OPERATION
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 hous-
ing. 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 and
sends the code over the CCD 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 pro-
grammed Smart Key transponders. The security code is
used by the assembly plant to access the SKIS for ini-
tialization, or by the dealer technician to access the sys-
tem for service. The SKIM also stores in its memory the
Vehicle Identification Number (VIN), which it learns
through a CCD 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 secu-
rity 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 then sends a “valid key” message to
the PCM over the CCD 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 instrument
cluster over the CCD data bus network to control 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 igni-
tion switch is turned to the ON position, it sends
messages to the instrument cluster to flash the VTSS
indicator LED. The SKIM can also send messages to
the instrument cluster to flash the LED and to gen-
erate a single audible chime tone. These functions
serve as an indication to the customer that the SKIS
has been placed in its “Customer Learn” program-
ming mode. See Sentry Key Immobilizer System
Transponder Programming in this group for more
information on the “Customer Learn” programming
mode.
For diagnosis or initialization of the SKIM and the
PCM, a DRB lll
t scan tool and the proper Body Diag-
nostic Procedures Manual are required. The SKIM
cannot be repaired, and if faulty or damaged, the
unit must be replaced.
SENTRY KEY IMMOBILIZER TRANSPONDER
DESCRIPTION
The Sentry Key Immobilizer System (SKIS) uses a
transponder that is integral to each of three ignition
key that are supplied with the vehicle when it is
shipped from the factory. The transponder chip is insu-
lated within a nylon mount inserted in the head of the
key, and invisible beneath a molded rubber cap.
Fig. 2 Sentry Key Immobilizer Transponder
1 – MOLDED CAP
2 – TRANSPONDER
3 – MOLDED CAP REMOVED
4 – SENTRY KEY
JX
VEHICLE THEFT/SECURITY SYSTEMS
8Q - 3
DESCRIPTION AND OPERATION (Continued)