state of some circuits and compares those states with expected values. Other systems are
monitored indirectly when the PCM conducts some type of a rationality test to identify problems.
Although most subsystems of the powertrain control module are either directly or indirectly
monitored, there may be occasions when diagnostic trouble codes are not immediately identified.
For a trouble code to set, specific conditions must be met and unless these conditions are
encountered, a code will not set. For example, the Speed Control Solenoid Circuit trouble code
will not set unless the system is active (speed control is turned on, the brake is off, and vehicle
speed greater than 55 kph/35 mph).
When a trouble code identifying a problem in an emission related system is set, the Check Engine
lamp in the dash will turn on. Conversely, any other trouble code will set without illuminating the
Check Engine lamp.
3.2.4
SKIS Overview
The Sentry Key Immobilizer System (SKIS) is an immobilizer system designed to prevent
unauthorized vehicle operation. The system consists of a Sentry Key Immobilizer Module (SKIM),
ignition key(s) equipped with a transponder chip, engine controller and body 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 engine controller indicating the presence of a valid ignition
key.
3.2.5
SKIS Operation
When ignition power is supplied to 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 an encrypted message to the transponder chip. This encrypted message is generated
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 plugged into an algorithm that encrypts the 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 I.D.
message to the SKIM. The SKIM compares the transponder I.D. 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 engine controller then waits for a engine controller response. If the
engine 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 engine
controller sends a seed response, the SKIM sends a valid/invalid key message to the engine
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 engine 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 engine 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 an indicator lamp located on the dash. The lamp receives switched ignition
voltage and is hardwired to the body controller. The lamp is actuated when the SKIM sends a PCI
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