Grand Cherokee 2WD V8-5.2L VIN Y (1998)
Body Control Module: Description and Operation
Airbag System
Functional Operation
FUNCTIONAL OPERATION
The airbag system is designed to provide increased driver and passenger protection if the vehicle is involved in a front-end collision. The system is
most effective when used in conjunction with the seat belt system. The passenger side air bag assembly utilizes a canister of a compressed, inert
gas to inflate the bag. This gas is expanded at a very high rate through a heating process. This allows a small quantity of gas under a very high
pressure to inflate the large passenger airbag.
These vehicles utilize an accelerometer and a safing sensor located in the Airbag Electronic Control Module (AECM) to control the deployment
of the airbags. The AECM is one type of Airbag Control Modules (ACM)
Diagnosis of the airbag system is done through the Data Link Connector (DLC) located under and just to the left of the steering column. The
airbag lamp is located in the cluster. Whenever the ignition key is turned to the 'run" position, the cluster illuminates the bulb for 7 seconds. This
illumination is called bulb prove-out. After the 7 seconds, the bulb will normally extinguish. The bulb will also illuminate if the ACM senses a
fault in the Airbag system and communicates this fact to the cluster via the CCD bus. The cluster will also light the bulb if the ACM fails to
communicate with it for 5 consecutive seconds. For most reasons the ACM commands the cluster to illuminate the airbag lamp and the ACM will
keep the lamp illuminated for a minimum of 12 seconds. The AIRBAG warning lamp is the only point at which "symptoms" of a system
malfunction can be observed by the customer. This lamp is not dimable.
Under some conditions the AECM can latch the airbag warning lamp on and keep it on, for a predetermined length of time, even if the code is not
active at the present time. The term latched is used to show that the presents of an active code is not needed to keep the airbag warning lamp on.
The warning lamp will be latched on for the duration of the ignition cycle in which the internal Diagnostic 3, Internal Diagnostic 4 and Ignition
Run/Only codes become active. if the AECM Accelerometer, Internal Diagnostic 1, or internal Diagnostic 2, codes are active for two consecutive
ignition cycles, the AECM will latch the warning lamp on permanently.
In the event that the cluster detects a failure in the airbag lamp circuit, the cluster will substitute the seat belt lamp as a backup. if the airbag lamp
is bad the cluster will flash the seat belt lamp for 20 seconds after prove-out time. If the airbag lamp must be turned on because of an airbag fault,
the seat belt lamp will be used instead, but will flash for 20 seconds and then remain on continuously until the fault is fixed.
The ACM monitors critical input and output circuits within the airbag system, making sure they are operating correctly. Some circuits are tested
continuously; others are checked only under certain circumstances. The ACM provides diagnostic information about the airbag system to the
technician through the DRB via the CCD bus.
The deceleration or G-force resulting from the impact of a front-end collision cause the AECM internal accelerometer, and the safing sensor are
closed, an electrical charge is sent to the airbag module. This causes the inflator to be actuated, thus deploying the airbags. The total time from the
closure of the sensors to deployment and deflation of the airbags is 1/10 of one second.
The ACM also contains an energy storage capacitor. This capacitor stores enough electrical energy to deploy the airbags for up to 300
milliseconds following a battery disconnect or failure, during an impact event. The purpose of this feature is to provide airbag deployment in a
severe secondary impact if the initial impact has damaged or disconnected the battery, but was not severe enough to deploy the airbags.
Airbag Diagnostic Trouble Codes
ACTIVE CODES
An active trouble code indicates an on-going malfunction. This means the defect exists every time the Airbag Control Module (ACM) checks that
circuit/function. It is impossible to erase an active code; active codes are automatically erased by the ACM when the reason for the code has been
corrected.
STORED CODES
Airbag codes are continuously stored in the ACM (AECM) code is stored 1 minute after the code was detected or when the ignition is turned off,
with the exception of the Loss of Ignition Run Only code, which is an active code only.
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 that 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 would be three minutes.
