Immobilizer Description and Operation: Notes

The vehicle theft deterrent (VTD) system functions are provided by the theft deterrent module (TDM). 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. This is the 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 module perform a calculation on the challenge. If the calculations match, the TDM 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 TDM will send the start disable message to the ECM via the serial data circuit. The components of the VTD system are as follows:

• TDM
• ECM
• Ignition key (Transponder)
• Ignition lock cylinder
• Security indicator