Control Unit, Car Access System (CAS)

Control Unit, Car Access System (CAS)

The CAS serves as an antitheft alarm system and enables the start of BMW vehicles.
A transponder chip is integrated in each of the vehicle keys. A ring coil is fitted about the ignition lock. The transponder chip is powered by this coil from the CAS control unit. This means that no battery is necessary in the key. The power supply and data transfer take place in the same way as a transformer between the loop aerial (coil) at the ignition lock and the transponder chip.
The key then sends data to the CAS control unit. If these data are correct, the CAS control unit enables the starter by means of a relay located in the control unit and additionally sends a coded start enable signal via a data link to the DME/DDE. These procedures may result in a start delay of up to half a second.

Components

Data link to DME/DDE
The CAS control unit sends a coded enable signal to the DME/DDE via the data link. The engine cannot be started before this signal has been transferred.

Engine control unit (DME/DDE) with coded start enable input
The engine control unit (DME/DDE) only enables engine start if a correct enable signal is received from the CAS control unit.

EWS-DME/DDE interface
Identical variable codes are stored in the CAS control unit and in the DME/DDE control unit. The value of these codes changes after every start procedure. Start enable only takes place if the code sent by the CAS control unit agrees with the code calculated in the DME/DDE control unit.
The control units are allocated only during initial programming of the DME/DDE control unit. The engine control unit then adopts the basic code of the CAS control unit.

IMPORTANT:
It is not possible to replace the DME/DDE or CAS control units for test purposes.
In rare cases, it is possible that the variable codes in both control units deviate from each other. In these cases, it is possible to reset both codes to the initial value via the service function 'DME(DDE) - CAS matching' (EWS matching).

Key identification and start procedure
The following procedure takes place after inserting the vehicle key in the ignition lock:
- The transponder in the key is powered via the loop aerial and sends the key data to the CAS control unit.
- The CAS control unit checks the key data to ensure it is correct and only then sends an enable signal to the starter.
- The CAS control unit sends the variable code to DME/DDE. There, the variable code is checked against the saved variable code; if they match, the fuel injection is released.
- After the engine has started, the CAS control unit generates new key data (change code) and transfers them to the transponder in the key.
- A new variable code is also created and stored in the DME/DDE control unit.

Fault recognition in engine control unit
The following faults are monitored in the engine control unit:
- Interface, i.e. line to CAS control unit: In this case, the check is carried out in order to establish whether a signal is received and whether this signal is not subject to excessive interference.
- Variable code: A check is conducted as to whether the variable code which is sent by the CAS control unit agrees with the value calculated in the DME(DDE) control unit.

Engine start is inhibited if a fault is detected.