RAM 1500 Truck 2WD V6-3.9L VIN X (1999)
Air Bag Control Module: Description and Operation
DESCRIPTION
The Airbag Control Module (ACM) is secured with screws to a mounting bracket located under the instrument panel center support bracket on the
floor panel transmission tunnel in the passenger compartment of the vehicle. A plastic trim cover on the floor panel transmission tunnel conceals
the ACM and the instrument panel center support bracket. The ACM contains an electronic microprocessor, an electronic impact sensor, an
electromechanical safing sensor, and an energy storage capacitor.
Club cab and quad cab versions of this vehicle are equipped with a structural seat system. The structural seat system uses an electronic seat belt
control system, which includes a Seatbelt Control Timer Module (SCTM). The SCTM has a hard wired fault circuit input to the ACM, which the
SCTM uses to signal the status of the electronic seat belt control system to the ACM. If the ACM detects a fault input or no input from the SCTM,
it sends messages to the instrument cluster over the CCD data bus to turn on the seat belt reminder lamp. Refer to Seat Belt Control Systems for
more information on the electronic seat belt control system and the SCTM.
The ACM cannot be repaired or adjusted and, if damaged or faulty, it must be replaced.
OPERATION
The microprocessor in the ACM contains the airbag system logic. The airbag system logic includes On-Board Diagnostics (OBD), and the ability
to communicate with the instrument cluster circuitry over the Chrysler Collision Detection (CCD) data bus to control the airbag indicator lamp.
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 CCD data bus to turn on the airbag indicator lamp. Refer to Instrument
Cluster in Instrument Panel Systems for more information 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 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.
A unique feature of the ACM used in this model is that it reads and learns the Vehicle Identification Number (VIN) through messages received
from the Powertrain Control Module (PCM) over the CCD data bus. This was done so that the ACM could learn whether the vehicle was equipped
with a conventional seat system or the structural seat system. It takes approximately 20 engine start cycles before the complete VIN number is
learned and stored in the ACM memory. Once the ACM has stored the VIN information in memory, it cannot be installed in any other vehicle
without setting a VIN mismatch fault.
