Big Rig Basics

John Baxter | April 01, 2011

Main Overload!

Proper fuse maintenance will help you contain the high costs incurred by electrical damage. Here’s how to replace them and ensure continued reliability


Level of difficulty (scale 1-5 with 5 being the hardest job): 2

Tools required: Narrow-jawed or needle-nose pliers (DSCN0790), rags for drying fuse box covers when under the hood

Time: Less than 15 minutes

When something goes wrong in wiring or an accessory, the flow of electrical current can create heat that damages wiring and accessories or even starts a fire. Fuses contain a soft conductor that melts easily. When a short circuit or overload happens, the fuse will quickly melt and break the circuit, killing power to it before there is too much heat.

Self-resetting circuit breakers are often substituted for fuses. They work like a thermostat, reacting to the heat created when current flows through them. Since wiring can handle an intermittent overload without overheating, these devices “reset” themselves after a short cooling off period, switching back on.

The latest vehicles may have small fuse boxes distributed around the vehicle, often incorporating electronic monitoring and control of circuits for protection, with fuses as backup. When a circuit fed by one of these starts to shut off intermittently, the problem is in the wiring, not a blown fuse. A

You can tell a fuse is blown by looking through the clear plastic B. You’ll see solid, bright metal when it’s OK and a separation in the metal when it’s blown. When a fuse blows, Freightliner of Philadelphia Service Manager George Bollinger says to suspect electrical trouble. But since fuses are softer than wiring and can fail, and they’re inexpensive, it’s worthwhile to try one replacement to see if that fixes the problem. If the replacement fuse blows, get the problem diagnosed and repaired, says Dave Zimmerman, service technician for Freightliner of Philadelphia.

Here’s how it’s done:

1. If the fuse box is under the hood, wipe dry the cover first. If part of the dash, wipe up dust or accumulated liquids around the cover.

2. Release the clips that attach the cover by squeezing them together or depress the cover release. Rotate the cover away from the attaching clips or cover release and unhook it from its hinges.

3. Look up the amperage rating of the fuse. This will be listed inside the fuse box lid, outside the lid or in the owner’s manual.

4. Get a fuse of the same amp rating. Using a fuse that’s even a few amps higher than the circuit’s capacity can cause serious trouble. Don’t attempt to bypass the fuse with anything other than a fuse.

5. Turn off the ignition, or if dealing with a circuit not controlled by the key, turn off all accessories on that circuit. strives to maintain an open forum for reader opinions. Click here to read our comment policy.