Immobilizer

Immobilizer Description and Operation

The immobilizer 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 a unique value, which is received by the TDM. The theft deterrent control module 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. If the calculations match, the TDM sends the fuel continue password 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 fuel disable password to the PCM via the serial data circuit.

In some instances, RF devices attached to a keychain or otherwise in close proximity of the ignition cylinder may cause interference in the immobilizer system and lead to a no-start condition. Some common RF devices that may be attached to a keychain are Speedpass gas station payment device, swipe devices used to gain access locked gates or 24-hour gyms, or other "smart" keys. These RF devices interfere with the communication between the transponder located in the key and the TDM. The immobilizer systems operates at a frequency of 125 kHz. RF devices that operate in the 120 to 135 kHz range, or a multiplier of these frequencies, are most likely to cause interference. If interference is suspected, removed the suspect device from the keychain and attempt to start the vehicle. If vehicle starting occurs with the device removed, instruct the customer to remove the device from the keychain to prevent further occurrences. It may also be necessary to inspect all of the customers keychains if the concern is intermittent and doesn't occur with every key.

The components of the immobilizer system are as follows:

* The TDM
* The ECM
* The ignition key (transponder)
* The ignition lock cylinder
* The security indicator

Theft Deterrent Module (TDM)

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 immobilizer system is unable to read the ignition key transponder value after the vehicle has started, the immobilizer system will consider itself malfunctioning. The immobilizer system will enter a fail enable state and will command the security indicator to illuminate. When the immobilizer system is in a fail enable state, the vehicle will NOT stall or stop running. If the immobilizer system is in a fail enable state when the ignition is switched OFF, the immobilizer system will remain fail enable until it is able to read a learned ignition key transponder value. When the immobilizer system is in a fail enable state, the immobilizer system is NOT active and the vehicle will start. This feature is NOT available on all GM vehicle lines.

Engine Control Module (ECM)

The engine control module (ECM) verifies that the password received from the theft deterrent module (TDM) via the serial data circuit is correct. The ECM can learn only one fuel continue password. If the fuel continue password is correct, the ECM enables the starting and fuel delivery systems.

The ECM disables the starting and fuel delivery systems if any of the following conditions occur:

* The fuel continue password is incorrect.
* The fuel disable password is sent by the TDM.
* No passwords are received. There is no communication with the TDM.

On some vehicles, if the ECM is unable to communicate with the immobilizer system after the vehicle has started, the ECM will consider the immobilizer system to be malfunctioning. The ECM will enter a fail enable state and will command the security indicator to illuminate. When the ECM is in a fail enable state, the vehicle will NOT stall or stop running. If the ECM is in a fail enable state when the ignition is switched OFF, the ECM will remain fail enable until communications with the immobilizer system has been restored. When the ECM is in a fail enable state, the immobilizer system is NOT active and the vehicle will start. This feature is NOT available on all GM vehicle lines.

The Ignition Key (Transponder)

Note: The ignition key will be stamped with a + or a + surrounded by a circle. This symbol only identifies the key as a PASS-Key III+ transponder key and is not a reliable way to determine if a particular key is the correct key for a vehicle. Service parts may have a different stamped symbol than the production part. The only way to determine the proper key for a vehicle is by referencing the parts catalog.

The ignition key for PASS-Key III+ equipped vehicles is a typical looking ignition key with a transponder located in the plastic head of the key. The transponder value is fixed and unable to be changed. The immobilizer system uses the ignition key transponder value to determine if a valid ignition key is being used to start the vehicle. There are approximately 3 trillion possible transponder values. There are no visible electrical contacts. Immobilizer systems use the following types of ignition keys:

Master Keys

Master keys have a black plastic head for full access operation of the vehicle. Master keys may perform the following functions:

* Start the vehicle.
* Lock/unlock all of the door locks.
* Lock/unlock all of the storage compartments.

Valet Keys

Important: Valet keys are NOT standard equipment on all GM vehicle lines.


Valet keys have a gray plastic head and are for restricted operation of the vehicle. Valet keys may perform the following functions:

* Start the vehicle.
* Lock/unlock all of the door locks.

Ignition Lock Cylinder

The ignition lock cylinder performs all of the functions of a lock cylinder on a non PASS-Key III+ equipped vehicle. The ignition lock cylinder for vehicles with PASS-Key III+ may be located on the steering column or on the instrument panel. In either location, the exciter coils surround the ignition lock cylinder such that they are very close to the head of the key which contains the transponder pellet.

If an ignition lock cylinder is replaced, the new ignition lock cylinder must be coded to match the mechanical coding of the PASS-Key III+ keys. When replacing an ignition lock cylinder, and new PASS-Key III+ keys are required, the new keys must be learned by the theft deterrent module (TDM). Refer to Replacing Keys Replacing Keys.

Security Indicator Operation

The theft deterrent module (TDM) can command the body control module (BCM) to illuminate the security indicator only when the ignition key is in the ON position. If the engine control module (ECM) loses communication with the TDM, the BCM will also detect the loss of communication and will illuminate the security indicator. The security indicator can indicate both malfunctions, indicator ON steady and tamper, indicator flashing. A flashing indicator indicates unauthorized operation.