Function chains
A large number of function chains can be identified in the bus system. Described below are four examples of important chains.
- Vehicle speed
The TC/ABS or ESP control module measures vehicle speed with the help of a Hall sensor at each wheel. The sensor supplies the control module with alternating current, the amplitude and frequency of which increase with increasing wheel speed. The ECM calculates the average speed of the two non-driven wheels, correcting this value for the type of tire. The ECM sends this value for vehicle speed over the bus making it available to other systems.
In the table of bus messages, you can see that the ECM sends the value for vehicle speed and that the majority of other control modules use this.
Examples:
- The ECM itself uses vehicle speed for cruise control, idle speed control and a number of diagnostic functions. Pulling off (certain markets only).
- The MIU uses the value for the speedometer.
- The ICM uses the value for trip computer functions.
- The CIM uses the value for the steering column lock.
- The ACC uses the value for activating recirculation at high temperatures and low speeds, to reduce the fan speed at high speeds, and to calculate the cabin temperature.
- The audio system uses the value for increasing the volume at high speeds.
- The BCM uses the value to automatically lock the doors after pulling off (certain markets only).
- Immobilization
When the ignition is switched on, the CIM checks that the correct key is inserted in the ignition switch. A circuit built into the remote control part of the key sends a code by means of inductance to a coil in the ISM. The code is passed on via a C-cable to the CIM, which checks that it is correct. The CIM also checks that the right MIU, BCM and PDM are fitted in the car. These are all part of the car's "immobilizer environment" and continuously send an ID number over the bus. If the key and control module ID numbers are OK, the CIM will send a message for "Immobilizer OFF" on the bus. The ECM acts on this information by cancelling fuel shut-off, which is always activated whenever the ignition is turned on.
- 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.
- Central locking system
BCM contains all logic for the central locking system. When the unlocking button on the remote control is pressed, CIM receives the radio signal. CIM then activates the other I-bus control modules and transmits which button was pressed and how it was pressed (e.g. single press). BCM transmits the unlock command on the bus, at which time the four door modules (DDM, PDM, RRDM, and RLDM) unlock their respective door. The Rear Electronic Module (REM) unlocks the boot lid/tailgate and BCM unlocks the fuel filler flap.
The BCM contains the central locking logic and can be programmed to unlock only the driver's door and fuel filler flap if the unlock button is only pressed once.