Grand Cherokee 4WD V8-4.7L VIN N (2000)
Alarm Module: Description and Operation
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
Communications Interface (PCI) data bus network with the Powertrain Control Module (PCM), the Electro-Mechanical Instrument Cluster (EMIC), the
Body Control Module (BCM) and/or the DRB scan tool.
The SKIM transmits and receives RF signals through a tuned antenna enclosed within a molded plastic ring formation that is integral to the SKIM
housing. When the SKIM is properly installed on the steering column, the antenna ring is oriented around the circumference of the ignition lock cylinder
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 and sends the code over the PCI data bus to the PCM, which also keeps this code in its 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
data 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.
For diagnosis or initialization of the SKIM and the PCM, a DRB scan tool and the proper Diagnostic Procedures are required. The SKIM cannot be
repaired and, if faulty or damaged, the unit must be replaced.
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 data 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 EMIC over the PCI data bus network to control the SKIS indicator lamp. The SKIM sends messages to the EMIC
to turn the lamp 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 lamp off for a duration of about one second. Then the SKIM sends messages to turn the lamp on or off based upon
the results of the SKIS self-tests. If the SKIS indicator lamp 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 the EMIC to flash the SKIS indicator lamp.
The SKIM can also send messages to the EMIC to flash the lamp and to the BCM to generate a single audible chime tone. These functions serve as an
indication to the customer that the SKIS has been placed in its "Customer Learn" programming mode. See Sentry Key Immobilizer System Transponder
Programming for more information on the "Customer Learn" programming mode.