1966 MGB, converted to negative ground (earth). I tried connecting the positive of the Jeep’s battery to the positive of one of the 6v batteries, and the negative to the other 6v battery’s negative post. That got me cables that were too hot to touch, and didn’t do anything useful. ISTR there’s a way of connecting cables in the engine bay, but my owner’s manual, Haynes manual, and Haynes restoration manual don’t mention it. (In actuality, none of them say anything at all about boost-starting.) Since I’m pretty inept when it comes to engines, I can use some detailed, step-by-step instructions.
There’s no simple wiring method that is going to get you around the 6 to 12 volt difference. If you put another 6 volt battery in series with the MGB’s battery then you can use a 12 volt system to charge them. Just about anything else is going to just make smoke.
See if you can find a portable 6 volt jump starter. Keep it charged and keep it in the trunk.
If it is currently in a garage and just needs a jump, get a 6 volt charger and charge the existing battery.
ETA: The MGB is still 6 volts even though it has been changed to negative ground, isn’t it? In other words, the 2 existing batteries are in parallel? If you can disconnect them and put them in series than a 12 volt system will charge them.
There’s this paragraph-challenged page that probably doesn’t discuss negative-ground conversion. But if the car has any after-market electrics installed to replace the points, you might give it a read.
It goes on with more scarey stuff. (I searched the page for jump start.)
Forget all the 6V stuff. The 2 x 6V batteries are connected to provide 12V and they are regularly replaced with a small single 12V battery. So the whole car is, and always was, 12V. It’s English. Go figure.
Why are you convinced you have a Pos/Neg “Conversion?” Is the larger connector from the first battery bolted to the frame…or is the smaller one? Larger is Pos ground and smaller is Neg. Usually the only thing you have to do to “convert” is mess with the radio and tach.
If you match your jumper cables or charger to the 2 batteries by hooking positive to big and negative to small and TREATING THEM AS A SINGLE 12V BATTERY, you’ll be fine.
If you’re worried about the alleged aftermarket ignition, disconnect it, charge the batteries, disconnect the charger, rehook the ignition and fire it up. Damage to the ignition can only occur if you feed it more than 15-20V from a charger or jumper.
Push starting it won’t work if it still has an electric fuel pump.
And after rereading your original question, once you have determined which ground you have, yes you can do the jumper cable connection in the engine compartment if you want. Loose/dirty cables at the batteries is just one of the downsides of the double battery setup.
In the engine compartment, one cable goes to the solenoid that leads to the starter or, if you have a newer starter, on the big post at the starter. The other goes to the engine or frame. If the MG batteries are DEAD DEAD, it’s possible to hook up wrong without sparks, but if the MG batteries have something in them, you’ll find out quick enough if you’ve got the cables backwards.You might want to disconnect the ignition for this part. Once you’ve got the jumper cables from another running car connected and the MG engine is spinning well on the key (with ignition disconnected) and the fuel pump is working and there’s gas at the carbs and you’re showing a 12V or better charge on the voltmeter/ammeter with the key switched on, you can turn off the jumper feeder car (because its alternator is feeding maybe 16V into the MG), rehook your ignition and start the car.
If the batteries won’t take a charge overnight, throw them away and buy a single 12V that will go in one of the holes and, if you have to, extend or remount the ground cable. It makes no difference where on the car it is bolted, as long as it has a CLEAN, BARE METAL connection, so you can either buy a longer cable, extend the original one or find a new place to bolt it.
Time to start cooking dinner, so I’ll try the advice tomorrow if it isn’t raining.
[ul][li]It’s a 12v system, only they use two 6v batteries.[/li][li]It was originally ‘positive earth’. I had it converted to negative as a concession to modernity.[/ul][/li] Meanwhile, back in India…
The dressing is made for tomorrow’s shaved beef salad. Now I can start the coals, season the rib-eyes, and start the escargots.
Apparently, you have two 6V batteries in series. One of the batteries is connected directly to the other, positive to negative, to get the 12V. Look and see which terminal on one battery is connected to the other battery:
car wiring —(+)battery-----(+)battery---------car frame: connect jumper cables to the positive of the first battery and to the negative terminal of the second battery (or the car frame).
Otherwise, you’re putting 12 volts across a six volt battery.
andyleonard’s idea to jump the car by hooking the cable to the starter is an excellent one, and I’d give that top priority. Once the engine is running the generator can charge the batteries (assuming they’re good).
An MGB doesn’t have many electrical accessories to keep running, and you probably only use it when the weather is warm so cold cranking amps are not a concern. I’ve run race cars on $20 lawnmower batteries and you can probably do the same. Motorcycle batteries are also compact, but often cost more. I use a U1R-7 that has 275 cranking amps at 32F. More than enough, and it’s close in size to one of your 6V batteries. Walmart has them cheap.
OP said he connected the cables to one battery’s positive and the other’s negative posts. If those are the posts that connect the two batteries together, the cables have just made a short circuit across the battery being used to jump the MG.
Are you sure about that? I don’t follow the discussion about 6-12 V and the negative earth stuff, but the general rule that I’ve always seen is that you connect positive to positive, negative post to the autoframe of the car with the dead battery. Connecting both set of posts isn’t advisable - at least, that’s what I’ve always been told.
Electrically it’s identical. Connecting to the frame rather than to the negative battery terminal is a precaution against a spark at the moment of connection or disconnection meeting with hydrogen gas being released from the battery, followed by “BOOM.” It’s one of those “yeah, ½ of 1% of the time it might matter” things, but it’s easier to instruct everyone to take the precaution than to teach them when it matters vs. when it doesn’t.
The batteries are wired as shown in your link. The car is restored and is very tidy, and I’m sure one of the cables isn’t shorting to ground. I’m sure I triple-checked the cables before connecting them. The conversion was done by a British car mechanic (from Britain) who has a couple/few decades of experience. That’s why I was so surprised at the outcome.
I used to have three MGBs at one time (in the '80s), a '66 and two '77s. ISTR that at least one had a button in the engine bay. Hook the jumper cables somehow, and press the button to start. That was a long time ago though, and it could have been a different car. Anyway, I connect Negative to the frame. When I connect Positive to the starter solenoid (I assume it’s the post with multiple wires connected to it? I haven’t looked yet, but I’ve seen pictures.) it will crank since I’m connecting electricity directly instead of going through the ignition switch?
Yes, I only drive it when it’s nice out. The last couple of years nice weather and my days off did not often coincide. I love it up here, but I’m running into weather issues that I never had in L.A.! Another issue is that the car will run fine for a few miles, and then quit and can’t be started for hours. That has discouraged me from driving to Seattle. Since everything is new, I assume it’s because of old gas. Poor weather and lack of confidence (I’m a much better cook than I am a mechanic!), along with other duties and responsibilities have kept me from attempting to drain the tank. (And what do I do with the old gas?)
Here’s the battery compartment on my car. The Lucas-style batteries are small and square. I don’t know the cold cranking amperage. I’ll have to measure the batteries and see if I can find a 12v of the same dimensions at Walmart, a you suggest.