Tools And Equipment Needed:
- Flashlight (traditional or LED)
- Small flathead screwdriver
- Fuse pulling tool piece or needle-nose pliers
- Multimeter / Voltmeter or test light
- Replacement fuses (if replacing is necessary)
Fuses are designed to stop excess current flow that can overheat circuits, damage equipment, or even cause a fire. To account for normal electrical spikes and surges, vehicle designers typically specify fuses with amp ratings of double the current flow a circuit will see under normal conditions. In this article, we'll examine how to remove, inspect, and test these fuses using electrical test equipment.
If an electrical item on your vehicle stops working, the first thing to do is check the fuse for the circuit that the non-working part runs on. When a fuse has more electricity flowing through it than it can safely handle, a conductive metal strip running through the center is designed to literally break apart to ensure all current is interrupted.
Common Types Of Fuses
"Blade” fuses are commonly found in automotive applications. A conductive (horseshoe-shaped) metal strip runs through an outer plastic housing, and will appear unbroken to the eye if the fuse is good.
Cylindrically shaped "glass” fuses are found in some automotive applications. Also known as "Buss fuses" after the manufacturer Buss, a conductive metal strip runs down the center and will appear unbroken to the eye if the fuse is good.
Older European cars often use "ceramic” fuses built with an outer conductive surface made of copper or other metals that surrounds a ceramic center support piece. The narrow part of the copper piece will break in the middle when the fuse blows.
Visibly Inspecting A Fuse
Using the owner's manual, locate where the fuses are. If you don't have an owner's manual, a quick internet search or call to the manufacturer should produce the information. On many vehicles, fuses may be lined up behind a panel under the dash or on a side of the dash obscured by the driver's door when closed.
On other makes and models, fuses may be contained in a box under the hood. Fuse panels and fuse boxes have a cover piece that may require a small flathead screwdriver to remove. On the back of the cover piece, you'll find a diagram which shows you the position of each fuse so you can locate the one you need to check.
Whatever their design, fuses will be clearly labeled with their ampere ratings. Physically remove the fuse using a small, specially shaped fuse puller tool. The tool serves to clamp around the numbered outer edge of the fuse that you can see. Some automakers include this tool within the fuse box.
Once you've got a grip on the fuse, wiggle it out by pulling gently toward you in the same fashion you'd pull a tooth. If you don't have a fuse-pulling tool, a pair of needle-nose pliers will provide needed grip and maneuverability. Use caution to ensure you do not "short" the metal part of the pliers against a live circuit.
Once the fuse is out, inspect the metal wire piece inside of it. A visibly broken metal strip indicates the fuse has blown and needs to be replaced. Install a new fuse with the same amperage rating - usually the vehicle manufacturer provides extras in the fuse panel. Under no circumstances should a replacement with a lower or greater amp rating be used.
While bad fuses are usually easy to spot by looking at them, don't assume a fuse is good or functioning perfectly if the metal strip appears intact. A fracture in the strip could be obscured out of sight behind the opaque plastic end cap. Testing the fuse with an appropriate tool is recommended in order to be sure.
Electrically Testing A Fuse That Has Been Removed
Testing a fuse out of the vehicle can be done with a multimeter set to "Ohms" or continuity. Touch the red lead to one metal end of the fuse and touch the black lead to the other metal end of the fuse. If the fuse is fine, the multimeter will show a resistance reading of zero (or very close to zero), or "continuity". A blown fuse will result in no reading, also known as an "open circuit".
Electrically Testing A Fuse Without Removing It
If you don't want to break electrical continuity to components a fuse handles by pulling it out, a fuse can be checked while still in place using a voltmeter or a multimeter set to "V" (Volts). Note! Do NOT use a multimeter set to "Ohms" to test an electrically live circuit! With the ignition switch in the ON position, touch or clamp the black test lead to a good known ground.
On the end cap of a blade fuse, you'll see two exposed areas of metal designed for testing. Touch the red test lead to the first metal square, then to the other. You should see a voltage reading in the 12-14 volts range. Readings on both sides of the fuse indicate it is working properly. If the fuse is good but the device controlled by the fuse does not function, then there is a problem somewhere downstream of the fuse. If only one side of the fuse generates a voltage reading, the fuse is bad. If neither side yields a reading, there is no current flowing into the fuse to begin with - indicating an electrical problem upstream of where the fuse is.
Replacing A Fuse
If you've determined a replacement fuse is necessary, gently press a new one of the same amerage into the designated slot and check to see if your dead part is now operative. Fuses blow from time to time, and there's no need for concern if the problem does not reoccur. Should the new fuse blow relatively quickly, more electrical diagnosis will be needed to test and track down the source of the improperly high current.