PCM Grounds − The PCM cannot detect a poor
system ground. However, a diagnostic trouble code
may be stored in the PCM as a result of this
condition.
Throttle Body Air Flow − The PCM cannot
detect a clogged or restricted air cleaner inlet or
filter element.(*)
Exhaust System − The PCM cannot detect a
plugged, restricted, or leaking exhaust system.(*)
Cylinder Compression − The PCM cannot de-
tect uneven, low, or high engine cylinder compres-
sion.(*)
Excessive Oil Consumption − Although the
PCM monitors the exhaust stream oxygen content
through the oxygen sensor when the system is in a
closed loop, it cannot determine excessive oil con-
sumption.
(*)NOTE: Any of these conditions could result in
a rich or lean condition causing an oxygen sensor
trouble code to be stored in the PCM, or the vehicle
may exhibit one or more of the driveability symp-
toms listed in the Table of Contents.
3.2.7
SKIS OVERVIEW
The Sentry Key Immobilizer System (SKIS) is
designed to prevent unauthorized vehicle opera-
tion. The system consists of a Sentry Key Immobi-
lizer Module (SKIM), ignition key(s) equipped with
a transponder chip and PCM. When the ignition
switch is turned on, the SKIM interrogates the
ignition key. If the ignition key is “Valid” or “In-
valid”, the SKIM sends a PCI Bus message to the
PCM indicating ignition key status. Upon receiving
this status message the PCM will terminate engine
operation, or allow the engine to continue to oper-
ate.
3.2.8
SKIM ON-BOARD DIAGNOSTICS
The SKIM has been programmed to transmit and
monitor many different coded messages as well as
PCI Bus messages. This monitoring is called “On
Board Diagnosis”.
Certain criteria must be met for a diagnostic
trouble code to be entered into the SKIM memory.
The criteria may be a range of; Input voltage, PCI
Bus message, or coded messages to the SKIM. If all
of the criteria for monitoring a circuit or function
are met and a fault is sensed, a diagnostic trouble
code will be stored in the SKIM memory.
3.2.9
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.
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 vari-
ables used in the algorithm that produces the
encrypted response message. The transponder uses
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
key codes in the SKIM memory (8 key maximum at
any one time). After validating the ignition 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 en-
crypted message that is generated using the follow-
ing:
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 VTSS LED located on
the dash. The LED receives switched ignition volt-
age and is hardwired to the body control module.
The LED is actuated when the SKIM sends a PCI
Bus message to the body controller requesting the
VTSS LED on. The body controller then provides
the ground for the LED. The SKIM will request
VTSS LED operation for the following:
– bulb checks at ignition on
– to alert the vehicle operator to a SKIS mal-
function
– customer key programming mode
For all faults except transponder faults, the LED
remains on steady. In the event of a transponder
fault the LED flashes at a rate of 1 Hz (once per
8
GENERAL INFORMATION