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Replacing FusesNotes From the Road

Replacing Fuses
By Tom Morr/

Automotive Circuit Protection -
Electrical problems can present one of the biggest troubleshooting challenges in automobiles. Unlike flat tires or other easily visible problems, electrical shortcomings can be tough to track down.

Fuse Function
Fuses are one electrical bug that the average motorist can diagnose and fix. Although many auto-electrical problems are better left to professionals in these days of computer-controlled vehicles, fuses are one common electrical bug that the average motorist can diagnose and fix. Basically, fuses are an intentional weak link in an electrical circuit. If too much current flows through the circuit for whatever reason, the fuse is designed to "blow" and stop the flow before electrical components are damaged, wires melt and cause a fire, and/or other nasty things. If a car's electrical accessory (such as the horn or radio) mysteriously stops working, the average motorist can potentially save a trip to the shop by checking the fuses first. Typically, fuse panels or "blocks" are located near the steering column, under or below the glovebox or in the engine compartment.
Locate the panel and look for a blown fuse. These days, most panels are labeled for easy fuse/circuit/device identification.
Locate the panel and look for a blown fuse. These days, most panels are labeled for easy fuse/circuit/device identification.
Fuse Types
Modern cars use either glass or blade-type fuses. Cylindrical-glass fuses aren't as common these days, and the more user-friendly blade-style "ATO" fuses began appearing in the '70s. (The two subspecies of ATO fuses are the early ATM style and smaller ATC configuration, which was introduced in the '90s.) They all perform the same function: When electrical current exceeds the circuit's capacity, this overload causes a wire inside the fuse to pop open, which stops the flow of electricity before further damage can be done.
Two Most Common Fuse Types
The two most common automotive fuses are the glass style (left) and blade-type (known as ATO, ATM or ATC). Glass fuses' amperage ratings are imprinted in tiny numbers on their metal ends. Blade fuses are color-coded for idiot-resistance (i.e., red is always 10 amps and so on).
Fuse diagnosis is straightforward: Look at the wire inside the fuse to see if it's continuous or "blown" open. If it's blown, remove and replace the fuse with one that has the same voltage and amperage rating. (Blade-type fuses are color-coded for easy selection.) Inexpensive plastic tools help remove the fuse without damaging it.
Look for Faulty Fuses
Faulty fuses can usually be spotted while still in the panel. Look for broken wire inside the fuse or discoloration.
Special Tools Can Help Pulling and Installing fuses
Special tools are available that assist in pulling and installing fuses without causing further damage; glass fuses are particularly fragile. Some pullers even have a built-in tester to check difficult-to-eyeball fuses.
Electrical Problems
Unfortunately, the caveat here is that the original fuse blew for a reason. Even worse, fuses normally don't malfunction due to age or wear-and-tear. These are the two prominent causes of auto-electrical problems:
  • Short Circuit: This occurs when the electrical current can't follow its normal path for some reason and flows directly to ground. Because the shorter route offers less resistance, the current can generate heat that degrades wiring and/or damages electrical components.

  • Overload: This occurs when the flow of the electrical current exceeds the wiring and/or equipment's capacity in the circuit. Two causes of overloads are too many components on one circuit or when one of the devices malfunctions. Like circuit breakers, properly functioning fuses protect the circuit by opening, blocking the current flow before it can damage electrical components.
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