Read especially the 2nd to last paragraph, which I have marked with an ====>, maybe that will help:
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 housing.
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.
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