Battery polarity reversed???

[William]

Hey all! Just got the rebuilt carbs put on, and the muffler hung, and Dad bought a new battery. We put it in, and connected the red cable to the positive and the black to the neg, and got major sparks, enough to start melting the battery posts. The black cable clearly attaches to the body as a ground, the red one is much longer and seems to disappear up the transmission tunnel.

Here's the thing: the car is a 1971 MGB. Car has the place for two batteries, but just the one (on the passenger's side. We were told the car had been converted to the single battery by the P.O.
Does this mean the car's polarity is reversed, and the cables have to go on the opposite posts??
Any help would be appreciated, and bear in mind I am easily confused by electrical things. (Boy, the one thing we didn't pay any attention to or take notes on was the battery. After all, how hard is it to replace the battery?)

-William

 

[piman]

Hello William,
the polarity is not really the issue. Sparks mean current, and such damage is a lot of current. Something is shorting the battery terminals when you try and connect. You need to try and find the cause of that. It's difficult to advise where to look.

Alec

 

[William]

The car definitely has an alternator. It's weird, when we parked the car for repairs, there weren't any electrical gremlins. The old battery even had a decent charge to it, but we decided to replace it because it was simply getting old. May have to send the car out for final repairs after all.

Many thanks!
-Wm.

 

[steve99]

William,

There used to be a guy in Geneva that owned and ran a shop called Jet Automatic Transmissions. He was on the main drag on top of the hill on the East side of the river. If hes still there, (his name is Dan) he would be of great help. Its been over ten years though so he may not be there. Just a suggestion.

Steve

 

[Dave Russell]

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by piman:
Hello William,
the polarity is not really the issue. Sparks mean current, and such damage is a lot of current. Something is shorting the battery terminals when you try and connect.
Alec

<hr></blockquote>
I think that polarity may be an issue. I believe that the car has an alternator. If a battery is connected in reverse polarity from the alternator design, the diodes will conduct & present a short to the battery. There would be three paralleled pairs of series diodes conducting.

Since the diodes can handle quite a bit of current before they blow. "Blow" could be either open circuited or short circuited. The "short" could be caused by reverse polarity on the alternator. If this is the case, the diodes may or may not be damaged. If by chance, the car does not have an alternator, you will obviously want to disregard the above info.

William,
You do need to carefully check the electrical system for polarity, grounds, short circuits, & check the alternator diodes. With your admittedly limited knowledge of things electrical, it would be hard to lead you through all of the checks by mail.
D

[ 03-27-2004: Message edited by: Dave Russell ]</p>

 

[Guest]

Originally posted by William:
The black cable clearly attaches to the body as a ground, the red one is much longer and seems to disappear up the transmission tunnel.

OK, then if the black cable is clearly connected to ground and you had the battery connected the way you say, then polarity couldn't be an issue. There has to be a direct short somewhere. If the diodes in the alternator are blown, then the battery had to have been connected backwards at some point if not the present situation , then some proir time.

I would look along the red wire to see if it is resting on something sharp, or pinched by a bracket. The red wire goes to a terminal on the starter before it goes elswhere so disconnect it there and see if the short dissapears. If not, then it is along the red wire somewhere back towards the battery.

Mine was converted by the previous owner to a single battery in the trunk and used a red wire for ground and a black for the hot. Dont think that wasn't fun the first time I replaced the battery. I have since moved the battery back in the tray where it belongs
[ 03-28-2004: Message edited by: mrbassman ]

[ 03-28-2004: Message edited by: mrbassman ]</p>

 

[Daniel H. Wilson]

Two observations on this situation.
1.It only takes a very few seconds to fry the alternator if the battery is connected in reverse when the ignition key is turned "on"-personal experience.
2. The positive battery should not run into the transmission/drive shaft tunnel. It should be clamped in the grooved rubber pads to the floor pan along side of the brake and fuel lines.

 

[Daniel H. Wilson]

Two observations on this situation.
1.It only takes a very few seconds to fry the alternator if the battery is connected in reverse when the ignition key is turned "on"-personal experience.
2. The positive battery should not run into the transmission/drive shaft tunnel. It should be clamped in the grooved rubber pads to the floor pan along side of the brake and fuel lines.

 

[William]

Thanks for the suggestions! Steve99-I don't think that Jet is still in bidness. Our dog gets groomed at a place up the street and I don't remember seeing it still open.
Mrbassman-we didn't connect the battery completely- just saw the nasty sparks and pulled out quick. But the zap was just enough to leave a bit of a mark on the battery posts. It's disconnected now and the caps are back on the posts of the new battery, and the leads are well away from them!
Daniel-to be honest, I don't quite know where the red cable goes! I just saw it disappear out of the battery compartment in the general direction of the trans tunnel!
I have tomorrow morning off, so I may have a better peek at the whole mess-battery removed of course! On the plus side, the carb job and new exhaust system and new plugs went in just fine!
Many thanks again!!!
-William

 

[Bugeye58]

William, just a gentle word of caution. It would be a good idea to disconnect the alternator before figuring out the rest of the problem. If you haven't already fried it, it would keep it from happening until you get it all sorted out. Or, go ahead and take it off now, and have it tested, and do the rest of the troubleshooting with it removed.
Jeff

 

[piman]

Hello all,
the reason I dismissed the reverse polarity theory is that it was correctly connected, i.e. negative to earth. To me it seemed that there is a load that shouldn't be there. There is a difference between mild sparking, which would occur if say the ignition was switched on, and melting of a battery terminal.

Alec

 

[piman]

Hello William,

I just re-read the original post. Did you by any chance drill the body to fix support brackets for the new exhaust system, if so could you have damaged the main battery cable?

Alec