Alarm Module: Description and Operation
SENTRY KEY IMMOBILIZER MODULE
Sentry Key Immobilizer Module - Typical
The Sentry Key Immobilizer Module (SKIM) is the primary component of the Sentry Key Immobilizer System (SKIS). The SKIM is located in the
steering column, below the ignition lock cylinder housing. The SKIM has an integral halo-like antenna ring that extends from one end. When the SKIM
is properly installed on the steering column, the antenna ring is oriented around the circumference of the ignition lock cylinder housing.
The SKIM cannot be adjusted or repaired. If faulty or damaged, the entire SKIM unit must be replaced.
The Sentry Key Immobilizer Module (SKIM) contains a Radio Frequency (RF) transceiver and a microprocessor. The SKIM transmits RF signals to,
and receives RF signals from the Sentry Key transponder through a tuned antenna enclosed within the molded plastic antenna ring integral to the SKIM
housing. If this antenna ring is not mounted properly around the ignition lock cylinder housing, communication problems between the SKIM and the
transponder may arise. These communication problems will result in Sentry Key transponder-related faults. The SKIM also communicates over the
Programmable Communications Interface (PCI) data bus with the Powertrain Control Module (PCM), the ElectroMechanical Instrument Cluster (EMIC)
and/or the DRBIII(R) scan tool.
The SKIM retains in memory the ID numbers of any Sentry Key transponder that is programmed into it. A maximum of eight Sentry Key transponders
can be programmed into the SKIM. For added system security, each SKIM is programmed with a unique Secret Key code. This code is stored in
memory, sent over the PCI data bus to the PCM, and is encoded to the transponder of every Sentry Key that is programmed into the SKIM. Therefore,
the Secret Key code is a common element that is found in every component of the Sentry Key Immobilizer System (SKIS). Another security code, called
a PIN, is used to gain access to the SKIM Secured Access Mode. The Secured Access Mode is required during service to perform the SKIS initialization
and Sentry Key transponder programming procedures. The SKIM also stores the Vehicle Identification Number (VIN) in its memory, which it learns
through a PCI data bus message from the PCM during SKIS initialization.
In the event that a SKIM replacement is required, the Secret Key code can be transferred to the new SKIM from the PCM using the DRBIII(R) scan tool
and the SKIS initialization procedure. Proper completion of the SKIS initialization will allow the existing Sentry Keys to be programmed into the new
SKIM so that new keys will not be required. In the event that the original Secret Key code cannot be recovered, SKIM replacement will also require new
Sentry Keys. The DRBIII(R) scan tool will alert the technician during the SKIS initialization procedure if new Sentry Keys are required.
When the ignition switch is On, the SKIM transmits an RF signal to the transponder in the ignition key. The SKIM then waits for an RF signal response
from the transponder. If the response received identifies the key as valid, the SKIM sends a valid key message to the PCM over the PCI data bus. If the
response received identifies the key as invalid, or if no response is received from the key transponder, the SKIM sends an invalid key message to the
PCM. The PCM will enable or disable engine operation based upon the status of the SKIM messages. It is important to note that the default condition in