RAM 2500 Truck 2WD V8-5.7L VIN D (2004)
Alarm Module: Description and Operation
SENTRY KEY IMMOBILIZER MODULE
The Sentry Key Immobilizer Module (SKIM) contains a Radio Frequency (RF) transceiver and a central processing unit, which includes the
Sentry Key Immobilizer System (SKIS) program logic. The SKIS programming enables the SKIM to program and retain in memory the codes of
at least two, but no more than eight electronically coded Sentry Key transponders. The SKIS programming also enables the SKIM to communicate
over the Programmable Communication Interface (PCI) bus network with the Powertrain Control Module (PCM), and/or the DRB III scan tool.
The SKIM transmits and receives RF signals through a tuned antenna enclosed within a molded plastic ring that is integral to the SKIM housing.
When the SKIM is properly installed on the steering column, the antenna ring is oriented around the ignition lock cylinder housing. This antenna
ring must be located within eight millimeters (0.31 inches) of the Sentry Key in order to ensure proper RF communication between the SKIM and
the Sentry Key transponder.
For added system security each SKIM is programmed with a unique "Secret Key" code and a security code. The SKIM keeps the "Secret Key"
code in memory The SKIM also sends the "Secret Key" code to each of the programmed Sentry Key transponders. The security code is used by
the assembly plant to access the SKIS for initialization, or by the dealer technician to access the system for service. The SKIM also stores in its
memory the Vehicle Identification Number (VIN), which it learns through a PCI bus message from the PCM during initialization.
The SKIM and the PCM both use software that includes a rolling code algorithm strategy, which helps to reduce the possibility of unauthorized
SKIS disarming. The rolling code algorithm ensures security by preventing an override of the SKIS through the unauthorized substitution of the
SKIM or the PCM. However, the use of this strategy also means that replacement of either the SKIM or the PCM units will require a system
initialization procedure to restore system operation.
When the ignition switch is turned to the ON or START positions, the SKIM transmits an RF signal to excite the Sentry Key transponder. The
SKIM then listens for a return RF signal from the transponder of the Sentry Key that is inserted in the ignition lock cylinder. If the SKIM receives
an RF signal with valid "Secret Key" and transponder identification codes, the SKIM sends a "valid key" message to the PCM over the PCI bus. If
the SKIM receives an invalid RF signal or no response, it sends "invalid key" messages to the PCM. The PCM will enable or disable engine
operation based upon the status of the SKIM messages.
The SKIM also sends messages to the Instrument Cluster which controls the VTSS indicator LED. The SKIM sends messages to the Instrument
Cluster to turn the LED ON for about three seconds when the ignition switch is turned to the ON position as a bulb test. After completion of the
bulb test, the SKIM sends bus messages to keep the LED OFF for a duration of about one second. Then the SKIM sends messages to turn the
LED ON or OFF based upon the results of the SKIS self-tests. If the VTSS indicator LED comes ON and stays ON after the bulb test, it indicates
that the SKIM has detected a system malfunction and/or that the SKIS has become inoperative.
If the SKIM detects an invalid key when the ignition switch is turned to the ON position, it sends messages to flash the VTSS indicator LED. The
SKIM can also send messages to flash the LED as an indication to the customer that the SKIS has been placed in it's "Customer Learn"
programming mode. See Sentry Key Immobilizer System Transponder Programming in this section for more information on the "Customer Learn"
For diagnosis or initialization of the SKIM and the PCM, a DRB III scan tool and the proper Powertrain Diagnostic Procedures manual are
The SKIM cannot be repaired and, if faulty or damaged, the unit must be replaced.