Positive Ground Cars
Since the beginning of automotive history, both negative and positive ground polarity have been used by car manufacturers. The fact is electricity doesn't care how it gets from point A to point B. It can flow either positive to negative or negative to positive. Some engineers chose one way, some chose the other.
Among the many positive ground cars is the most produced car ever built, the Volkswagen Beetle, which didn't switch to negative ground until the mid-sixties. All British vehicles imported after WWII were positive ground, many stayed that way until the early seventies. The Ford Motor Company used negative ground polarity on the Model-T, then went to positive ground with the Model A, and back to negative ground in 1956.
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With the advent of the electronic age, the automotive field embraced solid-state voltage regulators, electronic ignition, transistor radios, and other upgrades. Parts such as transistors and diodes were designed with negative ground, which became standardized. Positive-ground vehicles gradually became a thing of the past.
On positive ground cars, the cable that's attached to the positive post of the battery attaches to the engine block or chassis. The cable that attaches to the negative post of the battery attaches to the starter solenoid. The connections are opposite on negative ground cars. When charging positive ground cars, put the red (positive) cable on the ground and the black (negative) on the battery or starter lug.
As stated, polarity makes no difference at all to a car. Red is still positive, and black is still negative. Several minor differences include the coil terminals, which on positive ground cars are reversed (positive and negative ground coils are wound in different directions). The negative wire attaches to the power supply and the positive wire goes to the distributor/points. Light bulbs will work in either polarity, as will starters, relays, and heater blower motors. The radio and ammeter gauge will not; they are polarity specific.
Diagnosing Positive Ground Electrical Systems
The simplest way to diagnose a positive-ground system is to reverse any diagnostic procedure normally used on a negative-ground system. For example, instead of starting on the positive side of the battery and following the current that way, start at the negative side of the battery and follow the current the other way.
Jump-Starting Positive Ground Cars
If you want to jump start a 6 volt positive ground car, the best way is to use another 6 volt battery of either polarity. Positive goes to positive and negative goes to negative. This makes a parallel circuit. But make sure neither car touches each other!
You can use a 12 volt battery to jump a 6-volt system with two precautions. Do not run the vehicle with the 12 volts, and have someone ready to yank the cables as soon it starts.