Keyless Entry Module: Description and Operation

SENTRY KEY REMOTE ENTRY MODULE
The Sentry Key Remote Entry Module (SKREEM) performs the functions of the Sentry Key Immobilizer Module (SKIM), Remote Keyless Entry (RKE) Module, and the Tire Pressure Monitoring (TPM) System (previously part of the Electronic Vehicle Information Center (EVIC).

The SKREEM is located in the same location as the SKIM was and is mounted the same way. It looks identical, but has added capabilities.

SENTRY KEY IMMOBILIZER
The Sentry Key Immobilizer System (SKIS) authenticates an electronically coded Transponder Key placed into the ignition and sends a valid/invalid key message to the Powertrain Control Module (PCM) based upon the results. The "VALID/INVALID KEY" message communication is performed using a rolling code algorithm via the Programmable Communication Interface (PCI) data bus. A "VALID KEY" message must be sent to the Powertrain Control Module (PCM) within two seconds of ignition ON to free the engine from immobilization.

The SKREEM contains a Radio Frequency (RF) transceiver and a microprocessor. The SKREEM retains in memory the ID numbers of any Sentry Key that is programmed to it. The maximum number of keys that may be programmed to each module is eight 8. The SKREEM also communicates over the Programmable Communication Interface (PCI) data bus with the Powertrain Control Module (PCM), the Body Control Module (BCM), the Mechanical Instrument Cluster (MIC), and the DRBIII(R) scan tool. The SKREEM transmits and receives RF signals through a tuned antenna enclosed within a molded plastic ring formation that is integral to the SKREEM housing. When the SKREEM is properly installed on the steering column, the antenna ring fits snugly around the circumference of the ignition lock cylinder housing. If this ring is not mounted properly, communication problems may arise in the form of transponder-related faults.

For added system security, each SKREEM is programmed with a unique "Secret Key" code. This code is stored in memory and is sent over the PCI bus to the PCM and to each key that is programmed to work with the vehicle. The "Secret Key" code is therefore a common element found in all components of the Sentry Key Immobilizer System (SKIS). In the event that a SKREEM replacement is required, the "Secret Key" code can be restored from the PCM by following the SKIM replacement procedure found in the DRBIII(R) scan tool. Proper completion of this task will allow the existing ignition keys to be reprogrammed. Therefore, new keys will NOT be needed. In the event that the original "Secret Key" code can not be recovered, new ignition keys will be required. The DRBIII(R) scan tool will alert the technician if key replacement is necessary. Another security code, called a PIN, is used to gain secured access to the SKREEM for service. The SKREEM also stores in its memory the Vehicle Identification Number (VIN), which it learns through a bus message from the assembly plant tester. The SKIS scrambles the information that is communicated between its components in order to reduce the possibility of unauthorized SKREEM access and/or disabling.

The Sentry Key Remote Entry Module (SKREEM) receives an encrypted Radio Frequency (RF) signal from the transponder key. The SKREEM then decrypts the signal and broadcasts the requested remote commands to the appropriate modules in the vehicle over the Programmable Communication Interface (PCI) data bus. A valid transponder key ID must be incorporated into the RF signal in order for the SKREEM to pass the message on to the appropriate modules.

Automatic transponder key synchronization is done by the SKREEM if a valid transponder key is inserted into the ignition cylinder, and the ignition is turned ON. This provides a maximum operation window for RKE functions.

Each Sentry Key Remote Entry System (SKREES) consists of a SKREEM and a transponder key. Each system has a secret key code unique to that system. The secret key is electronically coded in the SKREEM and in all programmed transponder keys. It is used for immobilization and RKE functions for data security. In addition, each transponder key will have a unique identification.

When the ignition switch is moved to the RUN position, the SKREEM transmits an Radio Frequency (RF) signal to the transponder in the ignition key. The SKREEM then waits for a response RF signal from the transponder in the key. If the response received identifies the key as valid, the SKREEM sends a "valid key" message to the Powertrain Control Module (PCM) over the Programmable Communication Interface (PCI) data bus. If the response received identifies the key as invalid or no response is received from the transponder in the ignition key, 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 "invalid key." Therefore, if no response is received by the PCM, the engine will be immobilized after two (2) seconds of running.

The SKREEM also sends indicator light status messages to the Mechanical Instrument Cluster (MIC) to operate the light. This is the method used to turn the light ON solid or to flash it after the indicator light test is complete to signify a fault in the SKREES. If the light comes ON and stays ON solid after the indicator light test, this signifies that the SKREEM has detected a system malfunction and/or that the SKREES has become inoperative. If the SKREEM detects an invalid key OR a key-related fault exists, the indicator light will flash following the indicator light test. The SKREEM may also request an audible chime if the customer key programming feature is available and the procedure is being utilized.

REMOTE KEYLESS ENTRY (RKE)
The RKE transmitter uses radio frequency signals to communicate with the SKREEM. The SKREEM is on the PCI bus. When the operator presses a button on the transmitter, it sends a specific request to the SKREEM. In turn the SKREEM sends the appropriate request over the PCI Bus to the:
- Body Control Module (BCM) to control the door lock and unlock functions, the liftgate lock and unlock functions, the arming and disarming of the Vehicle Theft Security System (VTSS) (if equipped), and the activation of illuminated entry.
- Integrated Power Module (IPM) to activate the park lamps, the headlamps, and the horn for horn chirp. If requested, the BCM sends a request over the PCI Bus to the:

After pressing the lock button on the RKE transmitter, all of the door locks will lock, the illuminated entry will turn off (providing all doors are closed), and the VTSS (if equipped) will arm. After pressing the unlock button, on the RKE transmitter, one time, the driver door lock will unlock, the illuminated entry will turn on the courtesy lamps, and the VTSS (if equipped) will disarm. After pressing the unlock button a second time, the remaining door locks will unlock. The Electronic Vehicle Information Center (EVIC) or the DRBIII(R) scan tool can reprogram this feature to unlock all of the door locks with one press of the unlock button. If the vehicle is equipped with the memory system, the memory message will identify which transmitter (1 or 2) sent the signal.

The SKREEM is capable of retaining up to 8 individual access codes (8 transmitters). If the PRNDL is in any position except park, the SKREEM will disable the RKE. The 4 button transmitter uses 1-CR2032 battery. The minimum battery life is approximately 4.7 years based on 20 transmissions a day at 84°F (25°C). Use the DRBIII(R) scan tool or the Miller Tool 9001 RF Detector to test the RKE transmitter. Use the DRBIII(R) or the customer programming method to program the RKE system. However, the SKREEM will only allow RKE programming when the ignition is in the ON position, the PRNDL is in park position, and the VTSS (if equipped) is disarmed.

TIRE PRESSURE MONITORING (TPM)
If equipped with the Tire Pressure Monitoring (TPM) System, each of the vehicles four wheels will have a valve stem with a pressure sensor and radio transmitter built in. Signals from the tire pressure sensor/transmitter are received and interpreted by the SKREEM.

A sensor/transmitter in a mounted wheel will broadcast its detected pressure once per minute when the vehicle is moving faster than 15 mph (24 km/h). Each sensor/transmitter's broadcast is uniquely coded so that the SKREEM can determine the location.

The SKREEM monitors the signals from the tire pressure sensor/transmitters and determines if any tire has gone below the low pressure threshold LOW TIRE PRESSURE THRESHOLDS table.

Low Tire Pressure Thresholds:

CRITICAL AND NON-CRITICAL SYSTEM ALERTS

CRITICAL: A critical alert will be triggered when a tire pressure has gone below a set threshold pressure. The SKREEM will display "X TIRE(S) LOW PRESSURE". "X" will be the number of tires reporting low pressure. The message will display for the duration of the current ignition cycle or until an EVIC button is pressed. If the display is removed without correcting the condition, it will reappear 300 seconds to warn the driver of the low pressure condition.

NON-CRITICAL: A non-critical alert will be triggered when no signal is received from a sensor/transmitter. The EVIC display in the cluster will display "SERVICE TIRE SYSTEM SOON."