As stated above, the relay works as a large switch. You would'n need one if you ran a large fog light switch, but nobody wants a battery disconnect mounted to their dash to turn their lights on. You want a small switch that is easy to mount, and easy to flip. When terminal 85 is grounded and 86 has power, It creates an electromagnet inside the relay. This only requires about .3 of an amp of power. Your small switch can easily handle this load. The magnetic field inside the relay pulls against a spring inside the relay, and flips a much larger switch that connects terminal 87 and 30. Most relays can handle around 40 amps of load without a complaint. So basically, you are using a small switch to turn on a bigger switch, which turns on your lights. The beauty of this is that the power to your switch can come from another circuit on your truck (ignition switch, ect...). This will not overload the existing circuit on your truck to turn the relay on. Your relay will then connect your lights to your battery without adding any more load to the ignition circuit (or whatever else you are using to trigger the relay). This also works well on reverse lights, use your small reverse light wire to trigger the relay, run a large power wire (12 ga. or so) to the rear of your truck for power, ground the relay, and wire up some rear lights. This will not burn up your reverse light wires, and the lights will only come on when the truck is in reverse. You could also wire an aux trigger wire on the relay with a switch that is hot all the time. With this setup, they would come on anytime the switch is on, or if the switch is off, they will only come on when the truck is in reverse. Everything clear or do i need more details somewhere?
