3. Steering column lock
Turning the ignition OFF locks the steering column. The CIM first checks that vehicle speed does not exceed 5 km/h. The problem here is that the TC/ABS or ESP and ECM provide the value for vehicle speed. These control modules are connected to the P-bus and there is no communication on the P-bus when the ignition is switched OFF. To solve this problem, there is a direct lead from the TC/ABS or ESP providing a wheel speed signal. The TC/ABS or ESP can read the wheel sensor values and transmit a speed signal since it has a separate +X feed (battery voltage with ignition switch ON or OFF).
As well as checking that vehicle speed does not exceed 5 km/h, the CIM checks that the selector lever in cars with automatic transmission is in position P (I-bus from SLM). When these conditions are fulfilled, the CIM sends a signal on a C-cable to the ISM to release the key. The key can now be turned to LOCK and removed. Only now is the steering column locked.
When the key is inserted into the ignition switch, the steering column lock is released if the transponder is approved. If the car is parked in such a way that the steering shaft bears against the lock bolt, it may not be possible to release the lock. The CIM will in this case send a message on the bus that the ICM, via the SID, can instruct the driver to remove the key, turn the steering wheel and reinsert the key.