How to check a fuse in a car
Fuses are safety devices that protect electric circuits in a car from excessive current (overloading). Without a fuse an overloaded wire could melt or catch a fire. Each circuit has its own fuse. Several larger fuses with high Amp rating protect multiple or high-current circuits, such as electric power steering or radiator fan circuits. There is also at least one main fuse. See this fuse box diagram. Often a main and high-Amp fuses are installed closer to the battery. Modern cars have at least two fuse boxes.
In most cars, one fuse box is installed under the hood; the other is located inside the car.
Fuse removal tool and spare fuses inside the fuse box cover
When any electrical component in a car doesn't work, the fist step is to check the fuse that protects the circuit with this component. You can find the fuse map in the owner's manual, or on the fuse box cover. In many cars there is a tool to pull the fuses that might be located in the fuse box or in the fuse box cover. There are several ways to check fuses. The easiest way is to pull the fuse out and check it visually.
How to check a fuse visually
For example, we are going to check the fuse for the front accessory power socket, also known as the cigarette lighter socket. It stopped working. It's the fuse that blows the most often. In this car in the photo, the inside fuse panel is installed right above the driver's kick panel. We found the tool to pull fuses inside the fuse box in the engine compartment.
Turn the key to OFF position. Before pulling the fuse, It's always a good idea to mark its position, so you can install it on the same spot. We take the tool and pull the fuse with it. This fuse is blown (on the right side in the image). Fuses have a thin metal conductor inside that melts when the current is higher that the fuse rating. In this blown fuse the conductor is melted, see the photo.
If a fuse is blown, something has shorted the protected circuit. If the problem is not fixed, the fuse will blow again. In this car, it was a small screw that fell into the front cigarette lighter socket. Spare fuses in this car are also located inside the fuse box cover in the fuse box under the hood. When replacing a fuse, only the proper fuse type can be used. This one is a 15-Amp fuse; in most cars, it comes blue.
This main fuse is OK.
Some types of fuses, like these low-profile mini fuses in the photo above are universal and can be purchased in any auto parts stores. Larger fuses or panels with several fuses might need to be ordered from a dealer.
How to check a main fuse
All cars have at least one main fuse or fusible link. It's usually installed at the positive battery terminal or in the fuse box, connected to the battery positive cable. Often a main fuse blows when accidentally touching the wrong battery terminal when boosting a dead battery. The symptom of a blown main fuse is no power and no lights inside the car. Checking the main fuse is easy, it's usually clearly visible if it's blown. If the main fuse blew, there is a chance that a few other smaller fuses are blown too.
How to check a fuse with a multimeter
If one side shows 12 Volt, white the other shows no voltage, you found the blown fuse. This method works well, when many fuses need to be checked at once. Some mechanics use a power probe instead of a multimeter. A power probe lights up when there is 12 Volt power.
How to check fuse resistance with a multimeter
If you already pulled the fuse out, but it's not clear if it's blown or not, you can check its resistance. The resistance is opposite of the current flow. The lower the resistance, the higher the current flow. The resistance is measured in Ohms or Ω. A conductor, like a cooper or aluminum wire has very low resistance (close to 0 Ω). A good fuse will show 0 (or close to 0) Ohm, see the right image. In other words, there is a continuity between two pins (blades) of the fuse. A blown fuse will show very high resistance (infinity), see the left image. To measure resistance of any electric component, it has to be disconnected from the electric circuit. You can't measure resistance, while the component is plugged in or powered. Switch the multimeter to Ohms and connect the probes as in the photo.
What can cause a fuse to blow?
A fuse protects the circuit from higher current than the circuit can handle. If a fuse is blown, it means there is a short somewhere, whether between two wires or the power wire and the ground (car body). A fuse can also blow if a component draws a higher current than it's designed for. For example, if a wiper motor or a blower motor is jammed when powered on, it will draw higher electrical current and possibly pop the fuse. The same can happen if a winding shorted inside the motor. We noticed a few common problems that cause blown fuses in many cars:
1. The most common case is when a metal object (e.g. coin) falls inside or a shorted accessory is plugged into the front accessory power socket (cigarette lighter socket).
2. The wire harness that goes into the trunk lid or liftgate breaks in the place where it bends, shorts and pops the fuse related to taillights or brake lights.
3. A wrong bulb installed in one of the headlights or taillights can also pop the fuse.
4. A damaged or rubbed through trailer harness shorts out.
5. A wiring harness that is connected to some component inside the engine bay rubs through and shorts the fuse. In some older Mercedes-Benz cars, for example, it was a fairly common problem when an insulation on the engine wiring harness cracks shorting the wires.
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