Module, Sentry Key Immobilizer (Skim): Operation: Operation

The functions and features of the Sentry Key Immobilizer Module (SKIM) (also previously known as the Sentry Key REmote Entry Module/SKREEM, the Wireless Control Module/WCM or the Wireless Ignition Node/WIN) are all integral to the Radio Frequency Hub Module (RFHM) (also known as the RF Hub) and the IGnition Node Module/IGNM or the Keyless Ignition Node (KIN) in this vehicle. The RFHM contains a Radio Frequency (RF) transceiver and a microcontroller.

In vehicles with a rotary ignition switch the RFHM controls Low Frequency (LF) signaling with the Sentry Key transponder integral to the mechanical key or FOB with Integrated Key (FOBIK) through a tuned antenna internal to the IGNM. The IGNM transmits the received LF signal and the selected ignition switch status back to the RFHM. In vehicles with the push button Keyless Go (KG) system, the RFHM transmits LF signals to, and receives LF signals from the Sentry Key transponder integral to the FOBIK through a tuned antenna internal to the RFHM housing, while the KIN allows the vehicle operator to select and monitor the status of the ignition switch and also transmits the selected ignition switch status back to the RFHM.

The RFHM also serves as the Remote Keyless Entry (RKE) RF receiver and, if the vehicle is so equipped, the receiver for the remote start system and for the Tire Pressure Monitoring (TPM) system. The RFHM communicates over the Controller Area Network (CAN) data bus with the Body Control Module (BCM) (also known as the Common Body Controller/CBC), the Instrument Cluster (IC) (also known as the Common Instrument Cluster/CIC), the Powertrain Control Module (PCM) (also known as the Engine Control Module/ECM) or the diagnostic scan tool. It also communicates over a dedicated Local Interface Network (LIN) data bus circuit with the IGNM or the KIN.

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

The RFHM retains in memory the ID numbers of any Sentry Key or FOBIK transponder that is programmed into it. A maximum of eight Sentry Key transponders can be programmed into the RFHM. For added system security, each RFHM is programmed with a unique Secret Key code. This code is stored in memory, sent over the CAN data bus to the PCM, and is encoded to the transponder of every Sentry Key that is programmed into the RFHM. 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 RFHM Secured Access Mode. The Secured Access Mode is required during service to perform the SKIS initialization and Sentry Key transponder programming procedures. The RFHM also stores the Vehicle Identification Number (VIN) in its memory, which it learns through a CAN data bus message from the PCM during SKIS initialization.

In the event that a RFHM replacement is required, the Secret Key code can be transferred to the new RFHM from the PCM using the diagnostic scan tool and the SKIS initialization procedure. Proper completion of the SKIS initialization will allow the existing Sentry Key transponder units to be programmed into the new RFHM so that new Sentry Key units will not be required. In the event that the original Secret Key code cannot be recovered, RFHM replacement will also require new Sentry Key units. The diagnostic scan tool will alert the technician during the SKIS initialization procedure if new Sentry Key units are required. Refer to STANDARD PROCEDURE .

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

The RFHM also sends electronic security indicator  request messages to the IC over the CAN data bus to tell the IC how to operate the security indicator. The security indicator  request message from the RFHM tells the IC 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 RFHM sends security indicator  request messages to the IC 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 RFHM detects a system malfunction or the SKIS has become ineffective, the security indicator will stay On solid. If the RFHM detects an invalid key or FOBIK, or if a key or FOBIK 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 RFHM memory if a system malfunction is detected. The hard wired circuits of the RFHM 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 RFHM 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 RFHM or the electronic controls and communication related to RFHM operation requires the use of a diagnostic scan tool.