Immobilizer Description and Operation North America, except Canada

The Passlock system is provided in order to prevent vehicle theft if the ignition lock cylinder is forced to rotate or the ignition switch is operated while separated from the ignition lock cylinder case. If starting is attempted without authorization, the engine control module/powertrain control module (ECM/PCM) will disable engine starting.

The components of the Passlock system are as follows:

The ignition lock cylinder fits inside the ignition lock cylinder case and operates the ignition switch when turned by a key with the proper mechanical cut. When the ignition key is used to turn the ignition lock cylinder to crank, a magnet on the lock cylinder passes close to the Passlock sensor within the ignition lock cylinder case. The magnet activates the security hall effect sensor which completes a circuit from the security sensor signal circuit through a resistor to the security sensor low reference circuit. The resistance value will vary from vehicle to vehicle.

If a magnet from outside of the ignition lock cylinder case is used to attempt to steal the vehicle, the tamper hall effect sensor will be activated. This completes a circuit from the security sensor signal circuit through a tamper resistor to the security sensor low reference circuit, bypassing the security resistor. If the ignition switch is forced to rotate without the correct key, or if the ignition lock cylinder is removed by force, the Passlock sensor will be damaged rendered inoperable.

The BCM contains the logic of the immobilizer system. The BCM provides the 12 volt reference to operate the Passlock sensor. The BCM also measures the voltage of the security sensor signal circuit. The voltage measured will indicate whether the Passlock sensor has been activated and whether the resistance value from the sensor is a valid value or the tamper value. If voltage 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 the fuel continue password to the ECM/PCM. If the voltage codes do not match or the voltage is in the tamper range, the BCM will not send the fuel disable password to the ECM/PCM, disabling vehicle starting.

The ECM/PCM 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/PCM will continue to allow the fuel injectors to operate. The ECM/PCM will allow the fuel injectors to operate until it decides there is no valid password coming from the BCM. If the ECM/PCM does not receive a serial data message, receives a message with an incorrect or unknown password, or receives the fuel disable message from the BCM, engine starting will be disabled.

The instrument panel cluster (IPC) illuminates the security indicator as determined by the immobilizer system. The IPC receives a serial data message from the BCM requesting illumination.

When the ECM/PCM receives a password which is incorrect or a password which indicates tamper and starting is disable, the immobilizer system will disable engine starting for an additional 10 minutes, even if the ignition switch is turned from the RUN position to the OFF position.

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.