Alarm Module: Description and Operation
The Sentry Key Immobilizer Module (SKIM) (1) is the primary component of the Sentry Key Immobilizer System (SKIS). This module contains a Radio
Frequency (RF) transceiver and a central processing unit, which includes the Sentry Key Immobilizer System (SKIS) program logic. The SKIM is
located on the right side of the steering column, near the ignition lock cylinder housing and is concealed beneath the steering column shrouds. The
molded black plastic housing for the SKIM has an integral molded plastic halo-like antenna ring (4) that extends from the bottom. When the SKIM is
properly installed on the steering column, the antenna ring is oriented around the circumference of the ignition lock cylinder housing.
A single integral connector receptacle (3) is located just behind the antenna ring on the bottom of the SKIM housing. An integral molded plastic
mounting tab (2) on the rear corner of the SKIM housing has a hole in the center through which a screw passes to secure the unit to the steering column.
The SKIM is connected to the vehicle electrical system through a single take out and connector of the instrument panel wire harness.
The Sentry Key Immobilizer Module (SKIM) transmits Radio Frequency (RF) signals to, and receives RF signals from the Sentry Key transponder
through a tuned antenna enclosed within the molded plastic antenna ring. 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) for gasoline engines or the Engine Control Module (ECM) for diesel engines, the ElectroMechanical Instrument Cluster (EMIC) and/or a
diagnostic scan tool.
The SKIM and the PCM/ECM both use software that includes a rolling code algorithm strategy, which helps to reduce the possibility of unauthorized
Sentry Key Immobilizer System (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/ECM. However, the use of this strategy also means that replacement of either the SKIM or the
PCM/ECM units will require a system initialization procedure to restore system operation.
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 or ECM, 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 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 or ECM 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 a diagnostic 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 diagnostic scan tool will alert the technician during the SKIS initialization procedure if new Sentry Keys are required.
When the ignition switch is turned to the On or Start positions, the SKIM transmits an RF signal to excite the transponder in the ignition key. The SKIM
then listens for a return RF signal from the transponder. If the SKIM receives an RF signal with a valid "Secret Key" and transponder identification
codes, the SKIM sends a "valid key" message to the PCM/ECM over the PCI bus. If the SKIM receives an invalid RF signal or no response, it sends
"invalid key" messages to the PCM/ECM. The PCM/ECM will enable or disable engine operation based upon the status of the SKIM messages. It is
important to note that the default condition in the PCM or ECM is an invalid key. Therefore, if no message is received from the SKIM by the PCM or
ECM, the engine will be disabled and the vehicle immobilized after two seconds of running.