Working With a Positive Ground

December 1, 2009 | By John B. Hege

Some Tips to Help You Spot a Positive Ground System. And, If You’re Converting, Here’s What to Do.

THIS PAST WINTER I borrowed an old Ford 9N tractor from my father-in law to grade my driveway. Normally he keeps the old machine in his drive-in basement, so it never saw a night below about 50 degrees.

In my barn in February it would not be quite so pampered, so when the weatherman announced that temperatures were going to be around 15 degrees at night, I went out to make sure the tractor would be prepared.

After assuring myself of the potency of the anti-freeze solution, I disconnected the six-volt battery and carried it into the house for overnight storage.

A few days later, when things warmed up enough that the ground was no longer frozen, I went to reinstall the battery and get to work on the driveway.

Once the battery was in the box, I looked at the cables and wondered which was the ground.

As a retired auto mechanic, I knew that I couldn’t trust cable colors or even terminal sizing, so I called my father-in law to ask if the tractor had a positive or negative ground system.

He told me that he couldn’t say for sure. I was on my own.

Many Factors at Work Here

No doubt many old car enthusiasts have found themselves in this spot. I could have saved myself a bit of trouble by observing the cable connections before I took the battery out, something you should always do when working on older vehicles, but even then, unless you have a running vehicle and a functioning charging system, it can be hard to tell.

Of course, there is the old “red is positive” school of thought, and battery manufacturers now make their positive terminals larger than the negative terminals, so if everything is as it should be, there is no difficulty.

But by the time a car has been through decades of repairs and batteries, anything could have happened and probably has. Cables may have been changed, universal terminals installed that don’t even approximate the correct sizes, or the system may have already been converted in the past to a negative ground.

So where to begin?

Differing Opinions Regarding Power Flow

The grounding terminal is the terminal of the battery that is connected to the metal chassis of the vehicle. It essentially causes the entire vehicle chassis to behave as one terminal of the battery, making it necessary to run only one wire to the power consumers like the lights and motors.

Early electrical engineers held the belief that electricity flowed from positive to negative so it made sense to have the power flowing “downhill” to the ground or “earth” as the British called it.

This may have been the thinking that led Henry Ford to wire his first electric start model T’s with a negative ground. But in the first half of the twentieth century, everything automotive was still experimental and schools of thought were changing with regularity.

Many domestic car companies had positive ground systems right up into the ’50s.

So when Ford introduced the Model A, it came with a positive ground system, bringing them into line with most, but not all of the automotive industry of the time. Why the manufacturers chose positive ground over negative is not crystal clear, but there is a school of thought that says that because the positive side of an electrical circuit tends more toward corrosion, grounding the chassis to the positive would help preserve the wiring and electrical connections.

Checking For a “Positive” Alternator

Many domestic car companies had positive ground systems right up into the ’50s. Ford changed back to a negative ground shortly before they went to 12- volt systems in ’56. Chrysler products also used positive ground on their six-volt systems. With General Motors it can quickly become confusing.

For instance, while a 1950 Chevrolet truck may have a six-volt system with a negative ground, a similar GMC truck could have a positive ground system.The British continued to “earth” their systems to the positive pole until they switched their charging systems to alternators in the later ’60s.

Even experts will disagree on the changeover dates, and naturally exceptions abound, so if a car was built in or around 1956, it should be considered suspect and should be carefully checked out before hooking up the battery for the first time.

An American-made car with a 12-volt system is most likely a negative ground, unless it was converted from six-volt, in which case it’s anybody’s guess.

If it is a truck or tractor, it could go either way as well.

Does it have an alternator instead of a generator? If so, it’s going to be negative ground. There are some positive-ground alternators out there among the imports and tractors, but not many.

If there is any doubt, it’s easy to check the polarity of an alternator with a multimeter. Connect the probes to the large terminal and the body of the alternator. Measure the resistance. Then reverse the probes and measure the resistance again.

A negative-ground alternator will show high resistance when the positive probe is on the terminal and show low when they are reversed. A positive ground alternator, of course, will show the opposite.

That’s What Friends—and Manuals—Are For

If, as in my case with the tractor, the polarity of the system is unknown, you should look for clues on the vehicle. Some places to check are components that have a polarity marking on them such as an ignition coil or an ammeter.

If the negative side of the coil is wired to the distributor, then the vehicle should have a negative ground. If the ammeter has + and – marks, then the side that is connected farthest away from the battery will represent the ground.

The polarity of my tractor still proved elusive, however, as the ignition system used a magneto without a marking on the coil, and the ammeter was an induction type that was also unmarked.

A friend of mine saved the day with an old factory manual that had a wiring diagram clearly showing that the positive terminal was hooked to the ground.

Generating Some Problems

So a case like this begs the question, “What happens if I hook it up backward?” If it’s an alternator-type system and it’s connected incorrectly, there will be a large spark, and the alternator will most likely be ruined instantly. If it’s a late model car with computerized systems, there could be thousands of dollars in damages done in the blink of an eye. So in those cases, it is essential that the polarity of the system be correctly determined before the battery is connected.

But with an old-style generatortype system, unless the car is equipped with an electric fuel pump, there may be no immediate indication that anything is wrong.

If the car has an ammeter, try hooking the battery up and turning on the lights. If the ammeter moves in the discharge direction, the battery is hooked up correctly; otherwise, the terminals should be reversed before attempting to start the car. Even if the battery is hooked up incorrectly, the car will likely start and run but the charging system will not work. Lighting systems will work, motors will run, but permanent magnet motors like the heater fan or wiper motors will run backward.

In this case, the damage will occur after you shut the car off.

Generator systems have a cut-out relay. It’s usually built into the regulator, but sometimes it can be separate. Its job is to isolate the generator from the battery when the engine is not running.

Once the engine starts and the generator starts producing voltage, the relay is pulled shut and the generator is connected to the rest of the electrical system. If the generator is producing voltage of the same polarity as the system, all goes well and the battery is charged, but if the generator polarity is opposite that of the electrical system, excessive current will be drawn through the points, sometimes welding them together. While the generator is still turning, it may be able to handle the current, but when the engine is shut off and the generator remains connected to the system because of the damaged relay, it will become like a motor that is trying to turn something that is locked up. It quickly overheats and burns itself up, and the wiring connected to it produces distressing amounts of smoke.

In spite of the appearance, repairing the damage thus done is usually not as difficult as the billows of smoke produced might seem to suggest. It’s just a matter of replacing a few feet of large-gauge wire along with the generator and the regulator.

Some Pros and Cons of Positive Ground

The main reason for maintaining a positive ground electrical system is to preserve the authenticity of the car.

An original electrical system with a positive ground will function just as well as a negative ground system, but it may lead to complications if the car is operated in the real world.

Many restorers and hot rodders find that if they want to drive a car on a regular basis, it is more useful to be on the same page with the rest of the world and use a negative-ground twelve-volt system. It will allow you to easily install current sound systems and accessories and make it safer to jump off to another car.

If the car already uses 12 volts, converting the ground may not even require changing any components.

If you’re converting from six to twelve, however, it will be necessary to change over the generator, regulator and all the power consumers like light bulbs and accessory motors.

On older cars that don’t have any semiconductors, the only components that are polarity sensitive are the generator, the ignition coil, ammeters and any permanent magnet motors, any of which can easily be persuaded to function either way.

Large motors like starters have field windings and will turn in the same direction no matter which way the current flows. As noted above, smaller motors like heater fans will run backward when the polarity is reversed, but in most cases these motors have two-wire connections so fixing them is simply a matter of reversing the connections. Many British cars use an SU electric fuel pump that is polarity sensitive because of a spark-suppression diode mounted under the cap. This diode may be de-soldered and reversed, or an alternate version of the pump can be purchased. These pumps are available in both negative- and positive ground versions.

Most contact point-type ignition coils will operate either way, but will perform better if they are wired up correctly.

This brings up one good reason for having a negative-ground system, and that is spark plug life. When a spark jumps between electrodes, most erosion occurs on the positive side. While the outer electrodes of spark plugs are usually the same material as the outer shell, the center electrode can be made of different materials and be better designed to resist electrical erosion. Therefore, it will last longer if the center electrode is run as positive and the outer (grounding) electrode as negative.

Once these procedures are completed, the car can be operated as any other…

Most coils are clearly marked, but if they aren’t, there is a method for checking the polarity. Being careful not to put yourself in the circuit, hold a high tension lead about 1 ⁄4-inch away from a spark plug or engine ground. With the engine running or cranking, hold the point of a wooden pencil in the path of the spark. If the spark flares on the ground side of the pencil, the polarity is correct. If the flare occurs on the wire side of the pencil, the polarity is incorrect and the primary wires need to be reversed.

Dealing With a Generator’s “Memory”

This leaves the charging system. All that really needs to be done to persuade the generator to work backward is to repolarize it. Most auto mechanics over 40 will be familiar with this process because it used to be an essential part of a generator replacement. The generator contains large pole pieces or “shoes” in the housing, and the field coils wrap around these shoes. The shoes are not permanent magnets, but do have a certain residual magnetic field left by their use as electromagnets. This magnetic field will have the polarity of the last current to pass through it, and in this way the generator “remembers” whether it was running with a positive or a negative ground. Polarizing the generator is simply a matter of passing an electrical current through the field coils to correctly align the magnetic field.

Before attempting to polarize the generator, it is necessary to determine whether it has an internally or an externally grounded field. Some of the old Motors manuals have this information on the specifications page. Otherwise, you can examine the brushes. One will be connected to the armature (large) terminal, and the other brush will be grounded. If the grounding brush has no other connection, then the field is externally grounded through the voltage regulator (Figure 1). If the grounding brush has a wire coming from the field coils also connected, then the field is internally grounded (Figure 2).

If it is determined that the generator field is externally grounded, then it can be polarized by momentarily connecting a jumper lead between the BAT terminal and ARM or GEN terminals of the regulator. This is done with the key off and is only done for about half a second. A spark should be visible when the wire is pulled away.

If the generator field is internally grounded, disconnect the field wire from the regulator and momentarily touch it to the BAT terminal of the regulator. As with the other, there will be a spark when you pull the wire away. Again, this only takes a moment. Leaving the wire connected for too long will cause the windings to overheat just as they would with a damaged cut-out relay.

Once these procedures are completed, the car can be operated as any other with a negative ground system.

Operate all the accessories and make sure none of the motors are running backward; if they are, reverse their connections. Check the instruments; most will be operating correctly except for ammeters or electronic tachometers. Ammeter connections can be reversed. An electronic tachometer would be a rare find on a positive ground vehicle and might prove difficult to convert.

Once a successful conversion is done, it’s probably a good idea to follow British practice and affix a label to the car near the battery that warns that the car is wired with a negative ground. It could prevent someone from making an expensive mistake later on down the road.