A Bit Of A Gay Question About Jump Starting

[GLPoomobile 958]

I'm rubbish with electrics, so can anyone tell me why, when jump starting a vehicle, it is necessary to attach the negative lead to a ground on the vehicle body/engine block?

 

Both batteries are connected to earth anyway, so why can't you just connect both batteries pos to pos and neg to neg?

[Jonmurgie 2]

Never heard that before... just normally stick pos to pos and neg to neg on the battery and away you go!

[damien 22 1 Cars]

it works both ways so i dont think it would matter

[Ryan 99]

Same here. All the books say to attatch the negative leads to the bodywork, but I've never seen a single person actually do it that way.

[GLPoomobile 958]

Thank f*** for that!

 

I've never bothered and always connect the batteries together, but I went out at lunchtime to help a woman with a flat battery in her MX5. She told me I needed to connect the lead to a ground and I told her I never bother. Went ahead and connected the batteries and couldn't get it going. She calls her husband and he says I need to connect it to a ground otherwise my car could produce power spikes that would upset the ECU. Oh s*it, I think!

 

So connected the negative to the part of the rear suspension (the battery is in the boot on this MX5) and it worked.

 

I just had this horrible feeling that I may have damaged her ECU!

[jonah 1]

It's supposedly because batteries produce hydrogen when charging, and hydrogen is explosive. So you're supposed to have the negative lead connected remote from the battery so that any sparks it produces when you unclip it (first in sequence) won't cause an explosion...

 

Pretty unlikely to happen in reality though IMO!

[maturin23 10]

I think the reason for attaching the negative lead to the car to be charged is that it's the last lead to go on. I was taught connect in order of decreasing 'hotness' and given the same advice about the earthing point.

 

An old-style vented battery might have a pocket of hydrogen around it and if the final lead is connected to the -ve terminal the spark created by making the circuit could cause an explosion.

 

Just a theory - but given any earth is by definition connected to the -ve terminal I though you should be OK ECU-wise matey

 

Edited to say - SHOULD TYPE FASTER!

Edited February 28, 2006 by maturin23

[Steve.C 0]

cheap jump leads will affect the starting, if not enough power is able to run through the cables you will have prob;ems starting the car your trying to jump, depends how flat the battery is in it aswel

[DaveK 0]

when i was trying to jump start my mates old 309 i had to put the neg on an earth, wouldnt work neg to neg and pos to pos. had me baffled but it worked so what the hey

[tom_m 0]

I just had this horrible feeling that I may have damaged her ECU!

 

meh she shouldn't have let the thing go flat then should she!