Top banner

How to check a fuse in a car

January 03, 2019

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.

Fuse removal tool
Fuse removal tool and spare fuses inside the fuse box cover

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.

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

How to check a fuse in a car

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.

Main fuse in a car
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.

Advertisement - Continue reading below

How to check a fuse with a multimeter

Checking a fuse in a car with a multimeter
If there is 12 Volt at both sides of the fuse, the fuse is OK.

If you have a multimeter, there are two ways to check a fuse. The first way is to measure the voltage at both pins (blades) of the fuse. Small fuses in a car have the top portion of both pins protruding through the top of the fuse, see the photo. This allows measuring the voltage at each side of the fuse without pulling it out.

Set the multimeter to DC (Direct Current). Connect the COM (black) terminal to the negative battery terminal or a metal part that is connected to the vehicle chassis or body. Set the parking brake and turn the ignition to the ON position. The ignition needs to be ON, because with ignition OFF, not all fuses are powered. With the positive probe, check the voltage at both sides of each fuse. A fuse, is just an electrical conductor. If both sides show 12 Volt, the fuse is good.

How to find a blown fuse in a car with a multimeter
If there is 12 Volt on one side of the fuse, but there is no voltage at the other side, the fuse is blown.

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

How to check resistance with a multimeter
In the left photo, the fuse is blown, the multimeter shows OL, which means no continuity or the resistance is higher than can be measured. In the right photo, the fuse is OK, so the multimeter shows 0 Ω or Ohm. Click on the image for a larger view.

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).

Advertisement

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 and popping the fuses.


You might also be interested: