Car Fuses

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. Often the main and high-Amp fuses are installed closer to the battery. 

Fuse box

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.

A fuse box in the cabin of a 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 (if the local electrical technicians haven't stolen it yet). There are several ways to check fuses. The easiest way is to pull the fuse out and check it visually. 

Comparison of a good and blown fuse

Remember to 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. 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 picture above.

If a fuse is blown, something has shorted the protected circuit. If the problem is not fixed, the fuse will blow again. Spare fuses are also located inside the fuse box. When replacing a fuse, only the proper fuse type can be used. 

Checking a fuse in a car with a multimeter

If you have a multimeter, you can 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. This allows measuring the voltage at each side of the fuse without pulling it out.

A multimeter

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. If there is 12 Volt on one side of the fuse, but there is no voltage at the other side, the fuse is blown.

Some mechanics use a power probe instead of a multimeter. A power probe lights up when there is 12 Volt power.

A power probe

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. 

Common causes of blown fuses are;

1. 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 or door 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.

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Email; vverteautosevcs@gmail.com
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