Automobiles have an enormous amount of wire in them. Wire runs from the front headlights to the rear tail lights. The starter, alternator, air conditioner, power seats, radio and many other components are all connected with wires and switches. These connections are called circuits. Various methods are used to channel electricity through these circuits and protect them – fuses, relays, circuit breakers and fusible links.
Fuses are the most common and numerous method of circuit protection in an automobile. Older vehicles used glass fuses. Modern vehicles use plastic spade-type fuses. The fuse has a relatively thin filament that will melt when too much current flows through it. At some point, you have probably encountered a “burnt” fuse. Fuses are rated for a certain amperage and are used mainly on the lights, wipers, radio and other accessories. Even a momentary current surge will blow a fuse.
Relays are used to activate a circuit that requires more current than can be provided by a switch. Automobiles use relays in several applications, the most notable being the fuel pump relay on a fuel-injected vehicle. Headlights and fog lights are other components that use relays. Most automotive relays are rated for 30 amps. They are usually located in a panel in the engine compartment. This actually aids in power distribution because all the relays can be grouped together.
Circuit breakers are used in applications that require a lot of current such as power seats and windows. They do not break the circuit under momentary current surges such as a fuse but will “trip” if the current draw becomes too high. For instance, if you were to move your power seat forward while trying to hold it back with your legs, the excessive current draw would trip the breaker. They are similar to the circuit breakers in your house, except you have to manually reset residential breakers. Automotive circuit breakers generally reset themselves after they cool down. Circuit breakers can be hard to find. They may be in or or the fuse block in the engine compartment, or close to the part they protect.
A fusible link is a special type of wire that protects a circuit that has constant high current requirements and must resist high surges in the circuit. The circuit between the alternator and battery is a good example of a circuit where a fusible link would be used. Fusible links are always smaller in size than the wire of the circuit in which they are installed. For instance, for a circuit with 10 gauge wire, the fusible link is a gauge or two smaller than the circuit's gauge. Find a fusible link on older cars very close to the battery. When a fusible link melts, the vehicle will normally not have any power whatsoever. A melted fusible link can be tricky to detect because the wire will melt, but the insulation may appear unaffected. Most new cars use "Maxi" fuses in place of fusible links.
