy to help you.