Theft Deterrent Module

Vehicles with steering column mounted ignition switches have the exciter integral with the theft deterrent module (TDM) which is located within the steering column. Vehicles with instrument panel mounted ignition switches have exciter modules separate from the TDM with the vehicle wiring harness connecting the two. The TDM for vehicles with instrument panel mounted ignition switches is located within the instrument panel. The TDM can learn up to ten 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 PCM, fuel 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 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 PCM 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 PCM via the serial data circuit.
• If the TDM is unable to measure the ignition key transponder value for one second due to a damaged or missing pellet or a damaged exciter, the TDM will send the fuel disable password to the PCM via the serial data circuit.