Fuses have been designed to protect the most important parts of an electrical system from overheating and subsequent damage. When a sudden and dangerous wave of current flows through the circuit, the wire inside the fuses "breaks" and interrupts the connection. This important element protects the electrical system of the car and the house, but when it breaks it creates temporary discomfort. With the right tools and a little knowledge in the electrical field, you can quickly check the fuses and evaluate if they need to be replaced.
Steps
Part 1 of 4: Locate the Fuse Box
Step 1. Check the car's manual or look for the box in the most likely places
Many cars have two bays for these elements and there is no standardized place to hold them. You can save time if you check the vehicle manual (some are also available online), so you can know where the fuse that controls the car device that has lost power is located and you can test it directly. If you don't have a manual, then look in these common places for a large box or cluster of exposed fuses.
- Most cars have one or two fuse boxes under the hood, near the engine or battery. There may also be one inside the car, so keep looking if the fuse you need is not in that box.
- More modern models often have a box under the dashboard in an accessible place. Check the storage compartment ceiling to see if there is a hinged door that opens downwards. You may need a flat screwdriver to open the lid.
- Older models have the box open to the left of the brake pedal or parking brake, if the pedal is present. On some cars it's not easy to inspect fuses, so get a hand mirror and a flashlight.
- In rare cases, you can look for it in the trunk or under the back seat.
Step 2. If you need to check the fuses of the home system, look for the box inside the house
It could be in a closet, in the basement, in the laundry room or walled into an external wall. If you live in an apartment building, chances are it's somewhere common.
Step 3. Consult the user manual for other advice
If you need to test the fuses of the centralized motor of the air conditioning or an appliance, refer to the manual to find out where they are. In some cases, you need to turn off the device before you can safely access the fuse box.
Part 2 of 4: Check the Appearance of the Fuse
Step 1. Read the label, if any
Car fuse boxes often have a diagram on the outside or inside of the cover; this is however always described in the maintenance manual. It saves you a lot of time, as you can figure out which fuse is protecting the radio (or broken device) and test it instead of going through 40+ items. Household boxes, on the other hand, do not have a diagram, but do not contain many fuses, so you can test them all.
Find the fuse diagram in the manual or on the box; some are available online and the only thing you need to enter is the model of the car
Step 2. Leave the fuse connected
Do not unplug it for now - it could be dangerous, as the power is still connected, and you could cause damage to the circuit if you remove a working fuse. Look carefully at the appearance of each fuse.
Step 3. Check for broken threads or burn marks
A broken fuse may have a completely normal appearance, but most of the time there are visual signals that guide you towards identifying a blown element. Fuses come in three basic forms:
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Cartridge fuse: it is a transparent cylinder (plastic or glass) with a wire inside. If the wire is broken, then the fuse is blown. If it is completely black or has turned brown, then it may have suffered a rather serious short circuit and the system needs to be overhauled (especially if the fuse was recently replaced).
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Blade fuse: This is a small square element with two points or blades. This type is used a lot on cars and has a U-shaped wire inside that connects the two blades. If the wire is broken the fuse is broken, but it's not that easy to determine.
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Cartridge fuses but constructed of an opaque material (such as ceramic) must be tested in other ways.
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Step 4. Turn off the power and disconnect the fuse located in the house
If you need to test an element of the home electrical system, then you have to turn off the main switch from the electrical panel and remove the fuse to observe it closely. Read the next section if, despite the visual inspection, you are unable to understand if the fuse is blown or not. If you have identified the problem, go directly to the “Replace the Fuse” section.
This step is not recommended for car fuses, as disconnecting the wrong one could cause problems with the engine when idling, accessories or trip computer diagnostic devices
Part 3 of 4: Test the Circuit
With a Phase Finder
Step 1. Purchase a modern phase finder
This tool is available at hardware stores, electronics stores and do-it-yourself centers. Choose a model with an LED bulb inside the handle or with an incandescent bulb powered by a battery or electrical outlet. Never test the wiring of a car with an old phase detector (with the incandescent bulb), as it absorbs electricity from the circuit and could trigger the airbags and cause serious damage to the car.
Alternatively, if you have a multimeter, skip to the next section
Step 2. Check the fuse with the phase finder
Follow these instructions:
- Connect the black ground terminal to a conductive object (any piece of metal).
- Start the engine or check that the main switch of the home system is active, if you are testing a fuse in the house.
- Touch the end of the fuse with the red probe, then touch the other too (in the case of blade fuses, the two ends are the blades themselves).
Step 3. Interpret the results
If the fuse is good, the light bulb turns on at both contacts. If this does not happen and only lights up at one end, then the fuse is broken and needs to be replaced.
If the bulb never turns on, then there is no current in the fuse box, the black cable is not connected to the grounding object or the bulb is burned out. Please fix these problems and repeat the test, or alternatively use a multimeter
With a Multimeter
Step 1. Turn off the power and disconnect the fuse
Turn off the car engine or turn down the main switch of the home system. Remove the fuse by levering one end and then the other. You can use tweezers or thin-nose pliers for this. Sometimes you can find the special tool fixed to the car's fuse box.
If you need to check more than one item, then take a picture so you can correctly reposition the fuses
Step 2. Check the current flow, if possible
Most DMMs can be set with this function, which is labeled with a series of parallel arcs:))). Turn the adjustment knob to this symbol, then attach the probes to the ends of the fuse. If you hear a constant “beep” as you touch the fuse, then it is fully functional. If you don't feel anything, then it's broken.
If your multimeter does not have this setting or you want to check the result, go to the resistance test
Step 3. Set the tester to resistance
The symbol that distinguishes it is the Greek letter omega: Ω. The resistance test sends a small amount of current to the fuse and measures how much current is flowing. The value of the resistor itself is not important because if the fuse is broken you will have no reading as the current will not be able to pass through a broken wire.
If you have an analog multimeter, there are several resistance (Ω) settings. Choose the one identified with the symbol "Ωx1" - but some models may have the word "Rx1" on it
Step 4. Connect the probes to each other and read the number that appears on the display
This value (or the one indicated by the needle of the analog multimeter) tells you what the "zero" level is for the instrument (ie the internal resistance of the multimeter itself). If, when testing the fuse, you get a value very similar to this, then the fuse is perfectly functional.
The instrument may have a knob (in analog models) or a button (in digital ones) that allow you to set the scale with this minimum value. You can do this if you plan to use the tester a lot; but know that it is not essential for this purpose
Step 5. Touch the ends of the fuse with the probes and check the display
If you don't notice any changes while the probes are touching, then you need to change the fuse because it has blown. If the needle moves to a value similar to the one you read with the two contact probes (or a similar number appears on the display), then the fuse is fully functional and can be put back in its housing.
Part 4 of 4: Replace the Fuse
Step 1. Remove the power supply and unplug the fuse
When you remove or put a fuse, make sure that the electricity is off. On a car, this means turning off the engine.
Step 2. Get a new fuse
You can find them in hardware stores, auto parts stores, supermarkets and electronics stores. Bring the burnt one with you, so you can compare its size and characteristics, as you will read below.
Step 3. Choose a fuse that has the same amperage, the same shape and is the same type as the one that has blown
It is essential to replace the damaged element with an identical spare. Even more important is the amperage; it is a number that most of the time is written on the fuse itself: make sure it is identical to the old one. Each type of fuse is designed to break at a certain amperage (this is its function), which is why this value is so important. If you use a replacement with too low a number of amps, it could break during normal use of the device and interrupt the flow of electric current. If you use a replacement with too high amperes, the fuse may not break during a power surge, resulting in damage to important and hard-to-replace parts of the circuit.
Transparent cartridge fuses are available in two types: fast (with a straight wire) and delayed (with a coiled wire). Do not use the delayed one, unless the original fuse you are replacing was of this type; otherwise it would not protect the device quickly enough
Step 4. Insert the new fuse
Modern ones can be placed with light pressure, as they snap in. The old glass ones must be inserted first with one end and then with the other.
Advice
- Fuses, like any other device, fail over time. A broken fuse is not always a symptom of a circuit problem.
- If your car does not start due to a broken fuse, but you need the car to go to the auto parts store, consult the owner's manual and remove a fuse of the same amperage from a non-essential circuit, such as the radio.. Replace it momentarily for the broken one.
- If the replacement fuse blows shortly after installation and you are sure you have used one with the correct amperage, then there may be a serious problem with the electrical system. Call an electrician.
