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, 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 fuel continue password 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 fuel disable password to the ECM via the serial data circuit. The components of the VTD system are as follows:

Vehicles with steering column mounted ignition switches have the exciter integral with the theft deterrent control module which is located within the steering column. Vehicles with instrument panel mounted ignition switches have exciter modules separate from the theft deterrent control module with the vehicle wiring harness connecting the two. The theft deterrent control module for vehicles with instrument panel mounted ignition switches is located within the instrument panel. The theft deterrent control module can learn up to ten 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), 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 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, 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. The theft deterrent control module then performs one of the following functions:

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.

Module substitution deterrence is on some EXPORT vehicles and is an internal function of the theft deterrent control module. Module substitution deterrence will disable starting of the vehicle when a loss of battery power has been restored. This can be do to, the vehicles battery becoming discharged or the battery power being removed from thevehicle theft deterrent (VTD) system. The theft deterrent control module performs one of the following functions:

The engine control module (ECM) verifies that the password received from the theft deterrent control module via the serial data circuit is correct. The ECM can learn only one fuel continue password. If the fuel continue password is correct, 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:

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 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:

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:

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 for vehicles with PK3+ 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 PK3+ keys. 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 Immobilizer System Component Programming .

The theft deterrent control module can command the instrument cluster to illuminate the security indicator only when the ignition key is in the ON position. If the engine control module (ECM) loses communication with the theft deterrent control module, the instrument cluster 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 theft deterrent control module was unable to measure the ignition key transponder value or the 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 theft deterrent control modules VIN check timer is active, 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.