Ford Full-Size Vans 1989-1996 Repair Guide

Basic Electrical Theory


See Figure 1

For any 12 volt, negative ground, electrical system to operate, the electricity must travel in a complete circuit. This simply means that current (power) from the positive terminal (+) of the battery must eventually return to the negative terminal (-) of the battery. Along the way, this current will travel through wires, fuses, switches and components. If, for any reason, the flow of current through the circuit is interrupted, the component fed by that circuit will cease to function properly.

Perhaps the easiest way to visualize a circuit is to think of connecting a light bulb (with two wires attached to it) to the battery-one wire attached to the negative (-) terminal of the battery and the other wire to the positive (+) terminal. With the two wires touching the battery terminals, the circuit would be complete and the light bulb would illuminate. Electricity would follow a path from the battery to the bulb and back to the battery. It's easy to see that with longer wires on our light bulb, it could be mounted anywhere. Further, one wire could be fitted with a switch so that the light could be turned on and off.

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Fig. Fig. 1: This example illustrates a simple circuit. When the switch is closed, power from the positive (+) battery terminal flows through the fuse and the switch, and then to the light bulb. The light illuminates and the circuit is completed through the ground wire back to the negative (-) battery terminal. In reality, the two ground points shown in the illustration are attached to the metal chassis of the vehicle, which completes the circuit back to the battery

The normal automotive circuit differs from this simple example in two ways. First, instead of having a return wire from the bulb to the battery, the current travels through the chassis of the vehicle. Since the negative (-) battery cable is attached to the chassis and the chassis is made of electrically conductive metal, the chassis of the vehicle can serve as a ground wire to complete the circuit. Secondly, most automotive circuits contain multiple components which receive power from a single circuit. This lessens the amount of wire needed to power components on the vehicle.


Electricity is the flow of electrons-hypothetical particles thought to constitute the basic "stuff" of electricity. Many people have been taught electrical theory using an analogy with water. In a comparison with water flowing through a pipe, the electrons would be the water.

The flow of electricity can be measured much like the flow of water through a pipe. The unit of measurement used is amperes, frequently abbreviated as amps (a). When connected to a circuit, an ammeter will measure the actual amount of current flowing through the circuit. When relatively few electrons flow through a circuit, the amperage is low. When many electrons flow, the amperage is high.

Just as water pressure is measured in units such as pounds per square inch (psi), electrical pressure is measured in units called volts (v). When a voltmeter is connected to a circuit, it is measuring the electrical pressure. The higher the voltage, the more current will flow through the circuit. The lower the voltage, the less current will flow.

While increasing the voltage in a circuit will increase the flow of current, the actual flow depends not only on voltage, but also on the resistance of the circuit. Resistance is the amount of force necessary to push the current through the circuit. The standard unit for measuring resistance is an ohm (W or omega). Resistance in a circuit varies depending on the amount and type of components used in the circuit. The main factors which determine resistance are:

Material-some materials have more resistance than others. Those with high resistance are said to be insulators. Rubber is one of the best insulators available, as it allows little current to pass. Low resistance materials are said to be conductors. Copper wire is among the best conductors. Most vehicle wiring is made of copper.
Size-the larger the wire size being used, the less resistance the wire will have. This is why components which use large amounts of electricity usually have large wires supplying current to them.
Length-for a given thickness of wire, the longer the wire, the greater the resistance. The shorter the wire, the less the resistance. When determining the proper wire for a circuit, both size and length must be considered to design a circuit that can handle the current needs of the component.
Temperature-with many materials, the higher the temperature, the greater the resistance. This principle is used in many of the sensors on the engine.


The preceding definitions may lead the reader into believing that there is no relationship between current, voltage and resistance. Nothing can be further from the truth. The relationship between current, voltage and resistance can be summed up by a statement known as Ohm's law:

Voltage (E) is equal to amperage (I) times resistance (R): E=I x R Other forms of the formula are R=E/I and I=E/R

In each of these formulas, E is the voltage in volts, I is the current in amps and R is the resistance in ohms. The basic point to remember is that as the resistance of a circuit goes up, the amount of current that flows in the circuit will go down, if voltage remains the same.