Archimedes
Sorry in advance for the noob question.
Aside from making sure the right cables are on the right terminals etc, is there anything I should know/do before I try jumpstarting the Volvo with the MX-5? FYI, 1991 240 GLE & 2007 MX-5.
lasercowboy
Red cable on positive terminal of good battery.
Other end on positive terminal of dead battery.
Black cable on negative terminal of good battery.
Other end on something metal and unpainted in the engine bay of car with dead battery.
Start car with good battery, and run for 5 mins.
Turn over car with dead battery and cross fingers!
JamesM
really? i always just also connected the negative to the negative. why do you suggest grounding to the chassis/car?
lasercowboy
Mostly historical reasons I think: the battery used to be hard to reach, not a good idea to be waving a live wire around in there. Also, old lead acid cells tend to generate hydrogen. Don't want the spark nearby.
Practically: if the quality of the earth connection on the negative battery post is anything other than optimal, you'll get a higher voltage drop and a better result grounding to the chassis/block than to the battery directly.
Angus242164
Electrically it makes no difference whether you connect it to the battery negative or to the engine. The reason for it is because there will be a spark generated when you connect the lead, and in theory that spark could ignite the hydrogen gas given off by the battery, which vents through the vent holes in the caps of the cells.
It's more of a theoretical problem than a real world one, I've never heard of the gas igniting before. The gas is generated during charging, and a flat battery is not likely to have much internal activity, and shouldn't be venting much gas.
I wouldn't worry about running the car with the charged battery for five minutes, 30 seconds or so should be enough. It may be necessary to raise the revs of the car, some alternators don't have enough output at idle to jump another car.
Did you know that it's possible to jump start two HQ Holdens using a single jumper lead? Just touch the bumper bars together to form the negative connection.
bumperplates
What can that do to the electronics/computer in modern cars if it's connected to the engine body?
Agouti
bumperplates;81061 wroteWhat can that do to the electronics/computer in modern cars if it's connected to the engine body?
Again, there is no functional difference. The whole chassis is well connected to the negative of the battery.
In terms of the computers they should be OK - they survive your own car starting after all.
Personally I don't like jump starting with an already running car due to fears of causing an issue with electronics. I've heard unverified stories of alternators being fried this way, but plenty of people jump with the donor car running without issue. YMMV.
carnut1100
Used to be common wisdom not to jump start cars with electronic ignition....but I've done it hundreds of times.
Used to do old style cars with donor engine running, but just to be safe with modern electrickery cars I do switch the donor car off before hooking start the donor car, run with a few revs for 30 secs and try starting the other.
In best practice theory you run both until there is enough grunt to restart then shut both down and disconnect....in practice once both have been running for 30 secs or a min drop both to idle and there won't be any issue unhooking. The change in power flow will be no different to turning headlights off...
Angus242164
The myth about not jump starting cars with efi/electronic ignition/cats doesn't seem to have any real basis. If it were true that it kills modules, you'd probably also need a new ecu every time you turn on the headlights and the heater fan at the same time.
Modules and ecu's are pretty resistant to voltage changes.
Connecting leads backwards can blow main fuses and alternator diodes however, so avoid that.
lasercowboy
^ this. Modern cars have very effective voltage regulation and fuses everywhere. The worst you're likely to do is pop a fuse - and when did you last do that (when not doing something silly)? :)
Agouti
Funny story, I used to have a high end Viper alarm on a previous car, had the works - tilt sensor, glass break, GPS, microwave for intrusion (so you could leave the roof down and it would chirp at anyone who reached in).
Anyway, it didn't have input voltage filtering so my trickle charger used to set it off at odd times of the night. Took me a while to figure out that was causing it.
Solution was a little voltage regulator, but as a result I don't like to assume. While it is true that things like the ECU would be well protected I wonder about things like the infotainment stuff. It's probably fine but I don't like to risk it. They aren't bulletproof and people have had then die on them.
Generally when the car is running the battery and alternator can hold the voltage pretty steady but nothing in the car compares to the draw of the starter.
Starters can pull over 200amps and pull an old battery down to 10V, even the biggest suckers like heated seats would be a tenth of that.
(Also the climate control heater uses hot coolant not electric heating coils so only the fans draw power, winter pack might add them though).
Anyway, to me it doesn't cost much to disconnect my car from the positive and the other car will still start fine off of your battery alone so why risk it, even if the risk is tiny?
PaddlerEd
Off on a tangent... a lot of newer cars have dedicated jump start points in the engine bay - I suspect they are there not only to facilitate the process, but also protect the electrickery when you read some of the voltages involved now.
Major Ledfoot
PaddlerEd;81130 wroteOff on a tangent... a lot of newer cars have dedicated jump start points in the engine bay - I suspect they are there not only to facilitate the process, but also protect the electrickery when you read some of the voltages involved now.
It may also be due to keyless entry and the like needing juice before you can even unlock the thing.
Angus242164
Jump starting points in the engine bay are generally only provided when the battery is located elsewhere, eg: in the boot or under the back seat, which is becoming common. VE-VF Commodore, P2 Volvo, all BMW's for a long time etc. have this.
No need for them when the battery is located in the engine bay and accessible.
