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All About Car Restoration


Classic Car Automotive Electrical Systems - Part 1: Basic Automotive Electrical Theory

There's nothing difficult about electrical systems. The basic theory of electricity is simple and easily understood if you are just a little patient and curious. So, we're going to start off with a few definitions. Armed with an understanding of the following six terms, you will quickly learn to "think like an electron". Take your time and read these over until you understand the concept fully:

Electron: The basic unit of electricity. Think of these little guys as "bullets", traveling down the wire. It's the movement of electrons which runs the devices which make our lives - and our cars - so comfortable and convenient.

Voltage: This is the force (or pressure, if you like) of electricity in the wire. If you think of your garden hose as the wire, the water pressure would be equivalent to the voltage. Older cars run on six volt systems and newer (most 1956 and later) utilize twelve volt systems. All vehicles' manuals specify the system voltage.

Current: This is the movement of electrons in the wire, expressed in a unit called the Amp. The greater the rate of movement through the wire, the greater the number of amps. Think of this as the speed of the water coming out of the garden hose. When you tighten the nozzle the water shoots out further and faster.

Resistance: This is a restriction to the movement of electrons through the wire or circuit. The unit of resistance is called the OHM and you can think of it as a kink in that garden hose. The higher the resistance, the more current must flow to overcome it. The more current which flows through an area of high resistance, the hotter the wire will become, ultimately failing. Corrosion, loose terminals and too-small diameter wires are three very, very common causes of resistance.

IMPORTANT FACT! High resistance is the cause of ALL electrical failures - with the exception of broken wires and lack of grounding - both of which will be discussed later.

Watts: The unit of power in electricity and the product of Amps x Volts. Why is this important? Because designers of circuits need to know the amount of current required for a given device (such as a fan, horn, light, etc.) in order to figure out which diameter wire to use. Example: a 50-watt brake light, operating on 12 volts, will draw 4.1 amps (4.1 amps x 12 volts = 50 watts). The wire diameter must be large enough to carry the current without heating up and melting off its insulation.

IMPORTANT FACT! This is the only formula you will ever really need to understand basic electricity, be it in your car or in your house.

Ground: All electrical devices must be part of a circuit. That is, electrons must flow from the power source through the device to a ground. In cars, the metal chassis is the ground (that's why the battery's negative lead is bolted to the engine or frame) and the power source is the positive lead on the battery. Without a ground there is only a POTENTIAL circuit. No electrons will flow — and therefore nothing will work — unless the circuit ends in a ground. Note: Some cars and trucks utilized "positive ground" electrical systems, where the positive lead from the battery connects to the frame and the negative lead goes to the electrical wiring harness. This in no way makes it more difficult to wire or troubleshoot; all that's required is to remember that the system is the reverse of normal systems.


Ready to go on? Okay, let's start with the fact that all cars run on Direct Current (DC) electrical systems, as opposed to alternating current (AC) which runs your home. DC is a "single wire" system. That is, the flow of electricity always runs from the source of current through the device and then to ground. It may do this through any number of connections and through other devices, but tracing the path is straightforward if you always ask the question:

"Where is the power coming from and is there a path to ground?"

For practical purposes, the flow of electricity is now considered to be from positive (voltage, designated by a plus sign +) to negative (ground, designated by a minus sign - ). Therefore, your car's battery "negative" terminal is connected to the metal framework of the car (some older cars — mostly foreign — utilized 'positive ground' systems but this is no longer done).

Tool Time!
In order to measure voltage, resistance, direction of current flow and other electrical parameters you need a multimeter. These are devices which have been around for many years and are available at electronics stores and even most home centers. Inexpensive ($30 or so), reasonably high-quality meters are all the average hobbyist needs, so don't overspend. All these meters can measure DC, AC, resistance and even small amounts of current. Meters in this price range are fully capable of measuring your car's components accurately, as well as your household system and you can choose either analog or digital types, depending upon whether you like to read a dial or just a number display. After you purchase one, read the instructions and practice measuring voltages and resistances with it. An hour's practice should make you an expert. When you get accustomed to using a multimeter you will quickly come to appreciate its enormous versatility.

Since the source of electricity in a car is the battery, let's see how one works:

Automotive Battery Diagram

A battery is an electrochemical device which converts chemical energy into electrical energy. Cars use "lead-acid" batteries.
A lead-acid battery uses a series of lead dioxide plates for its positive (+) terminal and porous, soft lead for its negative plates. All the plates are arranged alternately and submerged in a solution of sulfuric acid and water. The positive plate's lead oxide is a compound of lead and oxygen. Sulfuric acid is a compound of hydrogen and the sulfate radical (SO4), so the acid's chemical designation is H2SO4.

Chemically, when a battery is connected to an external load (a device which uses electricity) it begins to discharge. As that happens, the lead in the positive plate combines with the sulfate of the acid, forming lead sulfate (PBSO4) in the positive plate. Oxygen in the positive plate combines with hydrogen from the acid to form water (H2O), which reduces the concentration of the acid in the electrolyte. Also, the pure lead in the negative plate combines with the sulfate, forming lead sulfate and making the positive and negative plates more alike in chemical composition. Electrons are released during this reaction, creating electric current at a specific voltage (2 volts per cell, with 6 cells in a 12-volt battery, described below).

The battery voltage depends upon the chemical difference between the two plate materials and the concentration of the acid. Because the plates have become more chemically alike and the electrolyte concentration has become weaker, the voltage output gets weaker and weaker until the battery is "dead", or discharged.

However, the battery can be re-charged by passing an electrical current through it in the opposite direction of the discharge. The chemical reactions during a charge cycle are the reverse of those that occur during discharge. As the battery is charged the positive plates become lead dioxide again, the negative plates become pure lead again and the electrolyte returns to its proper concentration. The charge-discharge cycle can be repeated over and over again, until fatigue and erosion of the electrodes and corrosion of the positive plates cause eventual failure.

Mechanically, batteries are composed of multiple "cells", each containing the positive and negative plates. A single cell will produce two volts, so your 12-volt battery has six cells grouped together in one case, for efficiency. The cells are connected in "series", or positive to negative to positive to negative; and so-on. When you connect something in series you add up each cell's voltage to get the overall battery's output.

So why does such a big, heavy thing like a battery only produce 12 volts? Well, it's the current (remember?) which does the work and all those plates immersed in that acid are capable of producing impressive amounts of amps, at least for short durations. A typical battery delivers 500-1000 amps and you need all that current to run the starter motor, not to mention other things.

Batteries fail to provide sufficient current generally in only a few ways:

1. The electrolyte and plates "wear out". The life of a battery (36 months, 48 months, etc.) is determined by the thickness and number of plates and you get what you pay for in that regard. Eventually the battery wears out and can't hold a charge. To test for this, have a service station test the cells with a hygrometer (a device which measures specific gravity ) or buy one for yourself (they're cheap). If the hygrometer says the battery is shot and it won't hold a charge, replace it.

2. The most common failure of batteries is loose or corroded cable connections. In either case, the reason for failure is HIGH RESISTANCE! (remember, a poor mechanical connection means that little or no current can pass through). If the cables are loose, tighten them thoroughly. If corroded, remove them and clean them with a file or sandpaper (clean both the cable connectors and the terminals!) It's a good practice to clean the connections at least once a year.

3. Overcharging, either through external chargers or faulty regulator, kills batteries by creating so much heat (due to current flow) that the water in the electrolyte is boiled off. In some cases the battery explodes. Of course, connecting jumper cables incorrectly can result in a dead short, with catastrophic consequences (A dead short is when all the current from the voltage source is connected directly to ground without passing through any device or resistance. In the case of a battery, it would be equivalent to connecting both terminals together, causing a huge current flow through the plates, in turn causing massive heating, then boiling, and finally the battery will blow up).

In Part II of our Automotive Electrical Systems, we'll explain How Generators and Alternators work.

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