Grand Cherokee 4WD V8-4.7L VIN N (2000)
Air Bag Control Module: Description and Operation
The Airbag Control Module (ACM) is secured with screws to a mount that is welded onto the floor panel transmission tunnel underneath the center floor
console in the passenger compartment of the vehicle. The ACM contains an electronic microprocessor, an electronic impact sensor, an electromechanical
safing sensor, and an energy storage capacitor.The ACM cannot be repaired or adjusted and, if damaged or faulty, it must be replaced.
The microprocessor in the ACM contains the airbag system logic. The airbag system logic includes On-Board Diagnostics (OBD), and the ability to
control the airbag indicator lamp by communicating with the instrument cluster circuitry over the Programmable Communication Interface (PCI) data
bus. The microprocessor continuously monitors all of 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 over the PCI data bus to turn on the airbag indicator lamp.
One electronic impact sensor is used in this airbag system. 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. A pre-programmed decision algorithm in the ACM microprocessor determines when the deceleration rate as signaled by the impact 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 airbags.
In addition to the electronic impact sensor, there is an electromechanical sensor within the ACM called a safing sensor. The safing sensor is a normally
open series switch located in the airbag deployment circuit of the ACM. This sensor detects impact energy of a lesser magnitude than the electronic
impact sensor and must be closed in order for the airbags to deploy.
The ACM also contains an energy-storage capacitor. This capacitor stores enough electrical energy to deploy the airbags for up to one second following
a battery disconnect or failure during an impact. The purpose of the capacitor is to provide backup airbag system protection in case there is a loss of
battery power supply to the ACM.
