For the purposes of this post I will use the term 'multiplexed' to refer to different data networks and protocols including for example:
VAN - Vehicle Area Network
CAN - Controller Area Network
LIN - Local Interconnect Network
On modern vehicles, the data networks can remain active for some time after the engine is switch off and longer if doors are opened and equipment is being operated. There is always the possibility that disconnecting a wiring block to an ECU that uses a multiplexed data connection can result in the ECU being left in an 'awakened' state, without having been told to 'sleep' by the controlling ECU like a BSI.
Depending on the type of ECU, data parameters may not be written to memory if power (as in the battery being disconnected) is suddenly removed. Examples of this are the suspension ECU (on a Hydractive Citroen), the fuel injection ECU, Telematics and Head Units, Auto Gear Box, as well as leaving electro valves in a certain position (EGR for example). Although most of these are robust enough to correct themselves when the power is reapplied, it can sometimes cause 'glitches' in the data network when the ECUs try and reinitialise.
So very much like a PC should never be shut down by pulling the mains out, the same can be said for a car's controlling computer and network of connected ECUs.
- Open bonnet and driver's side window (the window is left open for emergency access reasons only whilst battery is disconnected and to enable switching on of the sidelights).
- Ensure all doors are closed and remove key from the ignition (do not leave the key in the ignition as the transponder circuit will detect the key and this can prevent the networks from sleeping).
For C5 X7 Tourer V6 Engines, with / without motorised tailgate or vehicles where the battery is in the rear boot:
- Open the boot
- Disengage the motorised function (short manual action down then up)
- Mechanically lock the lock using a tool (which results in the boot light switching off)
This will fool the BSI into thinking the tailgate is closed. Then follow the procedure from shutting all the doors - but now you can leave the tailgate open.
It is essential though, that after refitting the battery, that you remember to flip the tailgate locking catch back over to the unlocked position.
Procedure continues for all vehicles:
- Wait for 5 minutes, - Don't press any buttons, open any doors or use the remote key fob during this time.
- Disconnect the vehicle battery negative lead and do NOT let it reattach or touch the battery negative terminal post again (use some cloth or other non-conductive material as a barrier). If it touches, the whole process needs to be repeated as you have effectively started the wake up procedure if it touches the terminal.
A) If applicable, now undertake any work or operations you are planning that needs the battery disconnected, OR:
B) If just resetting the BSI, wait 5 minutes whilst the negative terminal is disconnected.
- Ensure all doors and tailgate are shut (if you have been doing any work on the car).
- For Keyless Ignition / Stop Start Vehicles only: For these systems, when reconnecting the battery, ensure the fob is not within 2-3 metres of the car perimeter until the time of reconnecting the battery has lapsed.
- Reconnect the vehicle battery.
- Wait a further 3 minutes (do not open doors).
- Reach in through the open window and switch on the side lights only.
- For standard vehicles, switch on the ignition only with the key, don’t start vehicle yet and check ignition lights come on.
- For Keyless Ignition / Stop Start Vehicles only: With the Fob in the car or in the Fob Reader, press the Start Stop button without pressing any pedals. That will switch the ignition on only.
- For standard vehicles, with the key still in the ignition and within 10 seconds of switching the ignition on, hold lock button on key fob down for a few seconds (to synchronise the remote fob). Repeat this part for any other fobs if required (especially if the battery was previously completely flat).
- Switch off ignition, remove key through window, then operate Central locking several times.
- For Keyless Ignition / Stop Start Vehicles only: Press the Start Stop button again, without touching any pedals to switch the ignition off.
- Remove the Keyless fob from the reader through the window, then lock the car using the sensor on the door handle or the button on the fob. Now unlock the car either using the sensor on the door handle or the fob.
- Now open door, start the engine and check there are no errors etc.
- Switch off your side lights