I have an old focus diesel estate and carry big jump leads .On many occasions I’ve got people going and been helped out a few times too through jump starting. I know it’s old news by now but is there a rule of thumb re which vehicles are safe to give or receive jump starts these days from a damage limitation point of view? You hear the odd story of someone who’s kindly jumped a flat vehicle who happily drives away and then they find it’s buggered their own vehicle.
I know it’s all about sensitive electronics and voltage spikes etc but how do you decide whether it’s likely to an issue or not ?

A friend (yes a friend, not me) got the leads the wrong way arround between a transit and a vw caddy, appart from killing the transits stereo no harm was done.

So safe-ish to say transits and caddys! Ditto our focus and fiesta have always been fine depsite the focus having the fancy fast charging/higher voltage stuff.

I usually use a battery pack which solves the problem entirely and charges from the cigarette lighter.

We once jumped a 14 plate diesel focus from a similar vintage diesel astra using bits of cable, test leads and rubber gloves we had in the boot of each car.

I once connected the leads the wrong way round on a car and fried it …fortunately they were both mine and the one that got cooked was fairly worthless. It’s more the issue of modern (whatever that means) electronics which vapourize or throw a wobbly due to surges and if unlucky someone gets a nasty bill as a result (as well as being left stranded)
I do actually have a mini usb starter pack in the car…perhaps I’ll offer that in future!

We once jumped a 14 plate diesel focus from a similar vintage diesel astra using bits of cable, test leads and rubber gloves we had in the boot of each car.

Test leads! Surely they would normally get melted! As for the rubber gloves..was that for heat or sparks …or maybe I shouldn’t ask!

If you do and as soon as the car starts up turn the headlights on. Some smart charge alternators go mental n kick 17v out… if you’ve got the lights on it helps dissipate the surge

How can you get the leads the wrong way round?
They’re even colour co-ordinated for those that aren’t colour blind.

How can you get the leads the wrong way round?
They’re even colour co-ordinated for those that aren’t colour blind.

We questioned that!

I guess if you’re being absent minded then most batteries are actually + to – to increace the voltage, youre actually putting them in parallel with the intention of pushing current the “wrong” way into the dead battery, the battery/car end isnt labled and couple that with a 12h shift and youve a recipe for trouble.

Test leads! Surely they would normally get melted!

If you leave it a while the batteries will equalize and you can start it without the jump leads. That would also solve the problem of frying the electrics if you have enough patience.

Not recommended to jump start a Hybrid.

I won’t offer for lack of knowing exactly which car is safe.

If asked I’ll oblige.

I know who’s car will come off worse given mines has about 3 electronic componants.

singlespeedstu

Subscriber

How can you get the leads the wrong way round?

Most people know literally nothing about electricity. And even then, “connect like to like” could easily not be that obvious, like “maybe black is out and red is in so we have to connect the black on that one to the red on this one”.

The real fun comes when you can’t find the battery

You shouldn’t connect both to the battery anyway.

+ to + first

Then engine/chassis to engine/chassis. That way if you drop the second lead or tap it on something you cant make a short and dont make sparks arround the battery which if its flat is probably venting while it charges.

What Thisisnotaspoon says, unless the battery is in the cabin and vent connected to outside the car.

With regards to getting it wrong, the positive lead going to the battery is often black.

We questioned that!

Just a little hint here that my post was ripping the piss.

Come on

How can you get the leads the wrong way round?

when the battery has stickers on with terminals wrong way round.

The handbook will usually advise the jump points if the battery is not visible , and any special precautions, some cars have jump connections even if the battery is visible. Don’t rush pay attention.

when the battery has stickers on with terminals wrong way round.

That happens all the time doesn’t it.

I was camping in a farmer’s field (with his permission) and flattened battery on my Rover during the stay. The farmer kindly gave me a jump from his tractor.
I think tractors have 24v batteries. It did something nasty to my car.
Fortunately, the farmer’s son workaed at a Rover dealership, went into work on a Sunday night, brought back some kind of diagnostic/repair box, reset everything and gave me a jump from a battery pack and sent me on my way.
He was a very nice farmer.

We had a mk2 Fiat Punto that didn’t like being jump started. ECU light would come on and require some kind of dealer reset. Jumping other cars was fine.

when the battery has stickers on with terminals wrong way round.

That happens all the time doesn’t it.

It was possibly 1957.

The terminal polarity is moulded onto the battery case these days to stop dumdumitus

Not recommended to jump start a Hybrid.

Hybrids can be jump started, but care needs to be taken.
Some cars, in particular Mercedes, do not take kindly to jump-starting! If you jump-start them, they’ll often throw up an engine management warning on the dash, or sometimes refuse to start at all if the battery has gone flat.
We have one at work at the moment that won’t start, it’s waiting for a transporter, I’ve jump-started quite a few and then needed our bloke with his magic box to reset things, and another one was delivered by a plater, the battery died while it was in Park, and there was nothing we could do to start it or get it out of Park. Out tech bloke spent hours with a laptop and his diagnosis kit to try to sort it, and nothing worked; it was finally dragged onto a flat-bed transporter after it spent several weeks getting in everyone’s way.
The worst car for flat batteries? Kia Vengas. A little part of me dies inside every time I have to go fetch or move one, because more often than not the bloody thing is dead on its ass. 🤬

I was asked recently, I have leads in the boot but a new (to me) car.

I made them wait while I read my own car’s manual!

All was well, after two minutes’ caution.

Not jump start but I have a booster pack that I can use. Meant to be much safer to car electronics. I’d agree though that it’s much harder to know what’s best these days. Even replacement batteries have to be “coded” for some cars

My Dad actually inserted he battery back to front in our Fiat Strada . Killed random bits of electronics and mellted a few wires , but as it was a turd to start with the failures were nothing new
Then a driver at work left the lights on on our sprinter. he camme in and stated the battery had died overnight . Its ok , we keep a spare for situations like this says I
Left him to connect it backwards and fry an ECU in the process .
He never admitted to leaving the lights on
But had to confess to ‘ welding sparks ‘ as he strapped it together wrong.
V70’s have a battery in the boot and a jumping positive pole under a cover under the bonnet next to one of the fuse boards

Then a driver at work left the lights on on our sprinter. he camme in and stated the battery had died overnight . Its ok , we keep a spare for situations like this says I
Left him to connect it backwards and fry an ECU in the process .
He never admitted to leaving the lights on

I borrowed a van from work once that had a knackered alternator. No problem I said as I gaily bump started it, several times. Eventually that wouldn’t work after the last bump when it died in a spectacular fashion.

Turns out you’re not supposed to turn the starter when bumping. Nobody tells you that bit.

That totalled up to the original dead alternator, a fried battery, fried starter motor and a blown master fuse link.

Garage was not impressed.

Owning a kit car jump leads are standard issue.

Only had to ask for 3 jumps not in works carpark (about 7-8 there) and always target the oldest car possible as seem to be the easiest and most willing to help. Did have a lady in a Prius offer but after 2 mins we decided we didn’t know enough about what’s what to risk it

That’s why I never really understood kit cars.

What makes them need jump leads more than any other car *

* My current car is a home brew of at least 3 different cars (chassis and running gear from 1 , engine from another and the wiring and loom work from a third but it’s reliable.

I know that you’re not supposed to bump start “modern” cars that have Start buttons and that batteries for cars with Stop-start systems don’t like being jumped. This could be why CountZero found Merc’s don’t play nicely.

I’d be wary of jumping/bumping a hybrid unless the owner was sure it was fine. Big engines, particularly diesels, pull a lot of current so may require care depending on what is jumping them.

I quite often have to jump my motorcycle off my van, as I go for weeks without getting time to ride the bike and it’s reluctant to start.

My Hiace has two batteries and my Suzi 750 has titchy little terminals buried deep under the seat that are difficult to clip the massive crocodile clips onto.

I often wonder am I doing something I shouldn’t be but meh, both 12v and nothing seems wrong yet.

Slightly off topic but…..round at my mates over Christmas, his son is at work nearby, phones his dad to say that the three phase electricity is down, they have an emergency generator , a big f88k off diesel engined Deutz, the batteries were flat but he’s connected up two more ,it’s 24v and still won’t start. Us two old farts go to look, and in time honoured tradition he is aghast to see us short the starter motor across the connections (using a big old spanner) and fire it up first time. Do not do this at home.

My 2004 Transit has been ju.led started from a VW T4 and a huge batter from a recovery truck of some sort.
It has jumped started a Renault Clio, Porsche Boxter, Gnusmobile, Rover 75, VW Golf, no problems. It wouldn’t get an E-Class going though, reading that bit about Mercedes above I wonder what we did to it…
.
Not had to jump new van yet. What if you have split chargers and leisure batteries connected to it? Battery-battery charger so the alternator does the liesures too, solar panel which does both (and hopefully means it never needs jumped anyway)

Slightly more off topic but I lump cars that can’t easily be jumpstarted without reading a manual alongside headlamp and tail light bulbs that can’t be easily changed. Poor and potentially dangerous design.

Slightly more off topic but I lump cars that can’t easily be jumpstarted without reading a manual alongside headlamp and tail light bulbs that can’t be easily changed. Poor and potentially dangerous design.

You’ll probably be walking before long then.

Why? I have company vehicles and old cars, the company vehicles from new so still have the manuals. The old cars arent that fussy.
The company vehicles will eventually be old and moved on, manuals get lost and certainly will need a jump start in later life.

The light bulb thing is unforgivable. If it can’t be changed at the side of the road without tools, it’s a failed design.

I don’t even bother carrying jump leads,I think it’s a pastime from an olden era nowadays with the amount of electronics stuffed in a car being helpful could ruin someones day worse than having to wait an hour for the AA man.

(Awaiting but not everyone’s got AA cover onslaught.)

The light bulb thing is unforgivable. If it can’t be changed at the side of the road without tools, it’s a failed design.

Ah but the soft trim in the car sales it not the ease of light bulb change 🙂

Cars is all style over substance.

When my Jeep’s battery was on the way out I’d get a jump start from whoever wandered out of work at the right time. Was probably very luck not to fry any ECUs jumping the big 4.7 V8 off all sorts!

Last one I helped with was two cars outside work, all hooked up but the dead one wouldn’t start. Pointed out that the -ve connected to a bit of plastic wasn’t helping and it fired up straight away hooked up to metal 🙂

I might be being a bit thick, but can someone explain why jump starting can be bad for some cars? I mean, I think of it basically as lending a working car battery and charging system to the one that has dropped its voltage; it’s not like you’re exposing it to voltages or ampages that the flat cars system isn’t designed to cope with is it? I don’t get it 🤯

I might be being a bit thick, but can someone explain why jump starting can be bad for some cars? I mean, I think of it basically as lending a working car battery and charging system to the one that has dropped its voltage; it’s not like you’re exposing it to voltages or ampages that the flat cars system isn’t designed to cope with is it? I don’t get it 🤯

A lot of modern cars have alternators that operate at higher than 12V then have a signal from the ecu that reduces it to the correct voltage for the batteries state of charge via PWM. Whereas an old car the alternator was just set to a voltage and that was that.

But yea, you’d kinda think the impedance of a flat battery would prevent most problems.

The merc thing might not be as bad as its made out, we found whilst using them for filming that they didnt like going flat, but would happily ‘jump’ start off the aux or coms batteries (or the shore line). But once you did they came up with loads of errors. It wasn’t the jump that does it, its just a feature of the mercs ecu that flat batteries trigger fault codes.

The terminal polarity is moulded onto the battery case these days to stop dumdumitus

Except on my neighbours car where there were only backwards stickers. Multimeter to the rescue!