Break down of friend

A friend broke down today, basically the lights would come on the dash but it just would not crank, not clicking no attempt at the starter kicking in.

He had a go at checking things but could not see any thing obvious so he called the AA, he checked the battery first, then put a jump cable negative lead on the negative pole, and the other end on the chassis and it fired up and said straight away it was a broken or damaged earth somewhere but couldn't say where as he said theres about 15 earths on a mk4 golf and made a temporary earth to get him home which it did.

I'm guessing what he did created a earth with the jump cable, nice trick that I'll remember that.

Any ideas where would be a good place to look for engine earths to see which one is offending.

Reply to
Pete
Loading thread data ...

Well if he went from the battery to the chassis then it's that one that's corroded, next to check is the Gearbox strap at the bottom, it might not be disconnected, it could just be corroded at the crimp.

Reply to
Duncan Wood

In fact, I was always told when jump starting a car, not to go direct to the dead battery anyway, but to go to an earth point away from the battery so the power is more likely to go to the starter motor, not be absorbed by the dead battery.

Cheers

Peter

Reply to
puffernutter

puffernutter gurgled happily, sounding much like they were saying:

Think about it. Either you've got a lousy earth connection between the block/body/battery or there's no real electrical difference between connecting to the battery or the block or body.

What connecting to the block does is increase the likelihood of getting a decent quality connection, because you're trying to clamp onto something more substantial than a small, round terminal. With the live, you have no choice. It also reduces the chance of you managing to short the jumps together.

Reply to
Adrian

Assuming you make the earth connection last, and remove it first, it also moves the possible spark away from the battery, which reduces further the already minimal risk of hydrogen released from the battery exploding.

Reply to
John Williamson

Could also be a dead battery due to a cracked plate. They do sometimes = just die suddenly but remain capable of supplying enough current for = dash lights etc but not for the starter.

Tim

Reply to
Tim Downie

OK, I'll think about it!

When you are pulling several hundred amps, even the smallest amount of resistance will create a large volt drop. So, if the jumper cables are attached to the battery, there will be a volt drop between the battery and the alternator, ignoring of course the very low impedance being offered by the battery itself. If you attach a positive to the battery (assuming negative earth) and a negative directly to the engine, then you are ruling out the resistance of the battery/body to engine earth strap and the potential for volt drop and making it preferential for the external current to head to the starter motor and not into the battery.

Seems logical to me!

Cheers

Peter

Reply to
puffernutter

Which helps. But not having a spark next to the battery potentially really helps (odds are it makes no difference, and you won't know if it did. You'll never forget the other way round though).

Reply to
Duncan Wood

Its to protect the electronics. Most handbooks state the fact.

Reply to
Rob

puffernutter gurgled happily, sounding much like they were saying:

IF the starter is pulling several hundred amps, via jump leads, then it's doing so via the normal earth strap too...

Reply to
Adrian

Not if the battery lead is placed on the block.

Regards

Peter

Reply to
puffernutter

That's why they say to turn the headlamps & the rear window demister on.

Reply to
Duncan Wood

Apologies for posting this here, the thread has disappeared in outlook, which is a sin to use.

Rightywhooo... my and matey have had a good look, we discovered the earthe neart the starter was cracked, damaged and not making a great connection, replaced with a new one for £4 and it now works as intended.

We looked passed it many times looking everywhere, but it was all in the detail and luckily spotted it eventually.

Thanks for the advice all, especially Duncan *virtual beer on the way to you*

Reply to
Pete

puffernutter gurgled happily, sounding much like they were saying:

Eh?

Does the starter somehow miraculously "know" where the power source is connected, and use a different amount of power according to that?

No.

If it's spinning normally, the circuit feeding it is connected from the battery +ve, down the main starter lead, and back via the earth strap to the battery -ve.

How does that somehow miraculously differ if there's a pair of jumpleads connected to the battery terminals? It doesn't. So if the earth lead isn't up to the job for jumping...

Reply to
Adrian

Because the jump leads connected between the battery -ve & the block.

Reply to
Duncan Wood

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.