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Automotive Electrical System

Updated on December 2, 2016

The automotive electrical system consists of a storage battery, a direct-current (dc) or alternating-current (ac) generator, a starting motor, the ignition system, lighting, and such accessories as a radio, indicating gauges, and the driving motors for power windows, power seats, and heater circulating fans.

Battery and Generator

The primary current source for the electrical system is a 12-volt, 6-cell storage battery of the lead-plate and acid type. Six-volt systems were discontinued by all American car makers in 1956. The battery is kept charged by a d-c generator or an alternator (a-c generator) driven by a V-belt from the engine. Voltage and current regulators automatically increase or decrease the charging rate, depending on the condition of the battery and the electrical load on the system. A signal light on the instrument panel (or, less often, an ammeter) indicates whether the battery is being charged or discharged.

By the late 1950's the growing use of electrical accessories made it advisable to increase the capacity of the automobile generator, particularly at engine-idling speeds. However, because of the destructive high-speed effects of centrifugal force on the segmented commutator of conventional d-c generators, it was considered impractical to increase electrical output simply by increasing generator revolutions per minute (rpm).

The alternator first appeared on an American passenger car in 1960. It offers these advantages over a d-c generator of comparable performance: (1) a basic design that permits safe operation at high rpm; (2) half the size and weight; and (3) fewer parts. The alternator's a-c output is changed to the d-c required by automotive electrical systems by six silicon rectifiers pressed into one end of the alternator housing.

Starting Motor

The starter is a series-wound, high-torque electric motor that drives the engine until it is running on its own power. The starting motor is geared to the flywheel ring gear at a ratio of approximately 15:1, a reduction that provides sufficient torque for engine-cranking speeds up to 200 rpm. Current practice is to energize the starter through the contacts of a relay, the relay itself being controlled either by the ignition switch, a button on the instrument panel, or the accelerator pedal. When the starting motor begins to turn, a pinion gear on the motor shaft slides axially into engagement with the flywheel ring gear and turns the engine over. Once the engine starts, the pinion gear is automatically disconnected from the flywheel.

Ignition System

The function of the ignition system is to ignite the combustible mixture in each engine cylinder at the instant that will yield approximately maximum power and efficiency from the explosion. Ignition is accomplished by a high-tension spark that jumps a gap between two electrodes in the combustion chamber. The electrodes are contained in a spark plug, which is composed of two parts: a center electrode and an outer steel shell held together by a removable bushing (metal lining) or, more commonly, sealed at the time of manufacture. The outer steel shell, threaded into the cylinder head, is grounded to the engine; the center electrode, consisting of a steel wire surrounded by insulating ceramic, is connected to the high-tension source. The spark gap varies in width from 0.025 to 0.040 inch (0.6-1 mm), measured from the face of the center-electrode wire to a wire extending horizontally from the steel shell. The spark plug must be able to withstand high pressures without leaking, and the insulation between shell and electrode must be of high insulating quality, able to withstand high temperatures and shock loads, and be a good conductor of heat. If heat is not conducted away from the electrodes, pre-ignition may result.

The source of high-tension current is a coil that is sealed in a cylindrical metal case. The coil consists of primary and secondary windings wrapped around a central core of soft iron. When current is passed through the primary winding from the system's 12-volt battery, a strong magnetic field is formed around the windings. Every time the current flow in the primary winding is interrupted, the rapid collapse of the magnetic field induces an electrical potential of up to 20,000 volts in the secondary winding, sufficient to cause a spark to jump between the spark-plug electrodes.

The distributor has two functions: it periodically interrupts the flow of current through the primary winding of the coil, and it delivers high-tension current from the secondary winding to the spark plugs. In effect, the distributor is a rotary switch. Its insulated rotor, mounted atop a vertical shaft, conducts high-tension current from coil to spark plugs at the proper time and in the proper sequence. On the shaft just below the rotor is a cam equipped with as many lobes as the engine has cylinders. By successively opening and closing a set of tungsten breaker points, this cam interrupts the primary circuit at exactly the right instant to deliver a high-tension spark across the electrodes of each spark plug. Breaker points are positioned automatically for various conditions of speed and load (1) by centrifugal weights on the distributor shaft and (2) by an actuating diaphragm under the control of engine intake-manifold vacuum.

The fully transistorized ignition system was introduced as an optional equipment item in 1963. It utilizes a magnetic pulse distributor in which a rotating timer core and magnetic pickup assembly have eliminated the mechanical switching performed by the conventional cam and breaker-point arrangement. A modification of this system retains the cam and breaker points, but the points handle only the low current used for switching a transistor on and off. The transistor, in turn, controls the higher current flowing through the primary winding of the ignition coil. This reduced current flow through the points (which open and close from 10,000 to 12,000 times a mile) minimizes arcing and prolongs the useful life of the tungsten contacts. Other advantages are better cold-weather starting and improved engine performance at high speeds.


The front of every American vehicle displays either two or four headlights. Each assembly (filament, reflector, and lens) is hermetically sealed for better lighting and long lamp life. A control switch, either manual or automatic, also is provided to switch headlamps from low-beam illumination (city driving) to high-beam illumination (country driving). Mounted near the headlight on each side of the vehicle is a combination lamp for parking and turn signaling. (Pre-1963 cars used white-light units; cars built later use amber-light units.) The rear of the vehicle carries a shielded white lamp for license-plate illumination, and red combination tail, stop, and turn-signal lamps. Many cars also are equipped with white backup lamps that operate only when the transmission lever is moved to the reverse position.


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