Body Control Module (BCM)

When an ignition key is inserted into the ignition lock cylinder and the ignition is switched ON, the body control module (BCM) supplies battery voltage to the theft deterrent exciter module. The transponder embedded in the head of the key is energized by the theft deterrent exciter module which is surrounding the ignition lock cylinder. The energized transponder transmits a signal that contains its unique value, which is received by the theft deterrent exciter module. The BCM monitors the theft deterrent exciter module for the transponder value via the security system sensor signal circuit. The BCM then compares this value to a value stored in memory, learned key code. The BCM then performs one of the following functions:

If the transponder value is correct, the BCM will send the fuel continue password to the powertrain control module (PCM) via the serial data circuit.
If the transponders value is incorrect the BCM will send the fuel disable password to the PCM via the serial data circuit.
If the BCM is unable to measure the ignition key transponder value for one second due to a damaged or missing pellet or a damaged theft deterrent exciter module, the BCM will send the fuel disable password to the PCM via the serial data circuit.

The vehicle theft deterrent (VTD) system uses the following inputs, battery voltage, ignition switched voltage, security system sensor signal and ground circuit. The VTD system uses the following outputs, security system sensor supply voltage and password exchange with the PCM, fuel continue/disable via the serial data circuit.

IMPORTANT: On some vehicles, if the 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 driver information center (DIC) to display the service theft - VTD fail enable message. 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.