Immobilizer Description and Operation

Immobilizer Description and Operation

The vehicle theft deterrent (VTD) system functions are provided by the theft deterrent module (TDM). 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 coils surrounding the ignition lock cylinder. The energized transponder transmits a signal that contains a unique value, which is received by the TDM. The theft deterrent control module then compares this value to a value stored in memory, learned key code. If the value is correct, the TDM sends a random generated number to the transponder. This is called a challenge. Both the transponder and the TDM perform a calculation on the challenge. If the calculations match, the TDM sends the fuel continue password via the serial data circuit to the powertrain control module (PCM). If either the transponders unique value or the calculation to the challenge is incorrect, the TDM will send the fuel disable password to the PCM via the serial data circuit. The components of the VTD system are as follows:

* The TDM
* The PCM
* The ignition key (transponder)
* The ignition lock cylinder
* The security indicator

Theft Deterrent Module (TDM)

The theft deterrent module (TDM) uses the following inputs: battery voltage, ignition switched voltage, and ground circuit. The TDM uses the following outputs: password exchange with the powertrain control module (PCM), fuel continue/disable via the serial data circuit.

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 coils surrounding the ignition lock cylinder. The energized transponder transmits a signal that contains a unique value, which is received by the theft deterrent control module. The TDM then compares this value to a value stored in memory, learned key code. If the value is correct, the TDM sends a random generated number to the transponder. This is called a challenge. Both the transponder and the TDM perform a calculation on the challenge. The TDM then performs one of the following functions:

* If both the transponder value and the calculation to the challenge are correct, the TDM will send the fuel continue password to the PCM via the serial data circuit.
* If either the transponders unique value or the calculation to the challenge is incorrect, the TDM will send the fuel disable password to the PCM via the serial data circuit.
* If the TDM is unable to measure the ignition key transponder value for 1 second due to a damaged or missing pellet or a damaged exciter, the TDM will send the fuel disable password to the PCM via the serial data circuit.

On some vehicles, If the vehicle theft deterrent (VTD) system is unable to read the ignition key transponder value after the vehicle has started, the VTD system will consider itself malfunctioning. The VTD system will enter a fail enable state and will command the security indicator to illuminate. When the VTD system is in a fail enable state, the vehicle will NOT stall or stop running. If the VTD system is in a fail enable state when the ignition is switched OFF, the VTD system will remain fail enable until it is able to read a learned ignition key transponder value. When the VTD system is in a fail enable state, the VTD system is NOT active and the vehicle will start. This feature is NOT available on all GM vehicle lines.

Powertrain Control Module (PCM)

The powertrain control module (PCM) verifies that the password received from the theft deterrent module (TDM) via the serial data circuit is correct. The PCM can learn only one fuel continue password. If the fuel continue password is correct, the PCM enables the starting and fuel delivery systems.

The PCM disables the starting and fuel delivery systems if any of the following conditions occur:

* The fuel continue password is incorrect.
* The fuel disable password is sent by the TDM.
* No passwords are received. There is no communication with the TDM.

On some vehicles, if the PCM is unable to communicate with the vehicle theft deterrent (VTD) system after the vehicle has started, the PCM will consider the VTD system to be malfunctioning. The PCM will enter a fail enable state and will command the security indicator to illuminate. When the PCM is in a fail enable state, the vehicle will NOT stall or stop running. If the PCM is in a fail enable state when the ignition is switched OFF, the PCM will remain fail enable until communications with the VTD system has been restored. When the PCM is in a fail enable state, the VTD system is NOT active and the vehicle will start. This feature is NOT available on all GM vehicle lines.

The Ignition Key (Transponder)

The ignition key for PASS-Key III plus equipped vehicles is a typical looking ignition key with a transponder located in the plastic head of the key. The transponder value is fixed and unable to be changed. The vehicle theft deterrent (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. The keys may be identified by the + symbol stamped into the steel shank of the key. VTD systems use the following types of ignition keys:

Master Keys

Master keys have a black plastic head for full access operation of the vehicle. Master keys may perform the following functions:

* Start the vehicle.
* Lock/unlock all of the door locks.
* Lock/unlock all of the storage compartments.

Valet Keys

Important: Valet keys are NOT standard equipment on all GM vehicle lines.

Valet keys have a gray plastic head and are for restricted operation of the vehicle. Valet keys may perform the following functions:

* Start the vehicle.
* Lock/unlock all of the door locks.

Ignition Lock Cylinder

The ignition lock cylinder performs all of the functions of a lock cylinder on a non PASS-Key III+ equipped vehicle. The ignition lock cylinder for vehicles with PASS-Key III plus may be located on the steering column or on the instrument panel. In either location, the exciter coils surround the ignition lock cylinder such that they are very close to the head of the key which contains the transponder pellet.

If an ignition lock cylinder is replaced, the new ignition lock cylinder must be coded to match the mechanical coding of the PASS-Key III plus keys. When replacing an ignition lock cylinder, and new PASS-Key III plus keys are required, the new keys must be learned by the theft deterrent module (TDM). Refer to Programming Immobilizer System Components (Domestic).

Security Indicator Operation

The theft deterrent module (TDM) can command the body control module (BCM) to illuminate the security indicator only when the ignition key is in the ON position. If the powertrain control module (PCM) loses communication with the TDM, the BCM will also detect the loss of communication and will illuminate the security indicator. The security indicator can indicate both malfunctions, indicator ON steady and tamper, indicator flashing. A flashing indicator indicates unauthorized operation. Under the following conditions the security indicator may be commanded to illuminate or flash.

Security Indicator Stays Illuminated and Engine Starts

If the TDM was unable to measure the ignition key transponder value or the PCM lost communication with the vehicle theft deterrent (VTD) system while the engine was last running, the indicator will remain illuminated during subsequent ignition cycles. This is to remind the driver that the VTD system will not be functioning until the problem is corrected.

Security Indicator Illuminates when Engine is Running

If the TDM is unable to measure the ignition key transponder value or the PCM loses communication with the TDM while the engine is running, the security indicator will be illuminated.

Security Indicator Illuminated and Engine Does Not Start

If the TDM was unable to measure the ignition key transponder value, the TDM VIN Check Timer is active, the PCM has detected a problem with the VTD system, or the VTD system is in Learn Mode. The security indicator will be illuminated

Security Indicator Flashes and Engine Does Not Start

The TDM has measured an incorrect transponder value, the calculation to the challenge is incorrect, or the PCM has lost communication with the TDM. The VTD system considers this a tamper condition. The security indicator will flash.