Antitheft and Alarm Systems: Description and Operation

IMMOBILIZER SYSTEM

The immobilizer system consists of the ignition key, the key ring antenna, the immobilizer-ECU, and the engine control module (ECM). The ignition key has a built-in transponder. Only the registered ignition key permits the engine to start, therefore, the engine can never be started by means of a forged key or by connecting the ignition wiring directly. The system is significantly safe and reliable against theft. In addition, the driver has only to turn the ignition switch to the "ON" position to activate the immobilizer system. If the requirements for starting the engine are not satisfied, the engine will be immobilized. If a registered ignition key is lost, all your ignition keys need to be registered again using scan tool MB991958 (M.U.T-III Sub Assembly) to ensure security. An additional ignition key can be registered as follows (only if two registered ignition keys are available):
- Using scan tool MB991958 (M.U.T.-III Assembly).
- By operating two ignition keys that have been already registered.

OPERATION
1. After the ignition is switched on, the engine control module sends a control signal and transponder read command signal to the immobilizer-ECU.
2. When the immobilizer-ECU receives the control signal from the engine control module through the key ring antenna, the immobilizer-ECU supplies a current and sends random number data to the transponder in the ignition key.
3. The transponder uses the random number data to derive an ID code, which is sent to the immobilizer-ECU through the key ring antenna.




4. The immobilizer-ECU compares the ID code that was sent with pre-registered ID codes, and if it matches, a control signal approving ignition is sent to the engine control module. If the ID code does not match (in the case of counterfeit ignition keys, for example), the immobilizer-ECU sends a control signal denying ignition to the engine control module, preventing the engine from starting.




CAUTION WHEN REPLACING

IMMOBILIZER SYSTEM RELATED PARTS
To replace immobilizer system related parts, refer to the table below. When the ignition key is re-registered with scan tool MB991958, the originally registered ignition key registration information will be lost.

IGNITION KEY REMINDER TORN ALARM
The Ignition key reminder tone alarms is controlled by the Simplified Wiring System (SWS). For troubleshooting, refer to SWS Diagnosis.