ELECTRICAL REBUILDER’S ASSOCIATION—Technical Document MI201404B www.electricalrebuilders.org Positive or Negative Which ground is best? BY BOB THOMAS few months ago, the ERA Help Line received a call asking why the once-common _ positive-ground electrical system had been _ totally abandoned by the automotive world. The caller, John Prinvale of John’s Automotive Electrical in Quincy, CA, was asking, because a student in his high school auto shop class had asked him. I have to admit that I never gave this much thought before, although I had heard all kinds of rumors. He asked me about posting the question on the ERA Forums online, and I encouraged him to do so. The question elicited 24 responses in less than a week, with many different theories as to why the system was no longer used. They included positive- ground’s effects on corrosion of electrical connections, contact surface erosion, radio tubes, transistors and spark plugs. Over the course of a week, with help from ten of the forum’s question-responders, I believe I have busted some myths, learned some science, and provided Prinvale with enough information to satisfy his inquisitive students. For anyone who may be interested, here is what I learned. Positive Ground’s Effects On: Corrosion—Galvanic or bimetal corrosion takes place when dis-similar metals are in contact with each other while exposed to an electrolyte. The electrolyte could be moisture mixed with salt or other chemicals. When this happens, the less noble metal erodes much faster. Any current that might flow through a bimetal connection in a circuit in either direction, will not appreciably affect the galvanic corrosion that is already taking place. The direction in which a current flows does not have any effect on metal oxidation either—oxidation happens to metals when they are exposed to oxygen in the air. The presence of moisture accelerates oxidation, but it does not really cause it. The best way to prevent galvanic corrosion and _ oxidation is to seal out moisture and air using dielectric grease. For corrosion to take place, air or moisture must be present. Therefore, the grounding polarity of a circuit has little if any impact on the corrosion of the conductors in that circuit. Contact Wear— The amount of surface material lost from arcing when a set of contacts opens, will be the same, in whichever direction the current flows. One contact will lose more material, depending on the direction of the current. If the current is reversed, you will still lose the same amount of surface material—just on the opposite side. The only exception to this is when the contacts are made of two different metals, such as generator regulator-relay contacts. Most of these relay contacts are made of two different materials to extend the wear-life. These regulators are manufactured specifically for only one polarity or the other. It is usually printed right on the regulator. Voltage regulators that can be used with either positive or negative ground, will have contacts made of the same material. This applies to all relays on any vehicle. Electronics—Transistors and _ the radio tubes that preceded them, were manufactured to operate in only one type of system—either positive ground or negative ground, but not in both. Unless someone installs the wrong part, the ground polarity has no bearing on the cost or durability of these electronic devices. Spark Plugs—As it turns out, polarity does matter when it comes to spark plugs. Regardless of a vehicle’s polarity in relation to the battery, the secondary ignition system has to be positive ground. The spark plug was designed to emit negatively charged electrons from the center electrode at very high voltage. The side electrode must therefore be positively charged. This was difficult for me to grasp on a negatively grounded engine, but the high voltage secondary system does not necessarily have to be the same polarity as the rest of the vehicle. In essence, the secondary high voltage current is “its own world” Some ignition coils have been manufactured specifically for positive ground or for negative ground applications, while others will work either way, as long as the terminals «9 marked “+” and are connected correctly. Many coils could easily be hooked up backwards accidentally or as a prank. This would force the spark to jump backwards, from the side electrode to the center electrode. The effect of this on engine performance would be significant enough to cause hard starting and a lack of power. A backwards coil can easily be diagnosed on a scope, because the spark pattern will display upside down. When any vehicle is converted from positive to negative ground, the coil wires must be reversed, too. Even today’s vehicles that use an individual coil pack for each cylinder, fire a positive ground spark! So what really happened to positive ground? Why was it abandoned? We have negative ground today, not because it was necessarily better, but because of standardization. It was cost effective for the automotive industry to have all manufacturers use the same _ battery polarity. The Society of Automotive Engineers (SAE) tackled the problem in the 1950s, and they initiated an industry standard. Ford, General Motors and Chrysler were all “on board” within a few years. Not surprisingly, the last 6-volt systems were abandoned at about the same time, without the need for creating a standard. It had more to do with Ohm’s Law. Special thanks to Forum posters John Prinvale, Art Glass, Mohammad Samii, Juan Grube, Jerry Wine, Hank Henke, Lynn Gross, Bruce Schiroo, Morgan McDonald and Joe Davis (who called me). Bay «“ An ERA Publication. ©2014, Electrical Rebuilder’s Association. 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