Using my D2 to jump start another car

[tonupkid]

Adrian. Glad you got it started and everything is OK with yours.

This thread raises some issues, and as we A8 drivers would like to display our Samaritan qualities, when called upon, it would be good if we could get to the bottom of the risks involved in providing that vital spark to a fellow motorist.

As far as I can understand the risk in jumping to a dead battery is to the leads, they melt. So it makes sense to connect and run the donor for a while to get some juice into the recipient before turning it over.
If you could connect the running car to the dead one with massive leads, then surely the draw from a piddly Rover turning over would be loads less than the A8's own startup.

There must be some other reason why jump starting can mess up the ECU etc. Anyone know why this might be?

Incidentally I believe the connection sequence is. Get donor car running. Connect the + on each car first, then connect the - to an earthed (not - pole on the battery) part of the donor car, followed by the other end to an earthed (not - pole on battery) part of the donor.
Keeping the earth connection away from the battery reduces the risk of igniting the hydrogen/oxygen given out by a charging battery.

[Nollywood]

Originally Posted by tonupkid

There must be some other reason why jump starting can mess up the ECU etc. Anyone know why this might be?

01) Voltage spikes.
02) Current surges.
03) Short-circuit in dead car.
04) Short-circuit or high-resistance in dead battery, due to damaged plates.

I would invest in a set of jumper-cables with built-in surge protection. A lot safer for the donor car's semi-conductor electronics. I had a set, lent them out to a buddy, and never got them back. They cost me £60 at the time.

[graynada]

Whilst I can understand the advice to minimise sparks in the vicinity of possible explosive gases, if you connect the jump leads to anything other than the battery directly you will increase the chances of passing high current through something sensitive. Connecting battery to battery allows your battery to directly supply current to the point where their battery would provide current for starting, without going through any other circuit on your car, or theirs.

Unless you have had a battery on charge for some time in an unventilated area the chances of a build up of significant quantities of explosive gases I would say will be minimal and can easily be countered by opening your boot and their bonnet for 5 mins before attaching the leads.

Good quality leads are a must, heavier gauge the better.

[Adrian E]

I took the practical view that when the ignition was switched on there was clearly enough juice getting through for the fuel pump to prime as normal

As for connecting to anything other than battery terminals, I'm of the view that if it was necessary then a point would be provided. I know on some Japanese and German cars it is specified in the manual

My neighbour recently started his rover 75 using his dads Mazda 6 - both V6 - and that had sparks etc flying about with cheap jump leads. Since mine was clearly not straining to provide power I didnt feel too worried - I did also turn off all internal electronics I could to reduce load and risk

I'm usually more worried about frying immobilisers on dead cars from past experience

My leads are very chunky for 12v use - unfortunately the clips are mainly plastic so 1 broke when I dropped it on the floor.....

I've had a truck battery blow up on me when jump starting another so I do understand care is extremely important when dealing with batteries

Need to find some surge protected leads - never seen any! Mind you this is the 1st time I've used mine in 10 years.....

[Dezzy]

If you're going to spend that sort of cash on a piar of leads you'd probably be better off with a portable starter. Like or similarhttp://www.maplin.co.uk/portable-jum...mpressor-49038

[PsYcHe]

On the D3 it is HIDEOUSLY IMPORTANT that you use the earth point above the battery and not on the battery itself, or it will kill the power management unit.

[tonupkid]

Originally Posted by PsYcHe

On the D3 it is HIDEOUSLY IMPORTANT that you use the earth point above the battery and not on the battery itself, or it will kill the power management unit.

Arrgh! I was just going to say that I've always ignored the explosive risk thing, and just connect to the most convenient point, usually the battery pole. Then Psyche frightens me with that.

I bought my leads for jumping trucks. Trucks being 24v and the electric fork lift we jumped from being 36v. They had to be good to carry the current.

Always force clamp jaws closed against the lead battery pole if poss and work them around to dig into it. (Unless your jumping a truck, where you just jam the last lead on as fast as you can, eyes closed and extinguisher ready to put out your hair)

Thanks for the heads up Psyche