What tools should be carried in a modern car?

Why should a jump start take out the ECU and what or how would a surge arrestor help?.

After all one motahs 12 volt nominal system isn't a higher voltage than another ..

Or was this someone doing a 24 to 12 volt transfusion;?..

If your battery's intermittently OC because it's earth lead's loose or it's broken then when the 50A the alternator's dumping into it to charge it has to go elsewhere then you get a large spike. You can get the same effect if the car with the dead battery's trying to charge the live one & you disconnect the jump leads.

Starting a car draws down a large current , both vehicles try to supply the demand upon starting there is a powersurge as the recipients vehicles alternator kicks in , vehicle alternater can output a considerable voltage on a dead battery / no battery situation enough to fry electrics>

Most domestic jump leads are incapable of starting a car with no battery - or a totally flat one.

Still don't see how you get one of these surges. The internal impedance of the battery will stop anything like that.

Glad I'm not the only one who's sceptical. The impedance of a car battery is extremely low. It has to be to deliver the starting current - far higher than an alternator produces. Which would have the effect of damping any 'spike' on disconnecting. Of course, if the battery is disconnected and you jump start the car, the alternator can produce a high voltage with no load. But that's a few 'what ifs'

Yes and hence the point that it only occurs when that's not the case because the battery's open circuit or not properly connected.

I agree apart from the AA/RAc card. Check out Autoaid - £37 a year for me and the wife, full cover for the vehicles we drive, and a good standard of cover. Only issue, you pay up front and then reclaim, which has taken less than a week start to finish when I've used them. Post off a one page form and the bill, get a cheque back in the post.

Seconded; although I do carry a tow rope, and personally I find tow starting less hassle than messing around with jump leads.

An alternator doesn't produce '50A' unless told to do so by the load. I'll grant that disconnecting and re-connecting a battery with the engine running could well produce some spikes - but that's not got a lot to do with jump starting.

Right - I'll grant you that.

So just how is a surge arrester in jump leads going to help? As soon as those jump leads are removed the not properly connected battery will cause the surge again.

That's what you do when you jump start something with a broken/wobbly battery lead or an open circuit battery. But something as trivial as switching on the demister and the interior fan will stop it damaging anything, you need an OC battery, a racing engine & no resistive loads for it to be an issue. Which, given the 1st of those faults is probably on the dead car won't be affected by jump lead surge arrestors.

Yup. Although if you're the AA you can unplug them at the connecter so it's still connected to the donor car. I'm not suggesting it's a common issue, but it's not an unknown one, hence the instructions to turn on lights, fan & demister in the manual.

Hang on a sec, I've always jump started cars with the donor engine switched off. I guess someone is going to tell me that wrong now ;)

I think that's what the Haynes book says (from memory) and my logic for doing it that way is so the donor alternator doesn't end up trying to provide the starting power / current which will be way over it's design capacity. In other words I just use the donor battery to help the dead battery in turning the starter motor once some charge has been transferred. It's always worked for me so far. I've heard a fair few stories about dead donor alternators after a jump start with the donor engine on which I presume is down to overloaded rectifier diodes blowing. Of course running the donor engine to charge the dead battery is fine but I'd always switch off before the dead car tries to start.

Thanks,

Peter.

Alternators limit their maximum output.

Fortunately the alternator manufacturer (& just the way they work) stops that happening)

As per Daves disbelief, you've got to be unlucky & lots of other things are likely to die 1st.

And it's certainly not an urban myth...

Chris

I will, if you like!

If you run the engine on the donor car with the jump leads connected for at least five minutes before attempting to start, some charge will go into the dead battery. This will make it more likely the dead car will start, and also reduce the chance of spiking. (No childish name pun intended!)

Leaving the jump leads connected with the donor car not running will greatly reduce the amount of charge that the dead car will get.

Also, the problem at disconnection comes from the fact that the "dead" car is running. The absolutely safe way to do it is to leave the leads connected with both cars running for long enough to put a decent charge in the "dead" one, then switch both off before you disconnect. Of course, the "dead" car may not then re-start.

Chris

Don't try doing it to a modern twin-battery Transit; it will blow the main battery fuse!

Please don't ask how; I've never been able to work it out,but it does.

Chris

I have been impressed what RAC can do for me. On a cold frosty night with traffic jams and slow crawling traffic, the clutch pedal went bang down to the floor as a hydraulic pipe had burst, it it had apparently been chafing against the bulkhead. The RAC guy tried many attempts to load up the system with various pumps and whatnot, yelling at me to pump the pedal up and down. I had second thoughts if it would ever work, but went through the motions as I didn't want to upset or offend his valiant effort. After about 20 failed attempts, it finally worked! He had plastered the pipe as a temporary repair and managed finally to load pressure on the system. I gingerly drove home and had the pipe replaced the next day. What a guy, full marks.