Electrical System


When the automotive industry was in its infancy, it used electricity only to ignite the fuel inside the engine. By the late 1920’s, the electric starter replaced the hand crank, electric headlights made acetylene lamps obsolete and the braying of the electric horn drowned out the squeak of the hand-squeezed air horn. Today, an automobile requires an elaborate electrical system of circuits just to produce, store, and distribute all the electricity it requires simply for everyday operation.

The first major component in the electrical system is the battery. The battery is used to store power for starting, and for running auxiliary devices such as clocks, radios and alarms when the engine is off. The next major component is the starter motor, which is used to start the engine. The third component is a charging device powered by the engine, known as the alternator. It powers the electrical system when the car is running, and restores the charge within the battery. With these basic components, the car maintains its supply of electricity. A device called the voltage regulator keeps the power level stabilized, and the fuse box keeps minor problems from becoming major ones.

Many different auxiliary electrical devices are used in modern cars, such as radios, cellular phones, rear window defrosters and electric door locks, as well as a vast array of motors powering everything from the moonroof on down.


The car’s initial source of electricity is a battery, whose most important function is to start the engine. Once the engine is running, an alternator takes over to supply the car’s electrical needs and to restore energy to the battery.

A 12-volt storage battery consists of layers of positively and negatively charged lead plates that, together with their insulated separators, make up each of six two-volt cells. The cells are filled with an electricity-conducting liquid (electrolyte) that is usually two-thirds distilled water and one-third sulfuric acid. Spaces between the immersed plates provide the most exposure to the electrolyte. The interaction of the plates and the electrolyte produces chemical energy that becomes electricity when a circuit is formed between the negative and positive battery terminals.


The starter converts electricity to mechanical energy in two stages. Turning on the ignition switch releases a small amount of power from the battery to the solenoid above the starter. This creates a magnetic field that pulls the solenoid plunger forward, forcing the attached shift yoke to move the starter drive so that its pinion gear meshes with the engine’s crankshaft flywheel. When the plunger completes its travels, it strikes a contact that permits a greater amount of current to flow from the battery to the starter motor. The motor then spins the drive and turns the meshed gears to provide power to the crankshaft, which prepares each cylinder for ignition. After the engine starts, the ignition key is released to break the starting circuit. The solenoid’s magnetic field collapses and the return spring pulls the plunger back, automatically shutting off the starter motor and disengaging the starter drive.

When the starter is not in use, the drive unit is retracted so that its pinion is disengaged from the flywheel. As soon as the starter is activated, the forward movement of the solenoid plunger causes the shift yoke to move the drive in the opposite direction and engage the pinion and flywheel. The pinion is locked to its shaft by a clutch that unlocks if the engine starts up and the flywheel begins turning the pinion faster than its normal speed. By allowing the pinion to spin freely for a moment, the clutch protects the motor from damage until the drive is retracted.

Alternator or Generator

The alternating-current generator, or alternator, is the electrical system’s chief source of power while the engine is running. Its shaft is driven by the same belt that spins the fan. It converts mechanical energy into alternating-current electricity, which is then channeled through diodes that alter it to direct current for the electrical system and for recharging the battery.

Lighting Circuit

The automobile lighting circuit includes the wiring harness, all the lights, and the various switches that control their use. The complete circuit of the modern passenger car can be broken down into individual circuits, each having one or more lights and switches. In each separate circuit, the lights are connected in parallel, and the controlling switch is in series between the group of lights and the fuse box. The parking lights, are connected in parallel and controlled by a single switch. In some installations, one switch controls the connection to the fuse box, while a selector switch determines which of two circuits is energized. The headlights, with their upper and lower beams, are an example of this type of switch. Again, in some cases, such as the courtesy lights, several switches may be connected in parallel so that any switch may be used to turn on the lights.

Main Lighting Switch

The main lighting switch (sometimes called the headlight switch) is the heart of the lighting system. It controls the headlights, parking lights, side marker lights, taillights, license plate light, instrument panel lights, and interior lights. Individual switches are provided for special purpose lights such as directional signals, hazard warning flashers, back up lights, and courtesy lights. The main lighting switch may be of either the “push-pull” or “push-pull with rotary contact” types. A typical switch will have three positions: off, parking, and headlamps. Some switches also contain a rheostat to control the brightness of the instrument panel lights. The rheostat is operated by rotating the control knob, separating it from the push-pull action of the main lighting switch.

When the main lighting switch completes the circuit to the headlamps, the low beam lights the way for city driving and for use when meeting oncoming traffic on the highway. When the dimmer switch is actuated, the single filament headlamps go “on,” along with the high beam of the two filament headlamps. The next actuation of the dimmer switch returns the headlighting system to low beams only on the two filament lamps. Some cars are equipped with an electronic headlight dimming device, which automatically switches the headlights from high beam to low in response to light from an approaching vehicle or light from the taillight of a vehicle being overtaken. The dimmer switch in the automatic headlamp dimming system is a special override type. It is located in the steering column as part of a combination dimmer, horn, and turn signal switch. The override action occurs when a slight pull toward the driver on the switch lever provides high beam headlights regardless of the amount of light on the sensor-amplifier.

For some years there has been discussion about the advantages of a polarized headlight system. Such a system comprises headlights which produce polarized light in a particular plane. The windscreens of all cars would be fitted with polarizing glass, which would be oriented so that glare from an approaching vehicle would be essentially eliminated, while the forward vision would still be kept at the present levels. The advantages the system appear attractive, but the practical problems of making the transition are very great, since it would not be practical to convert all existing vehicles to this type of lighting. Also, any benefits would only be marginal because glare itself is not a frequent cause of accidents. However, many cars now have refracting or colored glass to cut down on glare.

Directional Signal Switch

The directional signal switch is installed just below the hub of the steering wheel. A manually controlled lever projecting from the switch permits the driver to signal the direction in which he wants to turn. Moving the switch handle down will light the “turn signal” lamps on the left front and left rear of the car, signaling a left turn. Moving the switch upward will light the turn signal lamps on the right (front and rear), signaling a right turn. With the switch in a position to signal a turn, lights are alternately turned “on” and “off” by a turn signal flasher. Incorporated in the directional signal switch is a “lane change switch mechanism.” This feature provides the driver the opportunity to signal a lane change by holding the turn lever against a detent, then releasing it to cancel the signal immediately after the maneuver is completed.

Stoplight Switch

In order to signal a stop, a brake pedal operated “stoplight switch” is provided to operate the vehicle’s stop lamps. In addition to lighting the conventional rear lights, the switch also operates the center high-mounted stop lamp, that became mandatory on later models. Cruise control equipped vehicles may also utilize a vacuum release valve. In this case, both the vacuum release valve and the stoplight switch are actuated by movement of the brake pedal.


The car horn on passenger cars provides the driver with a means of sounding an audible warning signal. The horn electrical circuit generally includes: battery, fuse or fusible link, horn relay, horn(s), steering column wiring harness, horn switch, and body sheet metal. Often, a cadmium plated screw is used to ground the horn to the body of the vehicle. Horns usually are located in the forward part of the engine compartment or in the front fender well. The horn switch is built into the steering wheel or incorporated into the multi-functional switch lever, which includes turn signal and dimmer switch.

Transistors and Resistors

A transistor is a solid state device used to switch and/or amplify the flow of electrons in a circuit. A typical automotive switching application would be a transistorized ignition system in which the transistor switches the primary system off and on. An amplifying application could be in a stereo system where a radio signal needed strengthening.

A transistor is a three-element device made of two semiconductor materials. The three elements are called “emitter,” “base,” and “collector.” The outer two elements (collector and emitter) are made of the same material; the other element (base) is different. Each has a conductor attached. The materials used are labeled for their properties: “P” for positive, meaning a lack of electrons. It has “holes” ready to receive electrons. “N” is for negative, which means the materials has a surplus of electrons. The movement of a free electron from atom to atom leaves a hole in the atom it left. This hole is quickly filled by another free electron. As this movement is transmitted throughout the conductor, an electric current is created from the negative to the positive. At the same time, the “hole” has been moved backward in the conductor as one free electron after another takes its place in a sort of chain reaction. “Hole flow” is from positive to negative. Current flow in a transistor, then, may be either electron movement or hole flow, depending on the type of material, and this determines the type of transistor it is as well.

In most 12 volt systems, a resistor is connected in series with the primary circuit of the ignition coil. During the cranking period, the resistor is cut out of the circuit so that full voltage is applied to the coil. This insures a strong spark during cranking, and quicker starting is provided. The starting circuit is designed so that as long as the starter motor is in use, full battery voltage is applied to the coil. When the starter is not cranking, the resistance wire is cut into the circuit to reduce the voltage applied to the coil. If the engine starts when the ignition switch is turned on, but stops when the switch is released to the run position, it can indicate that a resistor is bad and should be replaced.

At no time should the resistor be bypassed out of the circuit, as that would supply constant battery voltage and burn out the coil. The resistor and resistor wires should always be checked when the breaker points are burned, or when the ignition coil is bad.

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