Automotive AC – Fuses

Fuses protect electrical circuits from excessive current flow and are located in fuse panels and power distribution centers. If a circuit has a surge of high current, then the small metallic bar inside the fuse melts to cause an open circuit. It is easy to replace a blown fuse when one is found. The correct amperage fuse should always be used. Adding a fuse that is higher than the manufacturer rating poses a risk for an electrical fire. See the following steps for methods of testing fuses.

12-volt Unpowered Test Light Ground the lead of the test light to a known good body, chassis, or battery ground. Turn on the ignition switch and touch both metal tabs on every fuse in the fuse boxes and power distribution center. Good fuses will cause the test light to light up at both tabs. A blown fuse will only light the test light on one tab. If the light does not light up on either tab, then another test technique can be used.

Digital Multimeter (DMM) Voltmeter Turn the DMM to DC volts and ground the black lead to a known good body, chassis, or battery ground. Turn on the ignition switch and touch both metal tabs on every fuse on the vehicle. Good fuses will have voltage available at both tabs and blown fuses will only have voltage on one of the tabs. If there is no voltage on either tab, then another technique can be used.

Power Probe Connect the Power Probe to power and ground. Turn on the ignition switch and touch both metal tabs on every fuse on the vehicle. Good fuses will have voltage available at both tabs, and blown fuses will only have voltage on one of the tabs.

DMM Ohmmeter Disconnect the vehicle battery. Turn the DMM to the ohmmeter setting and then touch both tabs on every fuse on the vehicle. Good fuses will have very low resistance and blown fuses will have an OL reading, which means that the fuse is open.

DMM Continuity Disconnect the vehicle battery. Turn the DMM to the continuity setting and then touch both tabs of every fuse on the vehicle. Good fuses will cause the meter to make an audible beep to show a complete circuit, and blown fuses will give no audible response.

Fuses are available in many physical sizes as well as amperage ratings. Items from left to right are: a mini fuse, a standard ATC-type fuse, a maxi fuse, a fuse element, a fuse cartridge, and a fusible link. Fuses generally range in amp ratings from 2 amps up to more than 100 amps and are used in accordance with the expected load of the circuit that they will be

The power distribution center is located under the hood and it houses the electrical components in a convenient location that is usually easily accessible. This power distribution center has mini fuses, cartridge fuses, and relays. The electrical components can easily be tested and serviced in this central]

A fuse is designed to be the weak link in an electrical circuit to protect the system against too much amperage. The metallic strip inside the fuse will get hot and melt to create an open circuit when the amperage in the circuit rises above the amperage rating of the fuse. A blown fuse will show a break in the metallic strip inside the fuse. The continuity of the fuse should always be tested if a fuse is suspected to be blown but the metallic strip appears to be

Fuses can be tested with a 12-volt unpowered test light by grounding the test light at a known good ground and then touching both metallic tabs on each fuse. A good fuse will cause the test light to illuminate at both test points. An open fuse will only illuminate the test light at one of the points. The ignition switch needs to be on when testing

The fuse panel is located inside the vehicle and it contains mini fuses of various amperages. Some fuse panels will also have some relays, depending on the manufacturer. Having the fuses located in this central location provides a way to quickly test all of the fuses without having to search for each fuse in a different

A blown fuse will often appear to be burnt in the area of the metallic strip. This is caused by the strip melting when the current is increased to levels that exceed the rating of the

Fuses can be tested with a digital voltmeter by connecting the ground lead of the meter to a known good ground and then touching both metallic tabs on each fuse with the probe of the red lead. A good fuse will have voltage at both test points, but an open fuse will only have voltage at one of the points. The ignition switch needs to be on when testing

Fuses can be tested with an electronic testing device, such as a Power Probe. This tool needs to be connected to power and ground and then it can be used to measure voltage at the tip of the tool. With the ignition switch turned on, the tip of this tool needs to contact both metallic tabs on each fuse. A good fuse will have voltage at both test points, but an open fuse will only have voltage at one of the points. This tool also has a light that projects toward the tip, which is handy when using the tool in dark

Fuses can be tested with an ohmmeter without removing them from their mounting location. After disconnecting the battery, the ohmmeter leads should be inserted at the metallic tabs on each fuse to measure the resistance of the device. A good fuse will have a very low reading on the meter, and a bad fuse will have an OL reading on the meter, which means the fuse is blown

Fuses can be tested with the continuity tester on the digital multimeter without removing them from their mounting location. After disconnecting the battery, the test leads of the meter should be inserted at the metallic tabs on each fuse to test for continuity of the fuse. A good fuse will cause the meter to make an audible sound as well as have a 0.0 volt reading on the display. A bad fuse will cause the meter to have an OL volt reading on the display. No audible sound will be present when testing a bad

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