Body Control Module (BCM)

The immobilizer system is an integral part of the BCM and is controlled internally within the BCM. The BCM can learn up to 8 keys (transponder values). The BCM uses low frequency antennas in three different locations on the vehicle to determine the location of the transmitter. Multiple antennas are used to ensure complete coverage of the vehicle interior and rear compartment. The BCM monitors the ignition mode switch. When the ignition mode is changed, the BCM will command, or "ping", the low frequency antenna.

The BCM uses the following inputs:

• Environment identifier exchange with various modules
• Encrypted code from the vehicle key, received by the immobilizer coil antenna if the keyless entry transmitter signal isn't detected as indicated by the "No Remote Detected" message
• Encrypted code from the vehicle key, received by the Remote Control Door Lock Receiver

The BCM uses the following outputs:

• Prerelease Allowed signal communication with ECM
• Challenge/Response with ECM

When a transponder value is received by the BCM, the BCM will compare this value to the learned key code stored in memory. The BCM then performs one of the following functions:

• If the encrypted code value matches the values stored in the BCM memory, the BCM will send the Prerelease Allowed signal to the ECM via serial data.
• If the encrypted code unique value does not match the value stored in the BCM, the BCM will send the start disable message to the ECM via serial data.
• If the BCM is unable to receive the vehicle key encrypted code value, the BCM will not send any messages to the ECM.