Electric Fans and Relays

Burning down your dashboard does nothing for your reputation, not to mention your old Chevy, and connecting high-amperage cooling fans to an under-dash power source is asking for a meltdown. One solution is running rigid conduit and zero-gauge welding cable to a knife switch from Frankenstein's lab. Or, you could just run a small 14- or 18- gauge wire to a micro switch and use a relay to handle the load.

The idea behind fan relays, or any other relay for that matter, is to run the largest amount of current to the load as possible. To do this, the shortest possible amount of heavy-gauge wire must be used between the battery and the load. The role a relay plays is simple: It acts as a heavy-duty conductor between the fan and the battery. A switch inside the car operates the relay instead of the fan and carries a small amount of current (0.08 amp) to trip it. This accomplishes several things:

1. Long lengths of heavy-gauge wire cause a substantial voltage drop; a shorter wire between the battery and the fan reduces this drop and improves performance; 2. Control computers and other devices sensitive to large amounts of current can be employed to control the relay; and 3. Any light-gauge wire attached to an under-dash switch can be used to control the relay.

The most difficult part of installing electric fans properly is the wiring, and there are several different ways to wire relays depending on the trigger that's used. Switches used to control relays can supply either a ground or power to the relay to activate it. CHP will take you through a typical electric fan and relay installation and cover basic relay operation to make your job easier.