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.

The SKIM also sends electronic messages to the EMIC which control the VTSS indicator. The SKIM sends messages to the EMIC to turn the indicator 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 indicator off for a duration of about one second. Then the SKIM sends messages to turn the indicator on or off based upon the results of the SKIS self-tests. If the VTSS indicator 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 flash the VTSS indicator. The SKIM can also send messages to flash the indicator and to generate a single audible chime whenever the Customer Learn programming mode is being utilized.

The SKIS will store fault information in the form of Diagnostic Trouble Codes (DTC's) in SKIM memory if a system malfunction is detected. For diagnosis or initialization of the SKIM and the PCM/ECM, a diagnostic scan tool is required.