Antitheft and Alarm Systems: Description and Operation
Sentry Key Immobilizer System (SKIS) Overview
The Sentry Key Immobilizer System (SKIS) is an immobilizer system designed to prevent unauthorized vehicle operation. The system consists of Sentry Key Immobilizer Module (SKIM) sends a PCI Bus message to the engine controller indicating ignition key status. Upon receiving this message the PCM will terminate engine operation or allow the engine to continue to operate.
Sentry Key Immobilizer System (SKIS) Operation
When ignition power is supplied to the SKIM, the SKIM performs an internal self-test. After the self- testis completed, the SKIM energizes the antenna (this activates the transponder chip) and sends a challenge to the transponder chip. The transponder chip responds to the challenge by generating an encrypted response message using the following:
Secret Key - This is an electronically stored value (identification number) that is unique to each SKIS. The secret key is stored in the SKIM, PCM, and all ignition key transponders.
Challenge - This is a random number that is generated by the SKIM at each ignition key cycle. The secret key and challenge are the two variables used in the algorithm that produces the crypto algorithm to receive, decode and respond to the message sent by the SKIM. After responding to the coded message, the transponder sends a transponder
ID message to the SKIM. The SKIM compares the transponder ID to the available valid ignition key codes in the SKIM memory (8 key maximum). After validating the key, the SKIM sends a PCI Bus message called a "Seed Request" to the engine controller then waits for a PCM response. If the PCM does not respond, the SKIM will send the seed request again. After three failed attempts, the SKIM will stop sending the seed request and store a trouble code. If the PCM sends a seed response, the SKIM sends a valid/invalid key message to the PCM. This is an encrypted message that is generated using the following:
VIN - Vehicle Identification Number
Seed - This is a random number that is generated by the PCM at each ignition key cycle.
The VIN and seed are the two variables used in the rolling code algorithm that encrypts the "valid/invalid key" message. The PCM uses the rolling code algorithm to receive, decode and respond to the valid/invalid key message sent by the SKIM. After sending the valid/invalid key message the SKIM waits 3.5 seconds for a PCM status message from the PCM. If the PCM does not respond with a valid key message to the SKIM, a fault is detected and a trouble code is stored. The SKIS incorporates a warning lamp located in the instrument cluster. The lamp receives power and ground from the instrument cluster. The lamp is actuated when the SKIM sends a PCI Bus message to the instrument cluster requesting the lamp on. The SKIM will request warning lamp illumination for:
- bulb checks at ignition on
- to alert the vehicle operator to a SKIS malfunction
- customer key programming mode
For all faults except transponder faults and VIN mismatch, the lamp remains on steady. In the event of a transponder fault the light flashes at a rate of 1 Hz (once per second). If a fault is present the lamp will remain on or flashing for the complete ignition cycle. If a fault is stored in SKIM memory which prevents the system from operating properly, the PCM will allow the engine to start and run (for two seconds) up to six times. After the sixth attempt the PCM will not allow engine to start.
VEHICLE THEFT SECURITY SYSTEM (VTSS)
The VTSS will monitor the states of the ignition, door lock signal, and all other doors to alert against unauthorized entry into the vehicle. The VTSS will be enabled at the assembly plant. However to prevent the engine disable feature during assembly and the shipping process, the vehicle must be started 20 times before the VTSS will operate properly.
Arming The VTSS
To arm the system, the ignition must be off and all doors must be closed and locked using either front door key cylinder lock switch, power door lock switch or the RKE transmitter. Using a key in either rear door or pressing the mechanical lock will not initiate arming the VTSS. The delay lock feature will delay the arming process until all doors are closed.
Once the VTSS begins to arm, the CTM will activate the security lamp output at a 1 Hz rate. During this process the arming process will terminate if the ignition is turned on, if a door is opened, if the doors are unlocked using the door lock switch or the RKE transmitter, or if the front doors are unlocked using the key. After fifteen seconds of the arming process, the security lamp will go off and the VTSS will be armed. Once the vehicle is armed the ignition may be turned to the accessory position (to listen to radio, etc.) without triggering the alarm. The VTSS can only be disarmed using the key or the RKE transmitter.
Triggering The VTSS
Once the VTSS is armed, the alarm will trigger if any of the doors are opened or if the key is turned the Run/Start position. When triggered, the CTM will instruct the PCM to turn off the engine after two seconds and will pulse the horn and headlamps at a specified duty cycle and frequency. The audible (horn) warning will continue for three minutes then terminate. The visual (headlamps) warning will continue for fifteen additional minutes then terminate. The engine Will continue to start and quit until the VTSS is disarmed.
Tamper Alert
If the VTSS was triggered during the last armed period, the next time it is disarmed the CTM will activate three horn chirps. To determine what caused the alarm to be tripped, using the DRB select "Theft Alarm" "Monitor Display" and observe: "Alarm Tripped by". This is a great tool to use when a customer complains that the alarm went off and cannot determine what triggered it.