Immobilizer Description and Operation: Notes

The immobilizer system functions are provided by the steering column module (CIM) and the engine control module (ECM). When a remote key is inserted into the ignition lock cylinder and the ignition is switched ON, the transponder embedded in the head of the remote key is energized by the theft deterrent module (TDM) exciter coil surrounding the ignition lock cylinder. This exciter coil is part of the TDM, which is secured directly to the CIM. The TDM contains no immobilizer logic; it only contains the exciter coils. All immobilizer logic is housed in the CIM. The energized transponder transmits an encrypted signal that contains its unique value, which is received by the CIM. The CIM then compares this value to a value stored in memory. If the values match, the CIM will send the pre-release password via serial data to the ECM. If the transponders unique value is incorrect, the TDM will send the fuel disable password to the ECM.

When the ECM receives the CIM pre-release password, the ECM will challenge the password. The ECM sends this challenge back to the CIM via serial data. Both the ECM and CIM perform a calculation on this challenge. If the calculated response from the CIM equals the calculation performed by the ECM, the ECM will allow vehicle starting.

The components of the theft system are as follows:

• CIM
• ECM
• Remote key (Transponder)