Immobilizer Description and Operation North America, except Canada

The immobilizer system determines whether engine starting will be allowed based on a pull-down voltage created by the security resistor located in the ignition switch. When the ignition is in the START position, 5 volts is applied to the security resistor by the body control module (BCM) via the ignition key resistor signal circuit. A ground path is provided back to the BCM by the off/run/crank voltage circuit. This circuit is also used in power moding operations. With the ignition in the START position, a voltage value is produced on ignition key resistor signal circuit based on the resistance of the security resistor. Each available resistor will produce a unique voltage value seen at the BCM. During immobilizer programming, the BCM learns this voltage value. On a start attempt, the BCM will compare actual voltage seen to the learned value. If the voltage seen and the learned value do not match, vehicle starting will be disabled.

The components of the immobilizer system are as follows:

The ignition switch receives a 5-volt reference from the body control module (BCM) via the ignition key resistor signal circuit. When in the start position, the 5-volt reference travels through the security resistor within the ignition switch. This creates a unique voltage value which is returned to the BCM via the off/run/crank voltage circuit.

The BCM contains the logic of the theft deterrent system. The BCM provides the 5-volt reference to operate the security resistor. The BCM also measures the voltage of the off/run/crank voltage. The voltage measured will indicate whether the security resistor has been activated and whether the resistance value is a valid value or the tamper value. If voltage measured is in the valid range, the BCM compares this voltage to a previously learned voltage code. If the voltage codes match, the BCM sends a serial data message containing fuel enable password to the engine control module (ECM). If the voltage codes do not match, the voltage is in the tamper range, or there is a circuit fault, the BCM will not send a start disable password to the ECM, and the vehicle will not start.

The ECM contains the remainder of the logic of the immobilizer system. If a serial data message containing a valid fuel enable password is received from the BCM, the ECM will continue to allow the fuel injectors to operate. The ECM will allow the fuel injectors to operate until it decides there is no valid password coming from the BCM. If the ECM does not receive a serial data message, or receives a serial data message with an incorrect password, the engine will crank and will not run or will start and stall immediately.

The instrument panel cluster (IPC) illuminates the security indicator as determined by the immobilizer or content theft deterrent (CTD) system. The security indicator is illuminated when the BCM provides a ground on the security indicator control circuit.

Immobilizer Description and Operation Canada, Europe, Middle East, and Asia

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 TDM then compares this value to a value stored in memory. This value is known as the learned key code. If the value is correct, the TDM sends a random generated number to the transponder. This is the 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 transponder's unique value or the calculation to the challenge is incorrect, the TDM will send the fuel disable password to the ECM via serial data. The components of the immobilizer system are as follows:

The TDM uses the following inputs: battery voltage, ignition switched voltage, and ground circuit. The TDM uses the following outputs: password exchange with the 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:

The ECM verifies that the password received from the 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 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:

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:

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 plus is located on the steering column. The exciter coils surround the ignition lock cylinder such that they are very close to the head of the key which contains the transponder.

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 plus keys. When replacing an ignition lock cylinder, and new keys are required, the new keys must be learned by the TDM. Refer to Replacing Keys.

The 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 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.