Air Bag Control Module: Description and Operation

Airbag Control Module:

AIRBAG CONTROL MODULE
The Airbag Control Module (ACM) is bolted to the floor panel transmission tunnel rearward from the gear shift selector inside the vehicle. The ACM mounting bracket is welded to the tunnel and is not serviced with the ACM. The ACM contains a microprocessor, the impact sensor, safing sensor, and energy storage capacitor. The microprocessor contains the airbag system logic. The ACM system logic includes On-Board Diagnostics (OBD) capability, and communicates with the instrument cluster circuitry via the PCI data bus to control the airbag indicator lamp.

The microprocessor in the ACM monitors the impact sensor signal and the airbag system electrical circuits to determine the system readiness. If the ACM detects a monitored system fault, it sends messages to the instrument cluster on the PCI data bus to turn ON the airbag indicator lamp. A preprogrammed decision algorithm in the ACM microprocessor determines when the deceleration rate as signaled by the sensor indicates an impact that is severe enough to require airbag system protection. When the programmed conditions are met, the ACM sends an electrical signal to deploy the airbag system components.

The impact sensor is an accelerometer that senses the rate of vehicle deceleration, which provides verification of the direction and severity of an impact. The impact sensor is calibrated for the specific vehicle, and is only serviced as a unit with the ACM.

The ACM also contains an energy-storage capacitor. The purpose of the capacitor is to provide airbag system protection in a severe secondary impact if the initial impact has damaged or disconnected the battery, but was not severe enough to deploy the airbags.

THE AIRBAG SYSTEM IS A SENSITIVE, COMPLEX ELECTROMECHANICAL UNIT. BEFORE ATTEMPTING TO DIAGNOSE OR SERVICE ANY AIRBAG SYSTEM OR RELATED STEERING WHEEL, STEERING COLUMN, OR INSTRUMENT PANEL COMPONENTS YOU MUST FIRST DISCONNECT AND ISOLATE THE BATTERY NEGATIVE (GROUND) CABLE. WAIT TWO MINUTES FOR THE SYSTEM CAPACITOR TO DISCHARGE BEFORE FURTHER SYSTEM SERVICE. THIS IS THE ONLY SURE WAY TO DISABLE THE AIRBAG SYSTEM. FAILURE TO DO THIS COULD RESULT IN ACCIDENTAL AIRBAG DEPLOYMENT AND POSSIBLE PERSONAL INJURY.

NEVER STRIKE OR KICK THE AIRBAG CONTROL MODULE, AS IT CAN DAMAGE THE IMPACT SENSOR OR AFFECT ITS CALIBRATION. IF AN AIRBAG CONTROL MODULE IS ACCIDENTALLY DROPPED DURING SERVICE, THE MODULE MUST BE SCRAPPED AND REPLACED WITH A NEW UNIT.

The airbag warning lamp is the only point at which the customer can observe "symptoms" of system malfunction. Whenever the ignition key is turned to the "run" or "start" position, the MIC performs a lamp check by turning the airbag warning lamp ON for 6-8 seconds. If the lamp turns OFF, it means that the ACM has checked the system and found it to be free of discernible malfunctions. If the lamp remains ON, there could be an active fault in the system or the MIC lamp circuit may be internally shorted to ground. If the lamp comes on and stays on for a period longer than 6-8 seconds then goes OFF, there is usually an intermittent problem in the system.

The ACM cannot be repaired or adjusted and, if damaged or faulty, it must be replaced.

Fig.23 Side Impact Airbag Control Module (SIACM):

SIDE IMPACT AIRBAG CONTROL MODULES (SIACM)
Vehicles equipped with side impact airbags use two side impact airbag control modules. One is located on each side of the vehicle within the body B-pillar.

Each Side Impact Airbag Control Module (SIACM) serves as the impact sensor for its seat mounted airbag. The right side SIACM controls the right seat airbag. The left side SIACM controls the left seat air- bag. Each SIACM contains two accelerometers, both accelerometers must agree in order to deploy the side airbag. In the event of a side impact, the appropriate SIACM will send a electronic signal to its airbag, deploying the airbag. The SIACM communicates with the Airbag Control Module (ACM) via the PCI bus circuit.

The SIACM perform self-diagnostics and circuit tests to determine if the system is functioning properly. If the test finds a problem, the SIACM will set both active and stored Diagnostic Trouble Codes (DTC's). If a DTC is active, the SIACM will request that the airbag warning lamp be turned ON. The results of the system test are transmitted on the PCI bus to the ACM once each second or on a change in lamp state (ON/OFF). If the warning lamp status message from either SIACM contains a lamp ON request, the ACM will set an active DTC. At the same time as the DTC is set, the ACM sends a PCI bus message to the mechanical instrument cluster (MIC) requesting the airbag warning lamp to be turned ON. Observe all ACM warning and caution statements when servicing or handling the SIACM. The SIACM's are not serviceable and must be replaced even if they are dropped.

NOTE: When the Airbag Warning Indicator is illuminated, interrogate the ACM.

DIAGNOSTIC TROUBLE CODES
Airbag diagnostic trouble codes consist of active and stored codes. If more than one code exists, diagnostic priority should be given to the active codes.

Each diagnostic trouble code is diagnosed by following a specific testing procedure. The diagnostic test procedures contain step-by-step instructions for determining the cause of the trouble codes. It is not necessary to perform all of the tests in this book to diagnose an individual code.

Active diagnostic trouble codes for the airbag system are not permanent and will change the moment the reason for the code is corrected. In certain test procedures, diagnostic trouble codes are used as a diagnostic tool.

Active Codes
The code becomes active as soon as the malfunction is detected or key-off, whichever occurs first. An active trouble code indicates an on-going malfunction. This means that the malfunction is currently there every time the control module checks that circuit/function. It is impossible to erase an active code; active codes automatically erase by themselves when the reason for the code has been corrected.

With the exception of the warning lamp trouble codes or malfunctions, when a malfunction is detected, the airbag lamp remains lit for a minimum of 12 seconds or as long as the malfunction is present.

An "Interrogate Right SIACM or Interrogate left SIACM" diagnostic trouble code indicates an active trouble code in the respective module.

Stored Codes
Airbag codes are automatically stored in the ACM's memory as soon as the malfunction is detected. A "stored" code indicates there was an active code present at some time. However, the code currently may not be present as an active code, although another active code could be.

When a trouble code occurs, the airbag warning lamp illuminates for 12 seconds minimum (even if the problem existed for less than 12 seconds). The code is stored, along with the time in minutes it was active, and the number of times the ignition has been cycled since the problem was last detected. The minimum time shown for any code will be one minute, even if the code was actually present for less than one minute, Thus, the time shown for a code that was present for two minutes 13 seconds, for example, would be three minutes.

When and if the malfunction ceases to exist, an ignition cycle count will be initiated for that code. If the ignition cycle count reaches 100 without a reoccurrence of the same malfunction, the diagnostic trouble code is erased and that ignition cycle counter is reset to zero. If the malfunction reoccurs before the count reaches 100, then the ignition cycle counter will be reset and the diagnostic trouble code Will continue to be a stored code.

If a malfunction is not active while performing a diagnostic test procedure, the active code diagnostic test will not locate the source of the problem. In this case, the stored code can indicate an area to inspect.

If no obvious problems are found, erase stored codes, and with the ignition "ON" wiggle the wire harness and connectors, rotate the steering wheel from stop to stop. Recheck for codes periodically as you work through the system. This procedure may uncover a malfunction that is difficult to locate.