The vehicle theft deterrent (VTD) system functions are provided by the theft
deterrent module (TDM) and the engine control module (ECM). When an ignition key is
inserted into the ignition lock cylinder and the ignition is switched ON, the transponder
embedded in the head of the key is energized by the exciter coil surrounding the ignition
lock cylinder. This exciter coil is part of the TDM. The energized transponder transmits
a signal that contains its unique value, which is received by the TDM. The TDM then
compares this value to a value stored in memory. If the ignition key is recognised
as valid, the TDM performs a module substitution check where the identities of the
instrument panel cluster (IPC), body control module (BCM), infotainment unit (IRC)
and sensing diagnostic module (SDM) control units are checked. If they are recognised
as valid components, the TDM will send the fuel enable password to the ECM via the
BCM using the GMLAN. If the transponders unique value is incorrect or module substitution
has been detected, the TDM will send the fuel disable password to the ECM. It is important
to note that while it is referred to as the fuel enable/disable password, the vehicles
starter may also be disabled, preventing engine cranking.
When the ECM receives the TDM fuel enable password, the ECM will challenge the
password. The ECM sends this challenge back to the TDM via the BCM using the GMLAN.
Both the ECM and TDM perform a calculation on this challenge. If the calculated response
from the TDM equals the calculation performed by the ECM, the ECM will allow vehicle
starting.
The components of the VTD system are as follows:
• | Ignition key (Transponder) |
Theft Deterrent Module (TDM)
Vehicles with steering column mounted ignition switches have the exciter integral
with the TDM, which is located within the steering column. The TDM can learn up to
10 keys (transponder values).
The TDM uses the following inputs: battery voltage, ignition switched voltage
and ground circuit. The theft deterrent control module uses the following outputs:
password exchange and challenge/response with the ECM.
When an ignition key is inserted into the ignition lock cylinder and the ignition
is switched to accessories (ACC), the transponder embedded in the head of the key
is energized by the exciter coils surrounding the ignition lock cylinder. The energized
transponder transmits a signal that contains its unique value, which is received by
the theft deterrent control module. The theft deterrent control module then compares
this value to the learned key code stored in memory. If the ignition key is recognised
as valid, the TDM performs a module substitution check where the identities of the
IPC, BCM, IRC and SDM control units are checked. The theft deterrent control module
then performs one of the following functions:
• | If the transponder value and control units match the values and identification
stored in the TDM memory, the TDM will send the fuel enable message to the ECM via
the BCM using the GMLAN. |
• | If the transponders unique value does not match either or both the values
and/or identification stored in the TDM memory, the TDM will send the fuel disable
message to the ECM via the BCM using the GMLAN. |
• | If the TDM is unable to measure the ignition key transponder value, the
TDM will not send any messages to the ECM. |
Engine Control Module (ECM)
When the ECM receives the TDM fuel enable password, the ECM will challenge the
password. The ECM sends this challenge back to the TDM via the serial data circuit.
Both the ECM and TDM perform a calculation on this challenge. If the calculated response
from the TDM equals the calculation performed by the ECM, the ECM will allow vehicle
starting.
The ECM will disable vehicle starting if any of the following conditions occur:
• | The fuel enable password is invalid. |
• | The fuel disable password is sent by the TDM. |
• | No passwords are received--There is no communication with the TDM. |
• | The TDM calculated response to the challenge does not equal the calculation
performed by the ECM. |
• | Module substitution has occurred. |
The Ignition Key (Transponder)
The ignition key for Passkey III+ (PK3+) equipped vehicles is a standard ignition
key with a transponder located in the plastic head of the key. The transponder value
is fixed and unable to be changed. The VTD system uses the ignition key transponder
value to determine if a valid ignition key is being used to start the vehicle. There
are approximately 3 trillion possible transponder values. There are no visible electrical
contacts.
Ignition keys have a black plastic head for full access operation of the vehicle.
Ignition keys perform the following functions:
• | Lock/unlock all of the door locks and rear compartment. |
• | Lock/unlock all of the storage compartments. |
Module Substitution Detection
Module substitution detection is achieved by building an immobiliser chain made
out of five control units which contain the VIN. When the ignition is switched to
the ACC position, the TDM will request and compare the identities of each control
unit to the identity stored in the TDM memory. This is called a module substitution
check.
The following control units are in the immobiliser chain:
Two out of the four control units (IPC, BCM, IRC or SDM), are required to respond
with the correct VIN to allow vehicle starting. Therefore, if only one unit is missing
or reports with an incorrect VIN, vehicle starting is still enabled.