Theft Deterrent Module (TDM)

WARNING: This page is about a different car, the 2010 Pontiac G5 and 2010 Chevrolet Cobalt. However, it is still accessible from the selected car via links, so may be relevant.

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 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 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 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 fuel disable password 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 fuel disable password to the ECM 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.