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I just got a classic Ford 8N tractor running again. Someone trying to be helpful flipped the battery around, connecting negative to ground. This made the engine crank backwards! So I had spark, compression, and fuel but it wouldn't start. This got me thinking:

Positive ground was very common, especially on 6V system on work machines. Today almost all vehicles are negative ground.

Is this just a matter of convention, or are there reasons to choose one over the other?

  • Not sure, so correct me if im wrong.... seems cars and trucks with positive grounds dont rust like cars and trucks with negative ground, try to find an 80 car not rotted to the door handles, and not need a tetanus shot booster before you start working on it. – user22843 Oct 3 '16 at 15:05
  • It started backwards?? As far as i know, all starter motors are series motors, so it doesn't matter which polarity you maintain, It always rotates the same direction. You could even run it on AC. This makes rebuilding your vehicle to either positive or negative ground rather easy. Are you sure it cranked backwards? – Bart Nov 8 '16 at 9:59
  • You know, I came to this site searching for some answers to understanding the "positive ground" that some cars I used to drive had (a '31 Model A, a '59 TR3, and a '59 Morris Minor)had and now I think that I am MORE confused...??? Even tho' I have a background /degree in Computer Programming & Electronics...Where did I go wrong??? Sincerely, Patty PS:Also, I don't recall any of these cars having any more or less corrosion than any of the others I've had other than the normal rusting that goes with age... – Patricia Doucet Nov 21 '16 at 16:50
  • Interesting. I have learned that the earth itself (ground) is negatively charged and that is why there is a fad going on for grounding beds and shoes etc. for optimal body health. To make a bridge from a positive pole on the auto to ground should make a discharge condition which often is called a curcuit (?). So I guess the voltage factor comes into play. Could it be the reason for having chosen 6V positive terminal in the old cars there. If they knew that the higher the voltgage, the higher the current or electric "shock" would be. As most people don't particularly react hard to a circuit of – Elias Boysen Dec 18 '17 at 16:14
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    @EliasBoysen this is why the question is protected... – agentp Dec 19 '17 at 3:20
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It is partly a matter of convention and standards, and partly because positive earth encourages galvanic corrosion* of the surrounding bodywork - whearas with negative earth is is the much-easier-to-replace electrical contacts that corrode.

*Think back to school physics lessons with anodes and cathodes...

  • There's a lot more to it than this. A lot. See, for starters, chapter 1 of Modern Electrical Equipment for Automobiles: Motor Manuals Volume Six by A. Judge published in 1970 by Chapman and Hall; chapter 1 of Automobile Electrical and Electronic Systems by T. Denton published in 2013 by Routledge; and chapter 2 of Automotive Electrical Equipment by P.L. Kohli published in 1983 by Tata McGraw-Hill Education. It's not just corrosion of the chassis, and the conventions have switched twice in the history of at least one country. – JdeBP Jan 7 '17 at 15:09
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Hooray for Wikipedia:

Originally, the voltages on the wires were positive with respect to earth. This is called negative ground, since the negative side of the battery is grounded to earth. Then engineers discovered that with positive voltage on the copper wires, copper wires age quickly, due to electrolysis. With negative voltage on the wires, in respect to earth, (called positive ground) the copper is protected from corrosion. This is referred to as cathodic protection.

Clearly, there is a trade-off. You're going to have electrically-motivated corrosion on one set of components or another. As Nick points out, corrosion of electrical components (e.g., wires and connections) is much easier to replace on a vehicle than the bodywork or frame itself. Combine this with the fact that, frankly, we're better at protecting wires than we used to be and it makes a lot of sense to run a negative ground.

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Thirty years ago I asked the same question to an automotive engineer. Thirty minutes later his answer was complete. In the beginning there were basically two - Ford and Chevy. Ford was positive ground, Chevy was negative (mostly). When the ASAE (in the 50's I think) met to establish conventions, the big guy won. However, when you consider the electro-motive series of metals, negative ground is incorrect. Japanese and British manufacturers stick with positive ground in cars they manufacture for domestic sales. The advantages of positive ground lie mainly in the sacrificial anode being the vehicle itself. A few grams of lost iron is less devastating than the loss (or corrosion) of the brass or copper connections in the vehicle. I've seen electrical connectors on the doors of 30 year old RR's look just like the day they left the factory. The switch to 12 volts was a matter of economy (and watts required) to run starters and accessories. Small wires are cheaper. Unfortunately corrosion (think electromotive series again) is squared when the voltage doubles. Negative ground vehicles require a lot of grease and isolation from moist air to remain intact. Positive ground will always be better- but the big guy (GM) won. Conventions have been set that will be next to impossible to change. (Think Metric - that will never become standard here or in the UK. ) Think BETA vs VHS. BETA is better. But guess what- we like quantity over quality. In this case, the consumer made the decision. In the case of negative ground, GM bullied that without any convincing arguments. 12 (and 24) volts were wanted by everyone. It's cheaper. No argument there. Remember, Ben Franklin got positive and negative wrong. This was done because we didn't know what electrons were yet. 50 - 50 guess. Just got it wrong. Japan and UK got it right and are keeping it that way. When our cars rot away, we just buy a new one. Let me get off this little box I'm standing on.

  • This is fascinating! (And welcome to mechanics.SE, btw!) Do you have any sources you could link to for further reading? – anonymous2 Jun 1 '17 at 11:40
  • I find that very hard to believe Japanese and British home market cars are positive ground today. They would need to custom spec every bit of the electronics. Up to the early 70's maybe that was the case. – agentp Jun 1 '17 at 19:01
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I restore old tractors that were originally positive ground 6 volt, i think that is a better system than 12 v neg ground for these reasons. I lived where 40 below 0 was common and these tractors never failed to start unless there was a problem and since it has been mentioned about corrosion the units i see corroded the worst are the ones converted to neg ground and usually 12 v although i doubt that makes any diff. I think people have been sold a bill of goods on changing ground to neg and i would be interested to know who and why it came about, as far as computer industry they would naturally follow the trend for neg ground unless they found it was problematic. One reason i think tractor people changed is that 6 v cranks the engine over a lot slower then 12 v but 6v will crank for 30 minutes or more where 12v is on its face in a few minutes. been fun ron

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The positive ground extends the life of spark plugs. The electrons flow from the center to the edge of the plug. Then most of the damaging heat of the spark is put into the spark plug body and not the center electrode which has limited heat dissipation. Battery polarity has absolutely nothing to do with corrosion , unless you are underwater, which is probably going to cause bigger problems for a car than corrosion.

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There is also a safety factor.

If you have the positive terminal of your battery connected to the frame of the car, then it is very easy to make new circuits, through your body for instance.

There are a lot more negative ground sources out in the open then positive ones.

  • I'm not convinced. Even if you touch 2 vehicles with different grounds, you won't complete a circuit, as the non-ground side of the battery is isolated. Right? – Jay Bazuzi Sep 16 '11 at 0:02
  • Could be wrong, not an electrician, but connecting positive of the battery to any negative ground sure seems like a circuit to me. With a vehicle it has to wrap back to the battery because the whole vehicle is isolated from another ground by the tires. We are talking about the flow of electrons, they want to move from the positive to the negative to create balance, they don't care if that the negative is on the other side of the battery, the ground beneath your feet or anywhere else. – ManiacZX Sep 16 '11 at 7:14
  • I just thought of what might be a great example of what I'm talking about. A Lightning strike! When you get a strong enough positive charge in the sky it jumps to an available ground, in this case the earth. This isn't a cyclical circuit where it leaves the cloud to somewhere else and then goes back to the cloud. It leaves the cloud and goes into the earth, from a positive to a negative. – ManiacZX Sep 16 '11 at 7:23
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    Static electricity such as lightning, at tens of thousands of volts, is likely not a relevant example for what happens in dynamic electricity at the low tens of volts. Though I can't explain why fully. I imagine it has to do with the potential, the positive side of a battery doesn't have high potential relative to earth ground, it has high potential relative to the negative terminal of that same battery. An example of this is that you can't take two batteries and ONLY connect, say, a light bulb to the positive terminal of one and the negative of the other and have it work. – Sean Reifschneider Sep 23 '11 at 4:42
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    I think you've mistaken the difference between a negative terminal and a ground point. Bear in mind cars are not grounded because of the insulating effect of the rubber tyres. – Steve Matthews Oct 4 '16 at 8:53

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