The basic measurements of electricity are:

VOLTS Voltage is the difference in electrical potential between two points, or the amount of "push" that makes the electrons flow. It's also called the "electromotive force" (EMF). It's like the pressure that forces compressed air through a hose, but instead of being measured in pounds per square inch, voltage is measured in units called "volts."

AMPS
Current is the amount or volume of electrons that flow through a conductor or a circuit. It is a measure of volume, and is specified in units called "amperes" or "amps" for short. The analogy with an air hose would be the number of cubic feet per minute of air passing through the hose. One amp is equal to 6.3 million trillion electrons (6.3 with 18 zeros after it) flowing past a point in one second! That's a lot of electrons, but a relatively small current in many automotive circuits. A starter, for example, can draw several hundred amps when cranking the engine.

OHMS
Resistance is the opposition to the flow of current, or the restriction that impedes the flow of electrons. Resistance is measured in units called "ohms." The flow of air though a hose can be reduced by pinching it, by reducing the diameter of the hose or by holding your finger over the outlet. Likewise, current flow through a wire can be slowed or controlled by adding resistance.

OHM'S LAW
One volt equals the amount of force needed to push a one amp current through a circuit with a resistance of one ohm. They call this "Ohm's Law." It can be expressed in various ways:
AMPS = VOLTS\OHMS (Volts divided by Ohms)
OHMS = VOLTS\AMPS (Volts divided by Amps)
VOLTS = AMPS x OHMS (Amps times Ohms) Understanding Ohm's Law and the relationships between volts, ohms and amps is the key to understanding electrical currents and circuits. Ohm's Law explains why high resistance in a circuit chokes off the current and causes a voltage drop. It also explains why an electrical short can cause a wire to rapidly overheat and burn because of a runaway current.

VOLTAGE DROP
A voltage drop occurs when current flows through a component in a circuit. The resistance created by the device produces a corresponding drop in voltage which can be calculated using Ohm's Law if you know the resistance of the component and current flow.

VOLTAGE DROP = RESISTANCE x CURRENT
In the shop environment, voltage drop is measured with a voltmeter. The voltmeter's leads are connected on either side of the circuit component or connection that's being tested. If a connection is loose or corroded, it will create resistance in the circuit and restrict the flow of current causing an excessive voltage drop. As a rule of thumb, a voltage drop of more than one tenth volt (0.1v) across any connection means trouble. Measuring voltage drop is an effective means of quickly pinpointing circuit problems such as loose or corroded connectors, wires, switches, etc. because it doesn't require you to disassemble anything prior to testing.

TYPES OF CURRENT IN A CIRCUIT
Most automotive electrical circuits operate on Direct Current (DC). In a DC circuit, the polarity of the voltage and current do not change -- as opposed to Alternating Current (AC) circuits where they do.

In a DC circuit, positive (+) is always positive, and negative (-) is always negative. Positive or "hot" wires may be (but not always) color coded red, while negative or "ground" wires may be coded black (remember that electrons actually flow from negative to positive!).

All modern automotive electrical circuits are "negative ground" meaning the body is connected to the battery's negative terminal. In some antique vehicles and older British cars, the body is connected to the positive battery cable creating a "positive ground" electrical system.

DC current is used to drive the starter, solenoids, fuel injectors, relays, idle speed control motor, fuel pump and most other electrical and electronic components on the vehicle. Most sensors also operate on direct current.

The alternator, however, is an exception. It uses direct current to produce an alternating current charging voltage. But before the alternating current leaves the alternator, it is converted back to direct current again by the alternator's diodes.

In an AC circuit, voltage and current do not remain constant. An AC current reverses direction and goes from positive to negative and back to positive again in a cyclic fashion. If an AC current is plotted on a graph or viewed on an oscilloscope, it forms a "sine wave" pattern (a DC current would form a straight line). The AC wave starts at zero volts, increases to its maximum positive value, then falls back to zero and reverses to its maximum negative value.

Think of the difference this way: a DC current flows one-way (negative to positive) while an AC current surges back and forth.

ELECTRICAL CIRCUITS
An electrical circuit is basically a route or path through which electrons flow. As we said earlier, it must form a complete loop so the current will continue to flow. The electrons need a return path back to their source (the battery or alternator) otherwise they have no place to go. There are essentially two kinds of electrical circuits: SERIES CIRCUITS
A "series" circuit is one in which all the circuit elements are connected end-to-end in chain-like fashion. The current has only one path to follow so the amount of current passing through it will be the same throughout. The total resistance in a series circuit is equal to the sum of the individual resistances within each circuit element. If one element in a series circuit goes bad, continuity will be broken and the entire circuit will go dead.

PARALLEL CIRCUITS
A "parallel" circuit is one in which circuit elements are connected next to or parallel to one another. This creates multiple branches or pathways through which current can flow. The resistance in any given branch will determine the voltage drop and current flow through that branch and that branch alone. One of the advantages of a parallel circuit is that the various segments or pathways of the circuit can operate independently of one another. If one element goes open (breaks continuity), it won't disrupt the function of the other.

OPENS, GROUNDS & SHORTS
Since all electrical circuits require a continuous path for current to flow, most automotive electrical systems use the steel body for the "ground" path. The wiring makes up the other half of the circuit. The advantage of this approach is that it eliminates the need for two wires to many components. A parking light, for example, needs only one wire because it is grounded to the body.

On all modern vehicles, the body is connected to the negative battery terminal while the wiring is connected through the ignition switch and fuse box to the positive terminal. This is called a "negative ground" electrical system. In some antique vehicles and older British cars, the positive battery cable was connected to the body, creating a "positive ground" electrical system.

Although we usually think current flows through the positive "hot" wire to the various accessories and circuits in the car and then returns through the body ground back to the battery, the opposite is actually the case. Electric current flows from negative to positive so the electrons leave the battery through the negative terminal, travel through the body to the various circuits, and return to the battery through the so-called "hot" wires in the electrical system.

It may sound confusing, but a "hot" wire in a negative ground electrical system is really a return wire that completes a circuit. A "hot" wire always sparks when grounded because it completes a direct path back to the battery. It will also show battery voltage when checked with a grounded voltmeter. When a circuit is not complete (no continuity), it is said to be "open." Electricity obviously can't flow through an open circuit because there's no return path back to the power source. An open can be created intentionally by using a switch, circuit breaker or relay to turn a circuit off, or it may be unintentional due to a broken wire, or a loose or corroded connection.

A "short" occurs when a portion of an electrical circuit is bypassed unintentionally. It's called a short because it creates a shorter return path for the current to follow. An example of a short would be a break in the insulation on a wire touching metal.

When a short occurs, a couple of things happen. The first is that electricity always prefers the path of least resistance -- which is usually via the short rather than through the electrical circuit. The absence of resistance allows a runaway current that can quickly overheat and melt the wire, possibly starting a fire. To protect the vehicle against such mishaps, various types of protection devices are used to protect electrical circuits.

CIRCUIT PROTECTION
One of several safety devices may be used to protect an electrical circuit from shorts or overloads: A "fuse" is the most simple form of protection that's often used. A fuse contains a piece of wire that melts at low temperature. When the flow of current through a circuit approaches the limit of the fuse the fuse wire melts. This opens the circuit and stops the flow of electricity. The amp rating of any given fuse is determined by the vehicle manufacturer according to the size of wiring used and the electrical loads the circuit is designed to handle. Fuses are usually located in a central fuse box although in-line fuses may be used to protect certain accessories that have been added on.

A "fusible link" is a length of special wire that is designed to melt (like a fuse) when a circuit is overloaded. Fusible links are sometimes used in ignition circuits and other circuits that carry high amperage. The location of a fusible link will be specified in a vehicle's wiring diagram. You can tell if a fusible link has burned out by noting the condition of the insulating tape wrapped around it. If the tape appears to be blistered, the wire inside has burned out. Many late model import as well as domestic cars have gotten away from fusible links and gone to high current fuses in a centrally located "power distribution center" (usually separate from the fuse panel).

The third form of current overload protection is a "circuit breaker." A bimetallic switch is used to break the circuit when an overload occurs. When the amount of current flowing through the circuit exceeds the built-in limit of the circuit breaker, the bimetallic switch heats up and opens the contact points. Once the current stops, the circuit breaker begins to cool off and eventually recloses. If the opening of the circuit breaker was caused by a temporary overload, the circuit will resume functioning normally. But if the overload persists, the circuit breaker will continue to cycle on and off. Circuit breakers are often used on high amp load circuits such as the headlights and air conditioner.

RELAYS
A relay is nothing more than a switch in a box that's used to turn a circuit on or off. When voltage is supplied to the relay, the relay closes (or opens) and turns on (or off) whatever it is connected to.

For example, when the ignition key is turned on, power may be routed to the fuel pump relay. The ignition circuit is the control circuit in this case. The voltage from the ignition circuit flows through a small coil (actually an electromagnet) inside the relay. The magnetic field created by the coil pulls a set of contact points shut which completes the main relay circuit and turns the fuel pump on. When the key is turned off, the relay points reopen breaking the circuit to the pump which shuts the pump off.

Why use a relay to turn a circuit on and off? Because relays are usually designed to handle higher current loads. Thus, a relatively small control current can be used to energize a relay that in turn provides a much higher current to the component it controls.

Relays can be checked by applying voltage directly to the input terminals. If the relay is good, it will click and close the circuit. If it's no good, it won't click and close the circuit, or if it does there's too much resistance in the contact points to allow the circuit to function properly. Relays usually quit working because the wiring connectors to the coil break or the contact points wear out.

Relays are used for electric fuel pumps, the horn, the headlights, the heater blower motor, electric cooling fans, the A/C compressor clutch, electric rear window defrosters, power seats, power windows, you-name-it.

Here's a rule that makes no sense but is a fact of life anyway: Relays are seldom located anywhere near the device they operate. They're scattered throughout the vehicle and may be found anywhere under the hood, under the dash, under a seat, in the trunk or behind a kick panel. So to find out where a vehicle manufacturer hid a particular relay for a given circuit or accessory, it's almost always necessary to look up the relay's location on an electrical wiring diagram. The only exception is usually the horn relay which is almost always found in the main fuse box. Most relays are not identified with anything other than an OEM part number, so it's important to make sure your customer gets the correct replacement relay for the application.

SOLENOIDS
Like relays, solenoids are also switches in a box. They can be used for a variety of purposes. Some solenoids (like the starter solenoid) are used to operate an electrical circuit like a relay. Some (like ported vacuum switches for EGR valves or the charcoal canister purge valve) are used to open or block vacuum circuits. Some (like those in antilock brake systems or electronically controlled automatic transmissions) are used to open or block hydraulic circuits. Others (such as mixture control solenoids in electronic carburetors) are used to open and close fuel metering circuits.

A fuel injector, for example is nothing more than a solenoid connected to a fuel nozzle. When the solenoid is energized, it opens and allows pressurized fuel to spray out the nozzle. Like relays, solenoids contain an electromagnetic coil. Except for starter solenoids, which are essentially relays, most solenoids move a "pole piece" when energized instead of closing a set of contact points. The pole piece is nothing more than a small piece of iron that moves when pulled upon by a magnetic field. When the solenoid is energized, it pulls the pole piece up. When the solenoid's control voltage is cut, the spring-loaded pole piece snaps back to its original rest position.

With a fuel injector, the pole piece is connected to the pintle valve that opens the injector nozzle. With a ported vacuum switch, the pole piece uncovers or blocks vacuum passageways as it moves up and down.

The neat thing about solenoids is that they can be cycled on and off very rapidly. The buzzing that a fuel injector makes is nothing more than the rapid cycling of the solenoid and pintle valve. With antilock brake systems, the ABS solenoids that alternately hold, release and reapply brake pressure in the brake lines can cycle on and off 4 to 10 times a second depending on the system.

Another neat thing about solenoids is that the pole piece doesn't necessarily have to move all the way from its rest position to a fully retracted position. By varying the voltage to the solenoid, the pole piece can be slid partially open. This ability enables a solenoid to provide multiple operating positions rather than just on or off. Bosch, for example, uses a three-position solenoid in its antilock brake systems. How do you check a solenoid? Usually by measuring its resistance with an ohmmeter. If the reading isn't within specs, the solenoid coil is either open or shorted. You can also listen for a click when the solenoid is energized. No click or movement means it needs to be replaced.

VOLTAGE REQUIREMENTS
Every electrical device requires a minimum voltage to operate. A light bulb will glow with reduced brilliance as the voltage drops. But for some components there is a threshold voltage below which it won't operate at all. A starter motor, for example, will crank the engine more slowly if battery voltage is low but may not crank it at all if the voltage is below a certain threshold (usually less than 10 volts). Minimum threshold voltage is especially critical for such components as solenoids (which need a certain amount of voltage to overcome spring resistance), relays, timers, buzzers, horns, fuel injectors (which are solenoids, too) and most electronics (the ignition module, engine computer, ABS control module, radio, etc.).

Checking the load point for full battery voltage will tell you whether or not sufficient voltage is getting through, and to do that a voltmeter is necessary. The meter should read within one volt of battery voltage if the circuit's okay. Low circuit voltage is typically caused by excessive resistance at some point in the circuit. Usually this means a weak or corroded connector, a faulty switch, relay or poor ground. To find the point of high resistance, the voltmeter is used to do a "voltage drop test" at various points throughout the circuit. If the voltmeter shows a drop of more than a 0.1 volts across any connector, switch or ground contact, it means trouble.

Sometimes undersized wiring can cause low voltage. If rewiring a circuit or adding an accessory (driving lights, a killer stereo system, a rear window defogger, etc.), the wires must be heavy enough to carry the load. The higher the amp load in the circuit, the larger the required gauge size for the wiring. The following list includes recommended wire gauge sizes:
Wire size-----Amp Capacity
18------------6
16------------8
14------------15
12------------20
10------------30
8 ------------40
6 ------------50

CONTINUITY
Every electrical circuit requires a complete circuit to operate. Voltage to the load won't do any good unless there is also a complete ground path to the battery. The ground path in the case of all metal bodied cars is the body itself. In plastic bodies cars, a separate ground wire is needed to link the load to the chassis. In either case, a poor ground connection has the same effect as an open switch. The circuit isn't complete so current doesn't flow.

To check wiring continuity, all that's needed is an ohmmeter or a self-powered test light. An ohmmeter is probably the better of the two because it displays the exact amount of resistance between any two test points. A test light, on the other hand, will glow when there's continuity but the intensity of the bulb may vary depending on the amount of resistance in the circuit. A trained eye can usually detect the difference but an ohmmeter is more exact.

An ohmmeter should never be used to check resistance in a "live" circuit. Doing so can damage the ohmmeter. Before testing the circuit's resistance or continuity, therefore, the circuit should be isolated by disconnecting it from its power source. Pulling the circuit's fuse will usually do the trick. Ohmmeters are great for measuring resistance and checking continuity in normal electrical circuits but care must be used when working on electronic components. An ohmmeter works by applying a small voltage through its test leads, and this voltage can be enough to damage sensitive electronic components. Special high impedance 10 megohm meters are required for electronics testing, and even then caution must be used to avoid probing certain items directly such as control modules. Tracing wires is an essential element of checking continuity and finding wiring problems, but isn't as easy as it looks because a circuit wire will sometimes change color after passing through a connector, switch or relay. That's why technicians need detailed wiring diagrams when doing electrical work. Working without a wiring diagram is like working in the dark.

WIRING REPAIRS
Most wiring problems occur at harness connectors and terminals because these components are the point where the wiring is most vulnerable to environmental contamination, vibration and mechanical stress. Wiring connectors and terminals sometimes work loose or are damaged by careless handling, but rust and corrosion are usually the main culprits.

Though it is sometimes possible to clean a dirty wiring connector, the best cure is often replacement. Once the protective plating on the metal has been penetrated by corrosion, no amount of cleaning can fully restore it. Coating it with protective dielectric grease can help keep moisture out, but eventually corrosion will penetrate the connector and disrupt the circuit.

Damaged wiring is less common, but vibration, heat and physical abuse can damage insulation and break wires. If a circuit has shorted out, the wiring should be carefully inspected for heat damage (discolored, melted or burned insulation). If damaged, it must be replaced.

When replacing corroded connectors or damaged wiring, various methods may be used. Crimp connectors are a quick way to join wires. Soldering takes time and a certain amount of skill, but provides the best electrical path. Electrical tape or heat shrink tubing must always be used around wiring repairs to keep the electrons where they belong.