Communication Principle Between BCM And ECM

The immobilizer system needs a medial of communication between the BCM control unit (which manages the Immobilizer function, including the key recognizing) and the ECM control unit (which controls the engine). This communication is named: release message communication. The information exchange is necessary to lock or unlock the engine control, or to synchronize the Immobilizer system after the first key teaching.

This information is based on the Vehicle Specific Identification Number (VIN), coded in 2 bytes.

The VIN-code is calculated by the BCM at the first key teaching using data from PIN code; the value 0FFFFh is not authorized (reserved for virgin ECM).

The VIN-code is transmitted from the BCM in the release message communication (only in case of authorized key) using an encoding algorithm.

The ECM automatically learns the VIN-code after receiving the first release response message (that is: after the first key teaching, the ECM internal state being in virgin mode or neutral mode).

The ECM VIN-code is equal 0FFFFh when leaving electronic production.

To get a synchronized Immobilizer system (same VIN-codes in BCM and ECM control units, authorized and programmed keys), virgin keys and virgin control units have to be got and special procedures have to be executed (After Market teaching - see Key teaching procedures chapter). Some procedures use the Diagnostic tool equipment; the usage of this tool equipment is restricted to the authorized persons.

In addition BCM and ECM exchange another information: the Model Identification Number (MIN), coded in 1 byte. The MIN is fixed constant value(01H). The MIN-code is checked by the ECM to authorize the engine running, and checked by the BCM, in order to inhibit any unauthorized use of BCM or ECM in a different system model.