Air Bag Control Module: Description and Operation

Control Module




In the control module are a piezo-electric acceleration sensor, a processor, an energy reserve, a voltage transducer and an electromagnetic safety sensor.

The mounting of the control module is direction-dependent because of the acceleration sensor which senses the acceleration and retardation in the car's direction of travel. When retardation increases over a certain level, the processor begins to process the signal from the acceleration sensor more carefully. If retardation continues to increase and exceeds the levels defined for a collision, the processor will send and impulse to the electric detonators, inflating the airbags and activating the belt tensioners.

The energy reserve is supplied with energy from the voltage transducer which increases the voltage to just over twice that of the battery. There is one energy reserve for the detonators and another which keeps the processor going for a short period in the case of a voltage drop.

The control module can be programmed for the correct configuration, (airbag driver side or airbag driver side and passenger side) using ISAT




Self-diagnostics
When the ignition is switched on and the system is charged, the control module begins an initial self-test for up to 10 seconds. The airbags cannot be detonated during this time. During the self-test, the electric detonators, the cable assembly, the SRS lamp and the processor are checked.

The SRS lamp is lit for 3 to 4 seconds during the initial test when the voltage is switched on in order to check the lamp. The self-test is then continuous as long as the voltage is switched on. The system is then functioning normally and can sense a collision.

In case of fault, the SRS lamp stays lit.

NOTE: When switching on the ignition: The SRS is not functional for the first 6 seconds.

When a fault is registered for the first time, a diagnostic trouble code is generated. A "P" in front of the diagnostic trouble code means that the fault is permanent and an "I" that the fault is intermittent. At the same time that the diagnostic trouble code is registered, a fault counter and a fault time counter start. The fault counter counts how many times a particular fault is registered, up to 255 times.
The fault time counter shows how long a permanent fault has been registered (up to 40 hours), or alternatively how much time has elapsed since an intermittent fault was first registered. The fault time counter counts in intervals of five minutes of charged time. The first interval is however two and one half minutes long so that even faults registered after a short time driving can be included.




The control module stores up to five different diagnostic trouble codes plus diagnostic trouble code B1615 (detonator circuit activated). Further diagnostic trouble codes are not registered.

When the feed voltage is cut it takes a maximum of 5 seconds for the processor to disconnect the energy reserve from the detonator circuits.

The control module is of disposable type, and if it has activated the airbags it cannot be reused. If this has happened, the SRS lamp lights. Diagnostic trouble codes can be read after the airbags have detonated, but cannot be erased.

Control Module Voltage Feed
When the ignition key is in the drive position, the control module is supplied with +15 volts via fuse 4 to pin 5. The control module is specified to work with a voltage supply between 7 and 16 volts.

Connector
The control module connector is equipped with 5 short-circuiting straps which, in pairs, short-circuit the leads to the tour detonators and between the warning lamp lead and ground.

Diagnostics with ISAT
Communication with the ISAT is via pin 9. The data link connector is located under the passenger seat.




Control Module Grounding
The control module is grounded via the three securing bolts. No grounding leads from other systems may be connected to the control module securing bolts as the function of the airbag may be disrupted and a fault indication be received by the control module.

Insufficient grounding will not produce a diagnostic trouble code. When the grounding contact is reestablished after having been broken, this is equivalent to switching on the voltage and the control module restarts its self-test and the SRS lamp lights for 3 to 4 seconds.

If the grounding contact is again broken during these 3 to 4 seconds, this is equivalent to the voltage being switched off and the lamp goes out. If the grounding contact both breaks and returns during the 3 to 4 second period, this is equivalent to the voltage being switched on and off, which makes the SRS lamp flicker.