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to the message sent by SKIM. After responding to
the coded message, the transponder sends a tran-
sponder ID message to the SKIM. The SKIM com-
pares the transponder ID message to the available
valid key codes in SKIM memory (8 key maximum
at any one time). After validating the ignition key,
the SKIM sends a J1850 Bus message called a seed
request to the ECM, then waits for the ECM re-
sponse. If the ECM does not respond, the SKIM will
send the seed request again. After twenty failed
attempts, the SKIM will stop sending the seed
request and store a trouble code in memory. If the
ECM sends a seed response, the SKIM sends a
valid/invalid key message to the ECM. This is an
encrypted message that is generated using the
following:
•
VIN - Vehicle Identification Number.
•
Seed - a random number that is generated by the
ECM at each ignition key cycle.
The VIN and seed are two variables used in the
rolling code algorithm that encrypts the valid/
invalid key message. The ECM 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 an ECM status message from
the ECM. If the ECM does not respond with a valid
key message to the SKIM, a fault is detected and a
code is stored.
The SKIS incorporates a warning lamp located in
the instrument cluster. The lamp receives switched
ignition voltage and is hardwired to the instrument
cluster. The SKIS lamp is actuated when the SKIM
sends a J1850 Bus message to the instrument
cluster requesting the lamp on, off or flashing. The
instrument cluster then provides the ground for the
lamp.
The SKIM will request lamp operation for the
following:
– bulb check at ignition on
– to alert the vehicle operator to a SKIS malfunc-
tion
– when the SKIM is in customer key programming
mode
For all faults except transponder faults the lamp
remains on steady. In the event of a transponder
fault the lamp will flash at a rate of 1Hz (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 ECM will allow
the engine to start and idle for 2 seconds then stall.
This may occur up to six times. After the sixth
attempt, the ECM prevents the engine from crank-
ing until the fault is corrected.
GENERAL INFORMATION
3.3 DIAGNOSTIC TROUBLE CODES
Each diagnostic trouble code (DTC) is diagnosed
by following a specific procedure. The diagnostic
test procedure contains step-by-step instruction for
determining the cause of the DTC as well as no
trouble code problems. It is not necessary to per-
form all of the tests in this book to diagnose an
individual code.
Always begin diagnosis by reading the DTCs
using the DRBIII . This will direct you to the
specific test(s) that must be performed.
3.3.1 HARD CODE
A DTC that comes back within one cycle of the
ignition key is a hard code. This means that the
problem is current every time the ECM/SKIM
checks that circuit or function. Procedures in this
manual verify if the DTC is a hard code at the
beginning of each test. When the fault is not a hard
code, an intermittent test must be performed.
NOTE: If the DRBIII displays faults for multiple
components (i.e. ECT, VSS, IAT sensors) identify
and check the shared circuits for possible problems
before continuing (i.e. sensor grounds or 5-volt
supply circuits). Refer to the appropriate schematic
to identify shared circuits.
3.3.2 INTERMITTENT CODE
A DTC that is not current every time the ECM/
SKIM checks the circuit or function is an intermit-
tent code. Most intermittent DTCs are caused by
wiring or connector problems. Problems that come
and go like this are the most difficult to diagnose;
they must be looked for under specific conditions
that cause them. The following checks may assist
you in identifying a possible intermittent problem.
•
Visually inspect the related wire harness connec-
tors. Look for broken, bent, pushed out, or cor-
roded terminals.
•
Visually inspect the related wire harness. Look
for chafed, pierced, or partially broken wire.
•
Refer to hotlines or technical service bulletins
that may apply.
NOTE: Electromagnetic (radio) interference can
cause an intermittent system malfunction. This
interference can interrupt communication between
the ignition key transponder and the SKIM.
3.3.3 ECM DIAGNOSTIC TROUBLE CODES
IMPORTANT NOTE: Before replacing the ECM
for a failed driver, control circuit or ground circuit,
be sure to check the related component/circuit in-
tegrity for failures not detected due to a double fault
in the circuit. Most ECM driver/control circuit fail-
ures are caused by internal failures to components
3
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