System Description And Functional Operation; Airbag System - Chrysler 300M 1999 Manual

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3.0

SYSTEM DESCRIPTION AND FUNCTIONAL OPERATION

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The body system on the 1999 300M consists of a combination of modules that communicate over
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the PCI bus (Programmable Communication Interface multiplex system). Through the PCI bus,
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information about the operation of vehicle components and circuits is relayed quickly to the
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appropriate module(s). All modules receive all the information transmitted on the bus even though
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a module may not require all information to perform it's function. It will only respond to messages
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addressed to it through a binary coding process. This method of data transmission significantly
reduces the complexity of the wiring in the vehicle and the size of wiring harnesses. All of the
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information about the functioning of all the systems is organized, controlled, and communicated
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by the PCI bus, which is described in Section 3.14 (Vehicle Communication) of this general
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information.
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3.1

Airbag System

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The Airbag Control Module (ACM) is secured to a bracket on the floor panel transmission tunnel
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below the instrument panel inside the vehicle. The ACM mounting bracket also serves as the
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instrument panel center support. The ACM contains a microprocessor, the impact sensor, and
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energy storage capacitor. The microprocessor contains the airbag system logic. The ACM system
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logic includes On-Board Diagnostics (OBD) capability, and communicates with the instrument
cluster circuitry on the Programmable Communication Interface (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 pre-programmed decision algorithm in the ACM microprocessor
determines when the deceleration rate 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. This capacitor stores enough electrical energy to
deploy the airbags, for one second following a battery disconnect or failure during an impact. 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 a
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
remains 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.
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