Electric current is the flow of electrons from a point where there is a surplus of electrons to a point where there is a shortage.

To move between the two points the electrons need to follow a path. That path is called a circuit.

A complete circuit forms a circular path that eventually returns the free electrons to their point of origin.

For purposes of understanding the theory of electricity, the direction of electron flow doesn't really make any difference. The important thing is that the electrons are moving.

From a practical standpoint, however, the direction of flow can make a difference. In an electric motor, for instance, the direction of flow determines the direction in which the motor will turn. If the motor turns "backwards", the problem can be corrected by reversing the wires that supply the power at the point where they connect to the motor.

Many system components including those in electronic circuits are designed to carry current in a specific direction and could be damaged or fail to function properly if the connections are reversed.

In modern automotive systems, the generator/alternator generates a surplus of electrons that is passed onto the battery which, in turn, supplies the vehicle's electrical and electronic components.

The flow of current is carried by wires to the individual components. The circuit is completed by other wires and the metal in the vehicle's body.

If there is any break in the circuit such as a disconnected wire, open switch, or a component not properly grounded to the vehicle, the electrons will not flow.

Electrical circuits are described as having a "hot" side and a "cold" side. The "hot" side is the wire or connector where you can get a test light to glow or a VOM (volt/ohm meter) to produce a voltage reading by touching the red probe to the wire/connector and the black probe to a ground such as the body in a car thus completing the circuit. 

The concept can be illustrated by using an example familiar to anyone who has ever worked on a car. If you touch - (negative/black) terminal on the battery and touch the metal body of the vehicle at the same time, nothing happens. You are simply touching the same side of the circuit in two different places. If, however, you touch the + (positive/red) terminal and body of the vehicle at the same time, you get a shock. You have served as the "wire" that completes the circuit. That's what makes + the "hot"side the hot side.

In a residential 120v electrical circuit, the black wire will be the "hot" wire, a white wire will be the return wire that completes the path, and a green wire might be provided as a ground wire for safety.

A wiring diagram is nothing more that a road map that shows the components and connecting wires that make up an electrical system.

A wiring diagram  can be a bit confusing at first because it doesn't show the actual location of a wire or component, just its relative position in the circuit. Confusion also results from the fact that wiring diagrams typically use symbols to represent the different components rather than illustrations that show what the component looks like.

Wires are identified by their color "DK. BLU", for instance, would be a dark blue wire.

Some of the more common electrical system symbols are shown below. Electronic circuit diagrams use additional symbols to represent various electronic components. Note that the symbol for a given component can vary depending on the source of the wiring diagram.

A simple circuit is made up of three basic elements:

1. A device for creating a potential "difference" - a surplus of electrons at one point and a shortage at another point. That device can be an automotive alternator or other power generating device, or it can be a battery in which energy is stored.

2. A path for the electrons to follow - the wiring and in the case of a car or truck, the metal body.

3. A "Load" - something in the path that will use the energy of the flowing electrons.

If you remove the "difference", there is no electric current available to flow and do work. If you remove or interrupt the path, there's no way for the current to get from Point A to Point B. If you remove the "Load", there is nothing in the circuit to use the energy.

If no "Load is present, the result is a "short circuit". All the energy is returned to the source where it turns into heat at the first point where it encounters high resistance.

All circuits are made up of the same three basic elements. The thing that makes circuits seem complicated is that the path to and from the source can branch off to a variety of different devices included in the circuit to perform different tasks

The secret to unlocking the mysteries of circuits is to follow the path of the current from beginning to end.

To avoid unnecessary detail, electronic diagrams that include inaccessible, non-serviceable items are frequently drawn as block diagrams that show only the different modules that make up the system. In such instances, arrows are typically used to help identify the working relationship between the different modules.

Since a circuit must be complete to allow any electrical or electronic component to function, the troubleshooting process always begins with a circuit test.

The most basic test and most obvious test is to attempt to activate the system. If the load device (an electric fan motor, for instance) functions, the circuit is complete.

That, however, does not guarantee that the circuit is on good condition.

The possibility always exists that there are poor connections, high resistance, or leakage that would result in the load device receiving less power than intended.

When such problems become serious, the result will frequently take the form of a fan that turns too slowly or a light them is not as bright as it should be.

Two tools are commonly used to test circuits:

1. A Test Light.

2. A Volt/Ohm Meter (VOM).

A test Light can be used to verify that there is power at a given point in the circuit.

A VOM will give the actual voltage at the point in the circuit where the measurement is taken. A VOM can also be used to measure the resistance in a given wire or component.

The most common test routine is to perform a functional test of the system by turning it on and then systematically working back from the load device to the power source to identify the cause of any problems.

If there is power in the correct amount at the load device and it doesn't function, the problem lies within the device necessitating that it be repaired or replaced.

If there is no power, tests are conducted on each component in the circuit until the problem is found. Common problems include malfunctioning switches, poor grounds, and failed components such as resistors or capacitors.

Component tests are most frequently performed in one of two ways ways:

1. Using a Test Light/VOM connected to a good known ground and to successively to each terminal on the suspected component. Power in but no power out indicates that a problem exists in the component.

There are two instances in which testing using a jumper wire across the terminals of a component can produce problems:

1. Tests shouldn't normally be performed across any resistors in the circuit since the since the purpose of the resistor is to reduce the current flow to the load device. Bypassing the resistor could damage the load device.

2. In the case of a load device that uses a large amount of power, any jumper wire must be heavy enough gage wire to supply the needed power. If the wire is too small, it will most likely overheat and under any circumstances will not carry the minimum amount of current required for normal operation.