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SRS Systems require safe diagnostics

Tuesday 3rd November 2015
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We are getting a large number of enquiries on the best way to diagnose a fault on the SRS or airbag system. There is a lot of fear from technicians on what is the safe way to repairing SRS systems. 

First, remember you are dealing with explosvie components, and there are set rules and you must never become complacent. Saftey first... And any removed explosive component must be stored in an approved cabinet during repair work when not on the vehicle.

As always, scan the system for known fault codes to determine an area that will need to be diagnosed, not forgetting live data to confirm any codes retrieved .One of the most common faults is that the spiral or clock spring is damaged after steering system repairs.

Most squibs, the explosive component that inflates the airbags or retracts the pretensioners, have a very precise resistance, usually around 2.4 to 3.4 ohms. But you can't test these for resistance, as the meter will cause these to detonate. Use a substiute resistance value for the component and install this in place of the suspect part. Then try to delete the code or view the live data to confirm the circuit integrity.

For some vehicles, a seat recognition mat detects a passenger. This can be be substituted with a bypass module for test purposes. On BMW vehicles, a simple module constructed from 1 diode and 2 resistors will perform this very well, allowing accurate diagnosis without the need for replacing the seat mat.

Not forgetting some manufacturers are fitting explosive disconnection to main battery leads, which require main battery lead replacement after a collision detonation.

Before performing any circuit resistance tests, the system will need to be shut down by removing the battery and leaving a set time before work can commence. Always check the manufacturer’s data for the specific time, and disconnect all components from the system before resistance testing of the loom.

When working on vehicles with ESP, often the steering angle sensor is combined within the same unit as the spiral assembly. After disconnection or replacement, a steering angle sensor recalibration via your scan too must be donel.

There are various tools available on the market to substitute airbags and prestensioners. These have dedicated connectors with know resistance values which will save making up jumpers and resistor combinations.

Nearly all SRS modules now record crash and near event data that will need to be deleted, or the module replaced after an accident. This data can now be analysed to dertermine precrash information including speed, g force and direction,.brake position, accelerator position , engine load  trasmission information and much more.

And finally, when recommissioning the system, shut all doors leaving nobody in the vehicle  before reconnecting the vehice ground lead. Delete all codes and reset all adaptions.