Sentry Key Immobilizer System (Skis)

SENTRY KEY IMMOBILIZER SYSTEM (SKIS) OVERVIEW
The Sentry Key Immobilizer System (SKIS) is designed to prevent unauthorized vehicle operation. The system consists of a Sentry Key Immobilizer Module (SKIM), ignition key(s) equipped with a transponder chip and powertrain controller. When the ignition switch is turned ON, the SKIM interrogates the ignition key. If the ignition key is "Valid" the SKIM sends a PCI Bus message to the powertrain controller indicating the presence of a valid ignition key. With a valid key presence, the PCM allows the engine to continue to operate.

SKIS OPERATION
When ignition power is supplied to the SKIM, the SKIM performs an internal self-test. After the self- test is 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.

Challenger - This is a random number that is generated by the SKIM at each ignition key cycle.

The secret key and challenge are plugged into an algorithm that produces the encrypted response message. The transponder uses the crypto algorithm to receive, decode and respond to the message sent by 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 key codes in SKIM memory (8 key maximum). After validating the key the SKIM sends a PCI Bus message called a "Seed Request" to the powertrain controller then waits for a powertrain controller response. If the powertrain controller 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 powertrain controller sends a seed response, the SKIM sends a valid/invalid key message to the powertrain controller. 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 plugged into a rolling code algorithm that encrypts the "valid/invalid key" message. The powertrain controller uses the rolling code algorithm to receive, decode and respond to the valid/invalid key message sent by SKIM. After sending the valid/invalid key message the SKIM waits 3.5 seconds for an EMS status message from the powertrain controller. If the PCM does not respond with a valid key message to the SKIM, a fault is detected and a trouble code stored.

The SKIS incorporates a Red Warning lamp located in the Instrument Cluster. The lamp is illuminated when the SKIM sends a PCI Bus message to the instrument cluster requesting the lamp ON. The SKIM will request lamp operation for the following:
- bulb check at ignition ON
- to alert the vehicle operator to a SKIS malfunction

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 the rate of 1 Hz (once per second). If a fault is present the lamp will remain ON or flashing for the complete ignition cycle.