Items in a vehicle that require the highest amperage, such as air conditioners, engine fans, starters and heater fans use 12-volt relays to keep the high amperage wires under the hood. The higher the amperage, the thicker the wire, and the greater the odds of a fire, should a short in one of these wires occur.
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It is not desirable to run heavy-duty wire into the cabin of a vehicle. Most of the switches are not capable of handling high amperage. The electrical circuit in a car can run from up to 60 amps at the main fuses and 7.5 to 30 amps over the rest of the vehicle.
Anything over 10 amps is usually considered high amperage for the vehicle's interior. This is where the relay comes in. The relay is nothing more than a switch that can be operated remotely. All relays will have at least four terminals, and some may have as many as six. Two of these terminals are the connection from a direct battery source to the object being operated. As long as the relay is not activated, there is no power running to the object.
Once the relay is activated, it simply closes the switch and allows power to run to the object and turn it on. Activation of the relay just takes a switched source of power at very low amperage and a good ground. Although 30 amps may be passing through the relay to the object, it will only take one amp or less to activate it.
Some relays have more than four terminals so that there is more than one switched terminal that can activate it. For example, an engine cooling fan is made to come on when the vehicle comes to a stop and no wind is moving through the radiator, causing the engine temperature to rise. At the same time, the fan is designed to come on continuously when the air conditioning is on, so that it cools the condenser. There are two separate switching sources for the fan relay. When the engine begins to heat up, the coolant temperature sensor signals the computer, and the computer activates the relay by sending 12 volts to it. Now, the air conditioning is turned on, and the computer overrides the coolant temperature sensor and turns the fan on continuously as long as the air conditioner is on.