Alternator  The alternator is an AC generator which incorporates a set of diodes to convert the alternating current into direct current. When the engine is running, it serves as the source of electrical power for the vehicle's accessories and also recharges the battery. Alternators replaced DC generators in the early sixties when technology provided silicon diodes of adequate reliability. Alternators are able to produce power much more efficiently than DC generators.

Circuit Breaker  This is a device which opens (disconnects) a circuit when there is an overload or short, and may be reset manually, or in most cases, reset automatically; a "cycling type" circuit breaker will switch on and off repeatedly while an overload condition exists, while a "non-cycling type" will remain off until the overload or short is repaired.

Current  This term refers to the flow or movement of electricity, like the flow of water in a river. The volume of current is measured in "amps." To measure current flow, it is necessary to open the circuit and insert an ammeter (in series) or use a device which measures current flow by the strength of the magnetic field surrounding the wire.

Diode  For most automotive purposes, a diode is a semiconductor device that functions as an electrical "check valve," allowing current to flow in only one direction.

Drain  This term is used to refer to the current flowing out of the battery when the vehicle is shut down. If a battery repeatedly goes dead overnight or over a couple of days when a vehicle isn't being used (assuming the battery is of good quality) there is an excessive drain. A drain of 60mA or less is normal for PCM, radio and clock memories. The only reliable method for investigating a battery drain concern is to use an ammeter, and it may be necessary to wait up to forty minutes for time delay circuits to shut down before an accurate measurement can be obtained.

Fuse  This is an important device which is basically the weak link in a circuit, protecting the wiring and other devices. A fuse or circuit breaker must be located as close as possible to the power source, otherwise it won't protect very much.

Fusible link  This is a section of small gauge wire, usually with Hypalon insulation, which functions as a fuse. A fusible link should be of a gauge four numbers smaller than the wire it feeds in to.

Generator  Generally used to refer to a direct-current generator, although an alternator may also be accurately referred to as a [AC] generator.

Ignition  Noah Webster defined this term as, "the act of igniting," or, "the state of being ignited." Therefore, the ignition is located in the engine compartment; the place where one puts the key to start the car is the ignition switch. The ignition system includes the spark plugs, the spark plug wires, the coil(s), maybe a distributor, probably an ignition module and some sort of camshaft and/or crankshaft sensor.

Relay A relay is an electrically operated switch, which makes it possible for one circuit to control another circuit; often a weak circuit activates a relay which controls a high-current circuit.

Short  A short is when two (or more) circuits (wires) are connected together when they're not supposed to be. Most of the time a short results in a blown fuse or smoke.

Solenoid  This is an electro-mechanical device in which current flowing through a coil causes an iron core to move from one position to another. Some heavy-duty relays are referred to as, "solenoids," because they function in this manner (Ford starter solenoid).

Open  This is sort-of the opposite of a short. Usually if something's not working at all, there is an open circuit. This is when there is a break in a circuit or something's not connected that should be.

Voltage  This is the measurement of electrical potential. It is analogous to pressure in a hydraulic system.