How Does a Fuse Block Work?

Location

Electrical gremlins can be some of the hardest things to fix in a car, boat or plane, but sometimes you'll get lucky; it'll be a blown fuse. To find it, look for the fuse block. It's a small, organized panel of fuses, located in a cubbyhole, under the dashboard, or under the hood. The fuse block contacts are connected to a wiring harness, and the harness runs to the various instruments, motors and lights in the electrical system.

Cover

Fuse blocks are usually protected by a heavy plastic cover. This keeps grime and the elements away from the contacts. The cover has a snug fit and may snap or screw closed. Inside the cover may be a diagram or list of the fuse current ratings and what they're connected to.

Multiple Blocks

You may have more than one fuse block in the vehicle. For example, a fuse block under the hood will have fuses for the starter, headlights, A/C, ignition and other local functions. These fuses will be larger and have higher current ratings. One in the passenger compartment has fuses for the heater fan, radio, electric windows and other features local to it. These will have a smaller current rating.

Master

The vehicle may have a separate master fuse for the whole system. This will have a high current rating---50 amps or greater.

Extras

Most fuse blocks have a few spare fuses and a fuse puller. The puller is a little tool that lets you remove the fuse easily; it's hard to get them out with just your fingers. Remember to put the fuse puller back after use. If you use the spares, remember to get some more at an auto parts store. If the fuse puller is missing, and your fingers can't get a grip, long-nose pliers will work.

Fuse Types

SAE or blade-type fuses were introduced in the late 1970s. You can find them at any auto parts store and some hardware stores. They're a fuse in a slim, transparent plastic plug. Earlier fuses, like the 3AG glass-tube type, would be found in older vehicles. Either way, you can easily tell if a fuse is good or blown.

Oxidation

Despite the plastic cover, dirt and oxidation can build up on the fuse contacts, making the connection weak. The fuse is still good. To clean the contacts, all you need to do is remove and reseat the fuse. The friction from pulling and pushing the fuse cleans the oxidation, and makes for a better connection. It takes a while for the oxidation to form, so if your car is several years old and having electrical problems, this is worth a try.

Relays

Relays are small boxy plastic plugs sometimes located in the fuse block. These are miniature switches that route power to parts of the car such as the horn or electric windows. They sometimes go bad, and, like the fuses, are easily replaced. Unlike the fuses, the relays may not be in clear plastic; it will have a part number or miniature diagram on it. Spare relays will probably not be provided.