Learn About Airbag Control Modules

The airbag control module is the main controlling computer for a vehicles airbag system.

It is also referred to as the Airbag ECU or SRS ECU with SRS standing for Supplementary Restraint System.

They are typically located in the centre front of the cabin of a vehicle and bolted to the floor, usually somewhere near the gear lever or hand brake lever.

Below is an image of a typical airbag control module.

The airbag control modules purpose is to constantly monitor the vehicles airbag system for functionality and to recognise signals which would indicate a collision and initiate deployment of safety features such as airbags and seat belt pretensioners.

The module is usually a cast alloy case with an electrical socket and of an average size of 10cm square. They normally carry a sticker detailing information such as manufacturer, part number, serial number and the airbag or srs logo.

Monitoring The Airbag System For Correct Functionality

The airbag control module, whilst powered, will constantly monitor the vehicles airbag system for correct functionality.

The airbag system is made up of many individual circuits and components which all need to be kept functional incase airbag deployment is required.

The airbag module monitors all these circuits and components by looking for any circuit tolerances that are out of range, namely resistance measurements although other electrical measurements like capacitance can be measured.

The module is pre-programmed with data which is held in it's memory. This data will contain the set tolerances which are normal operating tolerances, so as soon as something looks unusual on a circuit and is out of tolerance the module can warn the driver of a possible system malfunction. Malfunctions will be indicated by the presence of the airbag warning light constantly illuminated or flashing.

To give an example lets take a common scenario.

It's autumn, the leaves are falling and your cars bulkhead drains become blocked. After heavy rainfall water has found it's way into the interior via the heater vent on the bulkhead and water is now in the cars footwells. This water has found it's way under the car seats and has introduced moisture into the seat airbag connectors. Over a short period of time the moisture causes corrosion to the airbag connectors which in turn creates a bad electrical connection in that connector. A bad connection has the effect of increasing resistance in that circuit because the current is struggling to pass through the bad connection. This means that in the event of an accident the airbag on that circuit may not deploy.

The airbag control module recognises the increase in resistance and as soon as it passes the allowable threshold as set in it's programmed limits, it will issue the command to illuminate the airbag warning light to inform the driver of a fault on the system.

Resistance measurements are a very effective way of monitoring electrical circuits and can be used to identify faults such as bad connections, broken wires, short circuits and defective components.

Airbag system monitoring is a vital function of the airbag control module and it not only monitors the whole system but it also monitors itself and can indentify internal faults which may affect the functionality of the system.

As soon as any fault is identified then the airbag warning lamp will be illuminated and remain lit until the fault is rectified.

System faults are usually logged in the airbag control modules memory as fault codes which can be accessed by a diagnostic computer connected through the vehicles OBD port.

Fault codes are usually displayed as a code followed by a short description of the type of circuit fault.

These will not give a diagnosis to the fault but will allow for the technician to use this information to investigate the circuit in question and find the root cause of the problem.

A useful point to remember is that diagnostic computers rarely diagnose faults, they merely pinpoint faulty circuits. It is up to vehicle technicians to carry out investigations and locate faults.

Vehicle State & Collision Monitoring

Another vital role of the airbag control module is to constantly monitor the state of the vehicle and look for collision signals so that it can deploy safety features in the event of an accident.

The module takes information from various inputs on the vehicle and the specific information used does depend on the vehicle manufacturer and airbag system in use.

Here is a list of some of the information airbags may use and where it is received from.

Vehicle occupancy - from occupancy sensors in seat base mats or from buckled seatbelts

Fastened seatbelts - from seatbelt buckles

Presence of child seat - from child seat transponder

Ignition status / Engine run - from engine electronic control unit (ECU)

Immobiliser status - from engine ECU

Fuel pump status -  from engine ECU

Engine speed - from engine ECU

Vehicle speed - from ABS ECU

Brake application - from ABS ECU

Rapid deceleration (crash condition)  -  from decelerometers or crash sensors at various points on the vehicle

Point & severity of impact - from crash sensors

Day / Night sensing from body control unit

Doors locked / unlocked - from body control unit

Position of front seats - from seat position sensors

Passenger airbag deactivation - from deactivation switch

Airbag systems are on a programme of continual development so modules and the data they handle constantly change with new developments and vehicle safety features.

The airbag control module monitors many lines of data and refers to the data programmed within it's memory in the same way it monitors for faults.

If any of the pre-set thresholds are reached, for example in the event of a collision rapid deceleration is detected by the module, it recognises this as an accident and can deploy the necessary safety features. All of this is done in nano seconds, so it's really quite amazing just how the work and the important role they play in modern vehicles.

By processing the data from it's inputs, the airbag control module can indentify when an accident as occurred, how serious the collision is, how many occupants need protecting and just where they are sat, the point of impact and which airbags or other safety features need deploying as a result of that impact. It will not only deploy airbags but also shut off fuel supply, unlock doors, turn on your hazzard lights and interior lighting. There are many very clever features integrated into the complexity of the modern airbag system.

Once a safety deployment has occurred  then this information is recorded to the memory of the airbag control module. This data can then be accessed by diagnostic equipment to assist with the repair or investigation of the vehicle. Many manufacturers have their airbag system set up in a way that once this data is written to the airbag control module, it cannot be erased. This type of data is commonly referred to as crash data and it will usually lock the module and put it into a read only state.

When repairing a vehicle which is fitted with an airbag module that is in a read only state or crash mode then that module either needs replacing or you need the specialist services of an airbag module resetting business.

Crash data can be removed from airbag modules by on the bench programming techniques which will then allow the module to reused as it will be back to it's fully function read/write state just as it left the factory.

With the cost of new airbag modules running into hundreds an airbag module resetting service can save you a small fortune when repairing a damaged vehicle and it's a service that many vehicle repairers use.

For more information or to order crash data removal and airbag module resetting services please visit our home page airbagreset.com