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 its unique value, which is received by the TDM. The TDM then compares this value to a value stored in memory. This is the 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 module perform a calculation on the challenge. If the calculations match, the TDM sends the start enable message via the serial data circuit to the engine control module (ECM). If either the transponders unique value or the calculation to the challenge is incorrect, the TDM will send the start disable message to the ECM via the serial data circuit. The components of the VTD system are as follows:

* TDM ECM
* Ignition key (Transponder)
* Ignition lock cylinder
* Security indicator

Theft Deterrent Module (TDM)

Vehicles with steering column mounted ignition switches have the exciter integral with the theft deterrent module (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 TDM uses the following outputs: password exchange with the engine control module (ECM), start enable/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 its unique value, which is received by the TDM. The TDM then compares this value to the learned key code stored in memory. If the value is correct, the TDM sends the challenge to the transponder. 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 start enable message to the ECM via the serial data circuit.
* If either the transponders unique value or the calculation to the challenge is incorrect, the TDM will send the start disable message to the ECM 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 start disable message to the ECM via the serial data circuit.

Engine Control Module (ECM)

Important: On some vehicles, if the ECM is unable to communicate with the vehicle theft deterrent (VTD) system after the vehicle has started, the ECM will consider the VTD system to be malfunctioning. The ECM will enter a fail enable state and will command the security indicator to illuminate. When the ECM is in a fail enable state, the vehicle will NOT stall or stop running. the ECM is in a fail enable state when the ignition is switched OFF, the ECM will remain fail enable until communications with the VTD system has been restored. When the ECM 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 engine control module (ECM) verifies that the message received from the TDM via the serial data circuit is valid. The ECM can learn only one start enable password. If the start enable message is valid, the ECM enables the starting and fuel delivery systems.

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

* The start enable message is invalid.
* The start disable message is sent by the TDM.
* No passwords are received-There is no communication with the TDM.

The Ignition Key (Transponder)

The ignition key for passkey III+ (PK3+) 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 plus in a circle symbol stamped into the steel shank of the key.

Ignition Lock Cylinder

The ignition lock cylinder is located on the steering column. 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 PK3+ keys. When replacing only an ignition lock cylinder, no programming procedure is necessary to either the key or the vehicle theft deterrent (VTD) system if existing keys are being used. When replacing an ignition lock cylinder and new PK3+ keys are required, the new keys must be learned by the theft deterrent control module.

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. 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 engine control module (ECM) 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 ECM 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 ECM 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 ECM has lost communication with the theft deterrent control module. The VTD system considers this a tamper condition. The security indicator will flash.