battery re-connection

[Tannaton]

Originally Posted by 100K+

LOL shows how much I know. (thought I know)

I always assumed ignition in P2 was to PREVENT spikes causing mischief in the various modules, by having the headlights coming on to "absorb" any spikes at the instant of reconnection.

You can tell lecky's not my strength


Cheers

Bob

There is no direct connection to the headlights though, it either goes through a relay or transistorised module (on later cars) and hence the CEM needs to be up and running to operate that... so it's most unlikely that they headlights could absorb a voltage spike.

The airbag theory sounds plausible but I think an urban myth - who has ever experienced spontaneous and erroneous deployment of airbags?

[4candles]

Originally Posted by Tannaton

who has ever experienced spontaneous and erroneous deployment of airbags?

Aint ya ever seen a hen night?

[Tannaton]

Originally Posted by 4candles

Aint ya ever seen a hen night?

Not with a full, spontaneous, explosive deployment of all airbags. That's still on my top ten things to do before I die...

[Mr steptoe]

How can you get a spike reconnecting a 12 volt battery.

It's 12 volts, it can't produce anymore volts simply by being connected to an inert electrical system.

[Tannaton]

Originally Posted by Mr steptoe

How can you get a spike reconnecting a 12 volt battery.

It's 12 volts, it can't produce anymore volts simply by being connected to an inert electrical system.

The risk is if you get any "contact bounce" when re-connecting the battery - i.e. the connection is made for 0.5 seconds and then broken for 0.1 seconds. Any inductive devices (motors, solenoids, relays) will give a "kick back" as the stored magnetism gets converted back into an electric charge - this can create a voltage spike considerably more then 12 volts and it is created in the opposite polarity to which the current was initially applied. The correct name for this is "back e.m.f.".

It's exactly this principle which is used to give a spark from a coil in a petrol engine and why you need a condenser to stop the primary back emf arcing and burning the points (for those of us who remember points anyway!).

This is why I don't understand Volvo's prescribed method of having the ignition on when re-connecting the battery - surely the fewer devices connected the better?

[Paulcv70]

Well after using the position 2 method I have to say I'm not keen on using this method again! There was a right good crack upon fitting the battery lead! But saying that everything is working again and all fault codes have been wiped and at least we have a car again for Christmas.

[blueacid]

Whenever I've reconnected the battery I've done it with the ignition off, then with the little 'zzt' spark and the boot lights fading on I've thought "oh yeah, I should have had the ignition on shouldn't I?"

So far nothing has gone wrong.. touch wood!