Big Rig Basics

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.

6. Pull the fuse straight out. If it doesn’t protrude far enough to grab it, gently pull it with a narrow pair of pliers.

7. Replace the new fuse by lining up its prongs with the holes in the box floor, then gently force it down into the box. Make sure it’s going in straight before applying pressure, and make sure to push it down firmly.

8. For under-hood fuse boxes, inspect the seal inside the groove in the cover and replace it if it’s broken to prevent water and road salt from damaging electrical circuits.

9. Replace the cover by inserting the prongs through the hinges, rocking it closed and forcing it downward until the attaching clips or locking mechanism latch tightly.


Sean Glen has a 2003 Kenworth W900 with a Caterpillar C16 engine. The truck has only self-resetting circuit breakers. “When a fuse blows, you can see it,” he says. “But there is no way to test a circuit breaker or know it’s bad.” He suggests carrying a duplicate set — one breaker of each amperage rating — in your fuse box. When a breaker starts to constantly reset or seems to have blown out, replace it. If the new breaker does the same thing, you will need to get the wiring problem diagnosed and repaired.


Freightliner of Philadelphia’s Dave Zimmerman and George Bollinger say small-amperage (15-25 amp) replacement fuses purchased in the aftermarket at reputable outlets are normally of equivalent quality to OEM fuses.

However, large circuits or circuit groups are protected by good-size fuses or fusible links that carry as much as 200 amps. These are more complex than standard fuses and work much harder, so when they fail, they need to be removed and tested for continuity with an ohmmeter, which measures electrical resistance . Zimmerman and Bollinger have found the failure rates are higher for aftermarket designs, so they recommend purchasing them only from the truck OEM.

When you’ve had trouble with a circuit blowing fuses only because of operating conditions, as when wiper motors stall with ice on the windshield, Zimmerman and Bollinger say you can often purchase self-resetting breakers to replace the original fuse. These will only work when there is enough room in the fuse box above the fuse panel.

We thank George Bollinger, service manager, and Dave Zimmerman, technician, of Freightliner of Philadelphia for their valuable assistance in the preparation of this article.

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