What is a CAS Module, and what it Does?
Control module Car Access System – CAS
The CAS serves as an antitheft alarm system and enables the start of BMW vehicles.
A transponder chip is integrated into 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 module. This means that no battery is necessary for the key. The power supply and data transfer occur in the same way as a transformer between the loop antenna (coil) at the ignition lock and the transponder chip.
The key then sends data to the CAS control module. If these data are correct, the CAS control module enables the starter utilizing a relay located in the control module. Additionally, it sends a coded start enable signal via an interface to the DME/DDE. These procedures may result in a start delay of up to half a second.
Interface to DME/DDE
The CAS control module sends a coded enable signal to the DME/DDE via CAN network or direct interface, depending on the EWS version.
The engine cannot be started before this signal has been transferred.
Engine control module (DME/DDE) with coded start enable input
The engine control module (DME/DDE) only enables the engine to start if a correct enable signal is received from the CAS control module.
EWS3 mode has 16bit ISN stored in CAS3 and DME/DDE (Examples are MS45, MSV70, MSS65, etc.).
Besides ISN, Identical random codes also are stored in the CAS control module and the DME/DDE control module.
The value of these codes changes after every start procedure. It starts to enable only if the code sent by the CAS control module matches the code stored in the DME/DDE control module.
The control modules are allocated only during the initial programming of the DME/DDE control module. The engine control module then adopts the CAS control module’s rolling code, a process called Synchronization.
In the case of EWS4 mode, there is no rolling code stored in CAS and DME/DDE. ISN (or S/K aka Secret Key) is 128bit long, 16Bytes, and it never changes.
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 antenna and sends the key data to the CAS control module.
The CAS control module checks the key data to ensure it is correct and sends an enable signal to the starter.
EWS3 mode – The CAS control module sends the random code to DME/DDE. The random code is checked against the saved random code; if they match, the fuel injection is released.
EWS4 mode – The CAS control module compares ISN (S/K Secret Key) with DME/DDE, same as in EWS3 mode; if they match, the fuel injection is released.
After the engine has started, the CAS control module generates new key data (random code) and transfers them to the transponder in the key.
In the case of EWS3 mode, A new random code is also created and stored in the DME/DDE control module.
Fault recognition in the engine control module
The following faults are monitored in the engine control module:
Interface, i.e., the line to CAS control module: In this case, the check is carried out to establish whether a signal is received and whether this signal is not subject to excessive interference.
Random code or ISN (S/K) A check is conducted on whether the random code sent by the CAS control module matches the value stored in the DME (DDE) control module.
Engine start is inhibited if a fault is detected.