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MG MGB Technical - jump starting positive earth

Hi folks not being up on electrics, what would be the sequence of connections for me to jump start my gt from another car, bearing in mind that the gt is positive earth, which cable go were and in what sequence, thanks guys.

Cheers Jack.
Jack New Forest

Try here for all you should need:

http://www.mgb-stuff.org.uk/electricstext2.htm#jump

Bernie
B Anderson

It's always positive to positive and negative to negative no matter what.

Don't let the cars touch, particularly with different polarities.

If a chrome bumper with twin 6v batteries make sure you know which are the 12v and earth terminals, and which are the link cable terminals, and on no account connect anything to the link terminals.

Make sure the clips at the MGB don't move and short to the edge of the holes on the back shelf.

And never, ever, clip the ends of the jump leads together.
PaulH Solihull

Thanks Bernie good info on that page, thanks Paul lots of good tips,
I have actually taken out my two 6V and replace with a 12V under the bonnet looks OK, it will still be + earth I have not changed the polarity, so will be puting a new cable from the Starter switch to the Negative on the battery and connecting the positive to the car body.
I Copied Joern from Germany's idea, looks good like it should have always been there, I have left in the old cable to the back boxes, just in case I or someone else want to go back to 6V can't see why.

Cheers Jack.
Jack New Forest

A suggestion to all MG owners, regardless of the polarity of the car - Get some stickers that say "This vehicle positive (negative ground) and place them in a conspicuous locations under the bonnet and near the batteries. This will help prevent someone from hooking things up backward. Secondly, if you have call for rescue service, be sure that the mechanic that answers the call hooks the jumpers up properly if they are giving you a jump start. A member of our local 'T' register recently had a service call on the road for his positive ground TD and the mechanic insisted that the negative lead "ALWAYS" goes to ground, even though our member told him that the car was positive ground. The result was a ruined battery after the jump start. Cheers - Dave
David DuBois

Sadly I'd imagine that is is only people of our persuasion who would know what that meant, and what to do about it. That's no reason not to add the stickers of course, quite the opposite, if a pro still does the wrong thing he can't say he wasn't warned and put the ball back in your court.
PaulH Solihull

All too true. Almost no mechanics, let alone owners, know what the terms negative and positive earth mean in this day and age. I had a customer who left his headlights on all day and ran down the battery in his '72 GT. Whoever had converted it from two six volt batteries to a single twelve volt battery had installed a red cable as the ground wire. The owner jump started the car and just assumed that the red cable was the positive lead. He fried his alternator diodes as well as the wiring harness from the starter to the alternator. When I converted my '67 over to negative earth, I installed one of the above mentioned stickers under the hood. Every time I open the hood and look at it I wonder: does anybody know what this means? RAY
rjm RAY

"installed a red cable as the ground wire"

On a negative earth car that's even worse than the factory installing black wires for both. When I got my roadster I had a length of red battery cable from somewhere, I cut the insulation off that in a spiral and wrapped it round the +ve cable. That was on the right-hand battery, still left two black cables on the left-hand though. You'da thought they could make a link cable black one end and red the other ...
PaulH Solihull

I recommend stickers for whatever polarity you have. My brother fried my petronics and the alternator in my 67 because he knew that the 67 was positive ground. He didn't realize it had been converted.

The nice moss sticker right when you open the hood is mostly to remind me so I don't do something stupid.

One recommendation on jumping from another car of a different polarity. Be very careful with jumping from a newer car. The computer system in these do not necessarily turn off when the key is off and frying a set of electronic points or even an alternator is nothing compared to frying the computer in a new car.
Bruce Cunha

I never jump start a car, with a dead battery, from another car. I have a jump start battery pack, that I bought 13 years ago for the motorcycle repair shop that I worked in, because we never had a spare battery around that held a charge, to test or start a bike with. I always use this unit when jump starting a car, primarily because it's so easy to use, but also to protect today's fragile electronics. RAY
rjm RAY

"Be very careful with jumping from a newer car. The computer system in these do not necessarily turn off when the key is off "

Really you should be jumping with the donor engine running, indeed having been connected *without* trying to start the dead car for some minutes to get some charge into the battery before trying.

It is beginning to be said not to use a modern car as a donor anyway, because of the potential for serious damage, i.e. use a jump-pack instead. In any event, advice is to turn on headlights, heated rear screen and heater fan full once the dead car has started but before disconnecting the jump leads or pack, to limit the chances of spikes.

You can get 'lighter socket to lighter socket' 'jump leads' but I'd imagine that would take ages to get anything useful into the battery of a car that won't start. I couldn't even get my son's BMW Coupe to turn over with jump leads off his Toyota, engine running for half an hour, it needed a jump pack with heavier duty leads than I had.
PaulH Solihull

Why not just switch to Negative Ground. It is very simple avoids confusion in the future and will allow the use of a modern stereo.

Frank Grimaldi
Frank Grimaldi

Tried to start my old +ve earth 67 Mk1 many years ago with my parents new Triumph 2000 (-ve earth)
Wondered why there was a spark when I clipped the lead on th the battery and then it was too hot to unclamp and i didnt have gloves on , etc, etc

Luckily these 2 strong British built machines survived with no noticable long term damage, but the same can't be said for the jumper leads!

I learnt a good lesson that day - what's the electrical equivalent of "measure twice - cut once"??

John Minchin

"what's the electrical equivalent of "measure twice - cut once"??"

Tap the last clip on briefly, if you get a big fat spark it is reversed or shorted. But you will always get a small spark connecting a good battery to a flat one. Incidentally that last connection shouldn't be to a battery post but to a substantial part of the engine so your face isn't over the battery, for the same reason Rolls Royce provided starting handles and men have nipples - 'just in case'.

Ironically it *is* relatively safe for the car as the two batteries in series would tend to drag each others voltages down, you wouldn't normally get a large reverse voltage to blow things like the alternator diodes, unless the dead battery was actually open-circuit (which can happen).

You are lucky neither battery exploded. Someone who should know better gave the spectacularly inappropriate advice to 'clip the ends of the two leads together' when connecting to the first battery in the MG Owners magazine a few months ago. OK, he had a drawing showing the one clipped to the insulation of the other (which is bad enough), but of course some poor soul didn't have that to hand when she went to jump-start her car and only remembered the words - one melted battery as well as her jump leads.
PaulH Solihull

I've witnessed a battery exploding. Not a pleasant sight. Nipples on men? That is redundant. RAY
rjm RAY

Whenever I connect anything with polarity to a car battery, be it another car for jump starting or a battery charger or whatever, I always look at the marking on the battery.

It's the only thing you can trust.
Mike Standring

Some 6v batteries have + and - markings, some have red and blue collars on the posts, some have neither. Of no use to a blind pal when he was reassembling his MGB, so I described how one post was slightly wider than the other. He got it right.

You can also reverse-charge batteries, so I'm told, making markings counter-productive. Hence the tap on the last post (to avoid a Last Post?) to judge the size of the spark.
PaulH Solihull

This thread was discussed between 25/01/2012 and 11/02/2012

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