Start enable

If the identification data is correct, the CAS control unit activates the starter motor via a relay in the control unit. At the same time, the CAS control unit the CAS control unit sends the DME control unit an encoded enable signal for the engine start. The DME control unit only enables the engine start if a correct enable signal is received from the CAS control unit.

The following test module is performed after insertion of the ignition key in the ignition lock or the remote key in the insertion slot:

• The transponder in the ignition key or in the remote key is supplied with power via the ring aerial and transmits the key data to the electronic immobilizer or CAS control unit.
• The EWS or CAS control unit checks the correctness of the key data and only then enables activation of the starter by the DME.
• The DME uses a random number and the secret key to calculate a request. The DME transmits this request via the CAN data line (PT-CAN and K-CAN) and the body bus (also called CAS bus) to the CAS control unit.
• The CAS control unit uses the request and the secret key to calculate the response. The response is transmitted by the CAS via the CAN data line (PT-CAN and K-CAN) and the body bus (also called CAS bus) to the DME.
• The DME itself also calculates the response that the CAS control unit expects. The DME then checks that the response it receives from the CAS control unit matches the response it has calculated itself. If the responses match, the engine start is enabled.

Identical rolling codes are stored in the control units whose value changes after each starting operation. The rolling code is generated from a random number and the secret key.