Vehicle Theft Deterrent (VTD)

VEHICLE THEFT DETERRENT (VTD) DESCRIPTION AND OPERATION

The vehicle theft deterrent (VTD) system functions are provided by the theft deterrent control module. 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 theft deterrent control module. The theft deterrent control module then compares this value to a value stored in memory. This is the learned key code. If the value is correct, the theft deterrent control module sends a random generated number to the transponder. This is called a challenge. Both the transponder and the theft deterrent control module perform a calculation on the challenge. If the calculations match, the theft deterrent control module 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 theft deterrent control module will send the start disable message to the ECM via the serial data circuit. The components of the VTD system are as follows:
- Theft deterrent control module
- ECM
- Ignition key (Transponder)
- Ignition lock cylinder
- Security indicator

Theft Deterrent Control Module
Vehicles with steering column mounted ignition switches have the exciter integral with the theft deterrent control module, which is located within the steering column. The theft deterrent control module can learn up to 10 keys (transponder values).

The theft deterrent control module uses the following inputs: battery voltage, ignition switched voltage and ground circuit. The theft deterrent control module 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 theft deterrent control module. The theft deterrent control module then compares this value to the learned key code stored in memory. If the value is correct, the theft deterrent control module sends the challenge to the transponder. Both the transponder and the theft deterrent control module perform a calculation on the challenge. The theft deterrent control module then performs one of the following functions:
- If both the transponder value and the calculation to the challenge are correct, the theft deterrent control module 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 theft deterrent control module will send the start disable message to the ECM via the serial data circuit.
- If the theft deterrent control module is unable to measure the ignition key transponder value for 1 second due to a damaged or missing pellet or a damaged exciter, the theft deterrent control module will send the start disable message to the ECM via the serial data circuit.

IMPORTANT: 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.

Engine Control Module (ECM)
The engine control module (ECM) verifies that the message received from the theft deterrent control module 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 theft deterrent control module.
- No passwords are received-There is no communication with the theft deterrent control module.

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. If 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 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 letters PK3+ 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 passkey III+(PK3+) equipped vehicle. 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. Refer to Programming Theft Deterrent System Components.

Security Indicator Operation
The theft deterrent control module 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 theft deterrent control module 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 theft deterrent control module is unable to measure the ignition key transponder value or the ECM loses communication with the theft deterrent control module while the engine is running, the security indicator will be illuminated.

Security Indicator Illuminated and Engine Does Not Start
If the theft deterrent control module 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 theft deterrent control module 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.