Alarm Module: Description and Operation
The Sentry Key REmote Entry Module (SKREEM) (1) is sometimes referred to as the Wireless Control Module (WCM). The SKREEM is the primary
component of the Sentry Key Immobilizer System (SKIS) and is also the receiver for the Remote Keyless Entry (RKE) system. The SKREEM 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 SKREEM has an integral molded plastic halo-like antenna ring (4) that extends from the bottom. When the SKREEM 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 SKREEM housing. An integral molded plastic
mounting tab (2) on the rear corner of the SKREEM housing has a hole in the center through which a screw passes to secure the unit to the steering
column. The SKREEM is connected to the vehicle electrical system through a single take out and connector of the instrument panel wire harness.
Two SKREEM modules are used: one for vehicles equipped with RKE only, and one for vehicles equipped with RKE and SKIS. The SKREEM cannot
be adjusted or repaired. If faulty or damaged, the entire SKREEM unit must be replaced.
The Sentry Key REmote Entry Module (SKREEM) contains a Radio Frequency (RF) transceiver and a microprocessor. The SKREEM transmits RF
signals to, and receives RF signals from the Sentry Key transponder through a tuned antenna enclosed within the molded plastic antenna ring integral to
the SKREEM housing. If this antenna ring is not mounted properly around the ignition lock cylinder housing, communication problems between the
SKREEM and the transponder may arise. These communication problems will result in Sentry Key transponder-related faults. The SKREEM also serves
as the Remote Keyless Entry (RKE) RF receiver. The SKREEM communicates over the Controller Area Network (CAN) data bus with the
ElectroMechanical Instrument Cluster (EMIC), the Powertrain Control Module (PCM), and/or the diagnostic scan tool.
The SKREEM 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 SKREEM. For added system security, each SKREEM 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
SKREEM. Therefore, the Secret Key code is a common element that is found in every component of the Sentry Key Immobilizer System (SKIS).
Another security code, called a PIN, is used to gain access to the SKREEM Secured Access Mode. The Secured Access Mode is required during service
to perform the SKIS initialization and Sentry Key transponder programming procedures. The SKREEM 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 SKREEM replacement is required, the Secret Key code can be transferred to the new SKREEM from the PCM using the 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 SKREEM so that new keys will not be required. In the event that the original Secret Key code cannot be recovered, SKREEM 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 position, the SKREEM transmits an RF signal to the transponder in the ignition key. The SKREEM then
waits for an RF signal response from the transponder. If the response received identifies the key as valid, the SKREEM sends a valid key message to the
PCM over the CAN data bus. If the response received identifies the key as invalid or if no response is received from the key transponder, the SKREEM
sends an invalid key message to the PCM. The PCM will enable or disable engine operation based upon the status of the SKREEM 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 SKREEM by the PCM, the engine
will be disabled and the vehicle immobilized after two seconds of running.
The SKREEM also sends security indicator status messages to the EMIC over the CAN data bus to tell the EMIC how to operate the security indicator.
The security indicator status message from the SKREEM tells the EMIC to turn the indicator on for about three seconds each time the ignition switch is
turned to the On position as a bulb test. After completion of the bulb test, the SKREEM sends security indicator status messages to the EMIC 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