Section 55 (Vehicle Theft Security (Service Information) 500 - Electric): Operation

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The Sentry Key Immobilizer System (SKIS) is designed to provide passive protection against unauthorized vehicle use by disabling the vehicle whenever any method other than a valid Sentry Key is used to transition the status of the ignition switch to On. The SKIS is considered a passive protection system because it is always active when the ignition system is energized and does not require any customer intervention. The SKIS uses Radio Frequency (RF) communication to obtain confirmation that the key in the ignition switch is a valid key for operating the vehicle. The microcontroller-based SKIS hardware and software also uses electronic messages to communicate with other electronic modules in the vehicle over the Controller Area Network (CAN) data bus.

The functions and features of the SKIS are performed by hardware and software integral to the Body Control Module (BCM) in this vehicle. The BCM contains a Radio Frequency (RF) transceiver and a microcontroller. The BCM transmits Low Frequency (LF) signals to, and receives LF signals from the Sentry Key transponder integral to the Sentry Key transponder through a tuned antenna external to the ignition switch lock housing. The BCM communicates over the Controller Area Network (CAN) data bus with the Instrument Panel Cluster (IPC), the Electric Vehicle Control Unit (EVCU) or the diagnostic scan tool.

The BCM and the EVCU 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 BCM or the EVCU. However, the use of this strategy also means that replacement of either the BCM or the EVCU units will require a system initialization procedure to restore system operation.

The BCM 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 BCM. For added system security, each BCM is programmed with a unique Secret Key code. This code is stored in memory, sent over the CAN data bus to the EVCU, and is encoded to the transponder of every Sentry Key that is programmed into the BCM. Therefore, the Secret Key code is a common element that is found in every component of the SKIS.

In the event that a BCM replacement is required, the new BCM must be obtained through an authorized Service Center. The Service Center will program and initialize the replacement BCM prior to installation in the vehicle. Proper completion of the BCM initialization will allow the existing Sentry Key transponders to be programmed into the new BCM so that new Sentry Key transponders will not be required.

When the status of the ignition switch transitions to On, the BCM transmits a LF signal to excite the transponder in the Sentry Key. The BCM then waits for a LF signal response from the transponder. If the response received identifies the transponder as valid, the BCM sends an electronic valid key message to the EVCU over the CAN data bus. If the response received identifies the transponder as invalid or if no response is received from the transponder, the BCM sends an invalid key message to the EVCU. The EVCU will enable or disable vehicle operation based upon the status of the BCM messages. It is important to note that the default condition in the EVCU is an invalid key ; therefore, if no message is received from the BCM by the EVCU, the vehicle will be disabled from operation.

The BCM also sends electronic security indicator request messages to the IPC over the CAN data bus to tell the IPC how to operate the security indicator. The security indicator request message from the BCM tells the IPC to turn the indicator On for about three seconds each time the status of the ignition switch transitions to On as a bulb test. After completion of the bulb test, the BCM sends security indicator request messages to the IPC to turn the indicator Off, turn the indicator On, or to flash the indicator On and Off. If the security indicator flashes or stays On solid after the bulb test, it signifies a SKIS fault. If the BCM detects a system malfunction or the SKIS has become ineffective, the security indicator will stay On solid. If the BCM detects an invalid Sentry Key transponder or if a transponder-related fault exists, the security indicator will flash.

The SKIS performs a self-test each time the status of the ignition switch transitions to On, and will store fault information in the form of a Diagnostic Trouble Code (DTC) in BCM memory if a system malfunction is detected. The hard wired circuits of the BCM may be diagnosed using conventional diagnostic tools and procedures. Refer to the appropriate wiring information. However, conventional diagnostic methods will not prove conclusive in the diagnosis of the BCM or the electronic controls and communication between other modules and devices that provide some features of the SKIS. The most reliable, efficient and accurate means to diagnose the BCM or the electronic controls and communication related to SKIS operation requires the use of a diagnostic scan tool.