Probably more than you want to know but since these security systems in the Silverado can be tempermental, you might as well learn how it all works:
EHICLE THEFT DETERRENT (VTD) DESCRIPTION AND OPERATION
The theft deterrent system has been incorporated into the body control
module (BCM). The theft deterrent is provided in order to prevent the
vehicle operation if the correct key is not used in order to start the
vehicle. The ignition key turns the lock cylinder. The cylinder
rotation produces an analog voltage code in the Passlock(tm) sensor.
This voltage code is received by the BCM. The BCM compares the voltage
code to the previously learned voltage code. If the codes match, a
class 2 message is sent from the BCM to the powertrain control module
(PCM)/vehicle control module (VCM). The message enables the fuel
The design of the Passlock(tm) system is to prevent vehicle theft by
disabling the engine unless the ignition lock cylinder rotates properly
by engaging the correct ignition key. The system is similar in concept
to the PASS-Key(r) system. However, the Passlock(tm) system eliminates
the need for a key mounted resistor pellet. The components of the
Passlock(tm) system are as follows:
- The ignition lock cylinder
- The ignition switch
- The Passlock(tm) sensor
- The Body control module (BCM)
- The security telltale
- The vehicle/powertrain control module (VCM/PCM)
Ignition Lock Cylinder And Housing
The ignition lock cylinder is located at the upper right side of the
steering column. The Passlock(tm) sensor is in the steering column. The
Passlock(tm) sensor is separate from the key and lock cylinder. The key
and the lock cylinder work together in order to determine if the proper
ignition key was used to start the vehicle.
In the event of an open Class 2 serial data line between the BCM and
the VCM/PCM, the vehicle will become fail-enabled if the VCM/PCM has
already received the password from the BCM for that ignition cycle, the
engine is running. In this event, the following conditions occur:
- The security telltale will be ON continuously.
- The VCM/PCM will become fail-enabled for future ignition cycles.
a failure in the Class 2 serial data line occurs before the ignition
cycle, when the VCM/PCM is not fail-enabled, the following conditions
- The VCM/PCM will never receive a valid password in order to enable the fuel injectors.
- The vehicle will not start.
The lock cylinder and the visible key insert portion of the ignition
switch are located at the upper right side of the steering column. The
electrical switching portion of the assembly is separate from the key
and lock cylinder. The electrical switch portion is hidden inside the
steering column. The electrical switch portion and the key and lock
cylinder synchronize and work in conjunction through the action of the
mechanical assembly between the 2 parts.
The Passlock(tm) sensor is inside the ignition cylinder lock housing.
The Passlock(tm) sensor contains 2 hall effect sensors. The tamper hall
effect sensor is on the top. The security hall effect sensor is under
the tamper hall effect sensor. Both of the hall effect sensors monitor
the magnet of the lock cylinder through an opening. The tamper hall
effect sensor is physically placed on top of the security hall effect
sensor. This arrangement enables the tamper hall effect sensor to
engage first if an intruder attempts to bypass the Passlock(tm) sensor
by placing a large magnet around that area of the steering column.
There is a tamper resistor inside the Passlock(tm) sensor in order to
help prevent tamper to the system. Passlock(tm) equipped vehicles have
a selection of 10 different security resistors ranging from 0.5K ohms up to 13.6K ohms
. Installing one of the security resistors inside the Passlock(tm)
sensor will generate a unique Passlock(tm) code. All 10 combinations of
the Passlock(tm) sensor have the same part number. However, you cannot
simply replace the Passlock(tm) sensor and expect the system to operate
properly. Always start by performing the Diagnostic System Check first
and following the instructions.
Body Control Module
The Body Control Module (BCM) contains the theft deterrent system
logic. The BCM reads the Passlock(tm) data from the Passlock(tm)
sensor. If the Passlock(tm) data is correct, the BCM will pass theft.
The BCM will then transmit the fuel continue password to the VCM/PCM.
During the tamper mode the vehicle may start. The vehicle will then
stall. If the BCM receives the wrong Passlock(tm) data, the VTD will
immediately go into the tamper mode. The tamper mode will lock-out the
vehicle fuel injectors for 10 minutes . The SECURITY indicator will flash while the VTD is in the tamper mode.
If the Passlock(tm) sensor sends a correct password to the BCM when the
ignition is in the ON position, the BCM will send a fuel enable signal
to the VCM/PCM. The VCM/PCM will not disable the fuel due to any BCM
message for the remainder of the ignition cycle.
The SECURITY telltale appears on the message center inside the
instrument cluster. If the SECURITY telltale flashes or if the SECURITY
telltale lights continuously during the vehicle operation, refer to the
Vehicle/Powertrain Control Module
The VCM/PCM communicates with the BCM over the Class 2 serial data
line. When the BCM determines a no start condition, it sends a Class 2
serial data password to the VCM/PCM in order to disable the fuel
injection system. If the BCM receives the expected voltage from the
Passlock(tm) sensor, the BCM sends a class 2 serial data password to
the VCM/PCM in order to enable the fuel injection system. The VCM/PCM
then allows the vehicle to start correctly. If the Class 2 serial data
password from the BCM to the VCM/PCM is not within the Vehicle Security
Status Message, the fuel injectors will shut OFF during a start
attempt. The SECURITY telltale will be illuminated for 10 minutes
. If the VCM/PCM does not receive the same password from the BCM as the
last learned one, the vehicle will start and then stalls due to the
FUEL LOCKOUT CYCLE
The Passlock(tm) system has a lockout cycle of approximately 10 minutes . Once the vehicle is in the lockout cycle, the vehicle remains in the lockout cycle for 10 minutes , even if the ignition switch is turned from the RUN position to the OFF position.
The Passlock(tm) system uses the lockout cycle in order to synchronize
all of the Passlock(tm) components when any Passlock(tm) related part
is changed. The Passlock(tm) system requires 3 consecutive lockout
cycles in order to complete the 30 minute learn procedure for a changed component.
CHANGING THE PASSLOCK(TM) COMPONENTS
The design of the Passlock(tm) system is to prevent theft even if the
various theft deterrent parts change. The parts that can no longer be
changed without the possibility of going into a tamper mode are:
- The Passlock(tm) sensor
- The BCM
- The VCM/PCM
If you replace any of these parts the vehicle may start and stall for 10 minutes
. This is the long tamper mode. If this occurs, the system must go
through a long tamper mode cycle. During this time the SECURITY
telltale will be flashing for the full 10 minutes and the DTC B3031 will be set. The BCM and the VCM/PCM require the full 10 minutes
in order to complete a learn cycle. The ignition switch must remain in
the RUN position until the SECURITY telltale stops flashing. You will
need to repeat the cycle if the ignition switch does not remain in the
RUN position. When replacing any of the above parts it is recommended
to perform the Programming Theft Deterrent System Components procedure.
The design of the Passlock(tm) system is to prevent the vehicle
operation if the proper ignition key is not used in order to start the
vehicle. The mechanical key, in normal operation, will turn the
ignition lock cylinder. The Passlock(tm) sensor monitoring the position
of the lock cylinder will relay the Passlock(tm) data to the body
control module (BCM). The BCM will determine the validity of the
Passlock(tm) data. The BCM will send a fuel continue password to the
vehicle/powertrain control module (VCM/PCM). When the VCM/PCM receives
the correct password the VCM/PCM allows the fuel injectors to operate
normally. The Passlock(tm) system requires the VCM/PCM and the BCM to
communicate the various functions in order to operate. These functions
transmit over the class 2 serial data line. For an explanation of the
class 2 serial data description and operation, refer to Data Link
Communications Description and Operation in Data Link Communications.