Regardless of the age of your favorite hot rod, it likely has more electrical items and accessories than ever. And if you own a late model street machine then it most definitely has a lot of electronics and factory relays. Relays are basically an electronic switch. They carry the big electrical load instead of running it through the switch. Here is an example. Let’s say you have a SPAL electric fan that draws 25 amps while running and 30 amps at turn on. You don’t want to run that through some little switch on the dash, so instead you should run it through a relay.
Here’s how that works. The relay has the battery power, a ground going to it. The power for the fan comes off the relay, and the ground goes to the chassis or battery ground. In this configuration, the fan has no way to turn on or off as the relay is not activated. Activating it requires either a ground or power “trigger” which is provided by any switch. The difference between running the fan directly off the switch, and through a relay is that when using a relay the switch only has to send a tiny bit of power to the relay to turn it on. The relay in turn handles the big amperage load that the fan pulls. It’s simple, reliable, and the way to really make your electrical accessories safe.