Immobilizer Description and Operation (North America, except Canada)
Immobilizer System
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:
? Ignition switch
? Body Control Module (BCM)
? Engine Control Module (ECM)
? Security indicator
Ignition Switch
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.
Body Control Module (BCM)
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.
Engine Control Module
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.
Security Indicator
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.
? The immobilizer system requests the IPC to illuminate the indicator only when the ignition switch is ON.
? The CTD system requests the IPC to illuminate the indicator only when the ignition switch is in the OFF or ACC positions or during retained accessory power (RAP).
? The BCM performs the displays test at the start of each ignition cycle. The indicator illuminates for approximately 3 seconds.
Originally Posted by SiriusC1024
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