About 4 months ago, in a drive way far far away, there once was a Hyundai Elantra. Now this Elantra was a very good running car and has been since it found it’s new home in 2007. But something happened that made it’s dome light stay on that ran the battery down when said owner went out to drive it one day. In a hurry and not using his brain, the owner (me) ran a small charger out to the car and hooked it up and left it charging all day.
So before the sun set, I went to go check to see how my little car fared.
But to my surprise, it was still not alive, so I left it still charging some more.
The next day I woke and sprang out of bed, and ran out to my four wheeled friend.
The meter said good, or so that I thought, I jumped in to give it a start.
It turned itself round and purred like a kitten.
Then I saw lights, it was Christmas inside, as my dashboard lit up like a tree.
The battery, brake, and air bag were lit.
And then the car sputtered and shook to a halt, and I thought that this can’t be good, not at all.
The battery was dead, I charged it again, it did the same thing.
I puzzled and wondered as to what it could be, so I took out the alternator to have it tested for free.
It tested out bad so I bought a used kind, from a salvage yard I had in mind.
After charging once more I put the part in, it ran for awhile and then it did end.
With the problem still there I misunderstood, so I went to the front and lifted the hood.
I stared at the engine just scratching my head, and then I did see that the positive was NOT red.
Someone had put a black hood on the post, and the negative had red…I felt like a dope.
I thought I was right when I hooked up the cables, but I guess I was wrong for not reading the label.
With the battery now fried, and the car brain might have died because the second alternator is now bad.
Could it be true that I ruined it too all from a faltering dome light.
So my question to you and to all those who read, is there a fix for my knuckle-headed deed?


First thing is to put in a new battery, and new alternator if that is actually dead. Then see what happens. The processor should be protected against reverse polarity, but perhaps not. keep your fingers crossed.

There are many other electric/electronic items in the car that could also be fried. There is more than one computer, as well as your radio/CD player/whatever, that could also be fried.


When you connected the charger to the battery with the leads reversed from the charger, you reversed the polarity of the battery–the negative post of the battery became the positive and the positive became the negative. On a modern car, I am not sure what damage may have been done. Some years ago,. I bought a 1948 Dodge where the battery had been installed backward. The Dodges of those years had the positive terminal as ground. I noticed the ammeter guage read backward. When I turned on the headlights with the engine off, the ammeter moved to charge. When I started the engine, the ammeter read discharge. The car had been driven that way for years. The fix was was easy–I left the headlights on overnight to completely discharge the battery. I then recharged the battery in the proper way. After reconnecting the battery the proper way, I followed the instructions I found to repolarize the generator. That was simply a matter of momentarily bridging two of the terminals on the voltage regulator(after 39 years, I don’t remember which two). I started the car and all was fine. In your case, you have an alternator rather than a generator. Repolarization isn’t possible as you probably burned out a diode in the alternator. Whether you can salvage the battery as I was able to do. Is iffy-----I would just buy a new battery. I think I would replace the battery and alternator and go from there.


Triedaq: Polarizing a replacement generator. The young mechanics today have no idea what we are talking about and I have no idea what computerized everything they are talking about. I can diagnose and repair vehicles 1976 and older. I have owned and maintained/repaired 4 vintage British sports cars. I am familiar with but never understood the “why” of positive ground.


Agree with Bill, is what I would start with, and a lesson to remember, don’t look at the color of the cap but the indicator on the battery. Sage advice, but I might have made the same mistake. Start simple, then work your way up the repair and $ chain.


There was no why for positive ground, there was just no agreement that one was better than the other. I blew up my first CB radio because a lot of the older trucks were positive ground. The next CB I fused both the neg. and pos. leads. If you worked for most freight lines you changed tractors every trip, sometimes every leg of every trip. Most drivers had a box with CB radios, AM/FM radios and speakers in it and an antenna mounted on vise grips to clamp on the mirror brackets.


There was no why for positive ground, there was just no agreement that one was better than the other. I blew up my first CB radio because a lot of the older trucks were positive ground. The next CB I fused both the neg. and pos. leads. If you worked for most freight lines you changed tractors every trip, sometimes every leg of every trip. Most drivers had a box with CB radios, AM/FM radios and speakers in it and an antenna mounted on vise grips to clamp on the mirror brackets.


HA, I once, many years ago, jumped my MG1100 backwards (it was + ground). This polarized the generator backwards and everything worked, including the radio. Car ran fine. However the generator charged the battery backwards, ie, discharged it. So it didn’t last long before the battery was dead.


@sgtrock21 Why positive ground? Why not positive ground? It really makes no difference in how a car’s electrical system operates whether it was set up as positive ground or negative ground. It seems to me that there was a theory that the current really flowed from what we consider the negative terminal to the positive terminal, so maybe that is the reason. Many 6 volt cars were positive ground. GM cars were the exception. My 6 volt 1947 Pontiac was negative ground. By 1956, all U.S. makes adopted the 12 volt negative ground standard.


Maybe you can figure it out from this

Very close Bob
You do have a few things mixed up. Yes electrons move from negative to positive. This is not a theory it is a known fact. It is also known that opposites attract. To get a better understanding of why a negative ground system was chosen we need to look at the molecular level.

In an atom there are three parts:neutron,proton, and electron. The electron orbits the nucleus(protons and neutrons). The property or willingness of an atom to give their electrons ultimately determines the materials conductivity(way more to it than that; this is a simple description). By natural law materials are or tend to stabilize in an equal state(once again there are exceptions and I’m just generalizing). We know that an atom has an equal number of protons and electrons. Therefore an atom doesn’t have a charge. Another important property we need to know about atoms is that they can gain or lose electrons and stay the same chemical makeup(ie copper is copper even when it is an ion) When an atom gives or gains an electron it becomes an ion and can be considered to have a charge.(This is important). The movement of these electrons is what creates a charge or difference/potential. The movement of these electrons is electricity! If the electrons flow in a uniform direction we have a current. That is a lay description of what electricity is.

Oops, I forgot to mention that an atom doesn’t give up protons. When you figure out how to do this let me know. I’ve got a lot of lead that needs changed into gold

Anyways, back to electricity. Since we have addressed the molecular level lets get some vernacular out of the way. Voltage is the potential of electron flow. When you have a battery sitting on a shelf not in a circuit it is said to have 12 volts. Think of this statement as there is a difference of electrons equal to 12 volts. Or there is a potential of electrons to flow from the negative side to the positive side(greater the potential the greater the voltage). This is important when you try to justify a 1 foot section of wiring during a voltage drop test. When we perform a voltage drop we measure a voltage. If there is a resistance we will see a voltage(for example maybe 1 or 2 volts). But wait! How can a piece of wire create a voltage?!! Only batteries or generators and stuff can do that right? Wrong! Voltage is not something we create it is a measurement of potential Now, the easiest property of electricity to define is current(in lay terms). Current is the amount of electrons flowing(quantity).

Also, it is important to understand how a battery works. The chemical reaction of the lead plates(calcium, antimony, and lead) and sulfuric acid creates a solution of ions. These ions donate their electrons and that is where the original electricity starts. Without the chemical process there would be no potential flow of electrons(voltage). It is not just electrons flowing from a negative plate to a positive plate. It is the gaining and losing of electrons of the plate. When a battery discharges the chemical process sulfates the plates(plates lose electrons). When a battery is charged the chemical process is the reverse and oxides the plates(electrons are gained).

Ok who is lost?
Alright so you are asking yourself what does this have to do with negative and positive ground systems. Quite simply it is a matter of effort. Every conductor in that vehicle that connects to ground essential becomes eligible to give up electrons.There is a potential huge imbalance of negative ions. Another way to comprehend what I am saying is to think of a negative ground system as allowing a chassis to be an amplifier. This is also the bane of negative ground systems! In a positive system the positive side will receive the electrons. Therefore only the negative feed wires will be donating the electrons on the current side of the load. This requires larger wires when a large amperage is required(this is true in ground systems but Im referring more to the momentary jumps like ignition coils and injectors). Therefore a positive system in essence will require better quality components. So, from a manufacturers stand point it is a lot cheaper to go negative. Also, standardizing the system played an important role in the decision. When computers came along it pretty much sealed the deal for negative ground systems in autos.

Ultimately it comes down to the fact that: negative ground immitates natural electrical flow, the industry needed a standard, and it can be cheaper for manufacturers. There is way more theory behind it, but I’m tired and want to go sleep.
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Old 07-30-2010, 11:15 PM


^^^ “Ok who is lost?”

Basically, everything after this sentence is nonsense.


Triedaq: Correct + or - ground will work if components are compatible. All 3 of my MGs 1960, 1962, and 1966 were 12 volt positive ground with two 6 volt batteries in series. Fortunately I never had to test it but how would a jump start have been accomplished?


Cars could be negative or positive ground, no theoretical reason to go either way. However, digital integrated circuits (ICs) almost exclusively require a negative ground, ie, a positive supply. Now there is electronics that can convert -12 to +12, but that is added expense and a reliability issue.

As to why ICs are negative ground, that involves semiconductor theory that we probably should not get into here. But remember that electrons and holes are totally different entities. (a hole is a missing electron in a crystal structure)

Barkydog: very good summary, but I lose you in the last paragraphs.


@sgtrock21 To jump start with the two 6 volt batteries in series, leave the interconnection between the batteries alone. Connect the black jumper cable to the negative post of the 12 volt battery supplying the power to the negative terminal in the MG that has the cable going to the starter. Connect the red cable to the positive post on the battery supplying the power to the engine block on the MG. When I was growing up we had a 6 volt positive ground Dodge and a 12 volt negative ground Buick,. We could jump start the Dodge from the Buick using this method. The 12 volts would really spin the starter. We made certain everything was turned off on the Dodge before we did this.


Triedaq: Thank you for confirming what I was thinking of as the correct procedure.


I would suggest checking all fusible links. These should be in the underhood fuse box. Quite often in cases of power surges or incorrect jumping the link will blow (as it should) instead of frying the alternator, wiring, and so on.

Remain optimistic and think of the glass as half full.


My 1960 Borgward Isabella wagon had a 6V + ground when I got it, I switched it to a neg ground just so I could jump start it from another car. The car didn’t have a radio so it didn’t care how the battery was grounded.

BTW semi conductors do not care whether the ground is positive or negative.


@keith As long as hou don’t allow the cars to touch, there would have been no need to change from positive to negative ground–just go positive to positive and negative to negative.


I only had one cable at the time.


How can you jumper a car with only one cable? takes two, one for each polarity.