Ok, so in a nutshell, your car has a central computer (the BSI), plus there are Electronic Control Units (ECUs) dotted throughout the car, that operate the windows, the mirrors, the lights, doors, engine management, tyre sensors, lane departure, external lamps, alarm, cruise, etc., etc.
In order to reduce wiring and provide system redundancy, the BSI communicates with these ECUs over multiplexed data networks. It allows the BSI to command various functions to work and for the various sensors to send data to other ECUs needed to run the car. The signals are coded in Binary so each ECU on the network knows who the message is meant for. Some computer networks are faster than others, for example the safety critical systems, whereas others may be slower or have less priority, like the comfort systems. There can typically be 4 data networks on the car all sending and receiving packets of data.
Normally, when you switch the engine off after a run, the BSI and certain other ECUs carry on operating until they have written their data into flash memory. After some time they go into sleep mode and shut down, but the BSI and several other circuits are always active - for example waiting for a remote central unlocking request, or to open the tailgate remotely. So imagine if you just went ahead and disconnected the battery all of a sudden without allowing the ECUs to finish shutting down...... this potentially can cause the ECU to get stuck or not start up again properly. This can result in some electrical glitches that can affect the lights, wipers, windows and other components for example.
So when we talk about a BSI reset, or Battery Disconnect / Connect procedure, we simply allowing all ECUs to go to sleep, remove power and allow power to dissipate, then reconnect and allow the ECUs to properly boot up. That’s why it’s important not to open any doors during the procedure, otherwise it will wake up the ECU and the whole excercise becomes pointless.
Here’s the BSI reset / Battery disconnection / connection procedure for reference if you’re still awake
- Open bonnet and Driver side window.
- Ensure all doors are closed and remove key from the ignition
- Wait for 5 minutes, then disconnect the vehicle battery negative and wait at least 5 minutes. Don't open any doors or press any buttons.
- Reconnect the vehicle battery, wait a further 3 minutes (do not open doors).
- Switch on the side lights through the driver's window.
- Switch on the ignition only, don’t start vehicle yet and check system's functionality.
- With key still in ignition hold lock button on key down for 5 seconds.
- Remove key through window, then operate Central locking several times.
- Now open door and start the engine and complete the system's check.
- Failure to follow this procedure could result in incorrect operation of many BSI related items.
