Without electricity, practically the entire modern vehicle fleet would be immobilized. No electricity means no electric starter, headlights or brake lights, or the latest tunes blasting from the radio. Fortunately, automotive electrical systems, including the battery, alternator/generator and wiring, make modern driving convenient and comfortable.
Automotive fuses make the sacrifice to protect thousands of feet of wiring, blowing themselves out to keep high current from melting insulation and causing fires. Usually, something “not working” indicates that you have an electrical problem, perhaps the radio, headlight or window. True, it could be a broken radio, burnt-out headlight bulb or dead window motor, but those are the last things to check and replace. The first thing you should check, no matter what problem you are experiencing, is the fuse for that circuit.
What Does an Automotive Fuse Do?
An automotive fuse is an overload safety device. Under normal circumstances, a certain automotive circuit might carry 5 A — ampere is a measurement of electrical current — while in use. Under certain circumstances, that current varies, so wire gauge is chosen at two or three times the expected load.
In case of overloading, such as a stuck electrical motor or short-circuit, the current in an unprotected circuit would go far beyond the capabilities of the wire to carry it. Theoretically, Ohm’s Law tells us a short-circuit would allow infinite current through our 15 A wire. The overloaded wire would overheat, melting the insulation, possibly the wire itself, as well as possibly melt or ignite surrounding wires or materials.
To prevent such damage from occurring, an automotive fuse is placed in the circuit. The fuse might be rated two times the expected load to allow for fluctuations during use. If something like an overload or short circuit occurs, the softer metal in the fuse overheats and melts, cutting off the rest of the circuit from current. This protects the wire from meltdown and prevents damage and fire in areas surrounding it. Of course, this prevents that circuit from operating again, until the fuse is replaced.
How Do You Check an Automotive Fuse?
Depending on the vehicle, fuses for different circuits could be in various places, such as fuse blocks in the engine bay, behind the dash, in kick panels or in the trunk. Fuse locations for your vehicle are usually in the owner’s manual. Additionally, on the fuse blocks or their covers or nearby, there may be labels with similar information.
First, locate the fuse related to the problem you’re experiencing. Some fuses may be related to multiple circuits, such as the radio and clock or brake lights and turn signals. The owner’s manual usually explains which circuits are affected by each fuse. Once you’ve located the fuse in question, turn the key on or attempt to use the component, then choose one of a few ways to check the fuse.
- Multimeter: A high-impedance digital multimeter (DMM) can be used to check an automotive fuse.
- Available voltage: Set the DMM to measure voltage, in the 12 V range or auto-range, and connect the negative test lead to ground, such as the battery negative terminal or any bare metal. Use the positive test lead to probe both sides of the fuse. A good fuse will measure around 12 V, or more than 13.5 V if the engine is running, on both sides. A blown fuse will show voltage on one side, but zero voltage on the other side.
- Voltage drop: Measure across the fuse, using both positive and negative probes. A good fuse will show 0 V, no voltage drop, and a bad fuse will show 12 V, full voltage drop, which means no voltage is available to the rest of the circuit.
- Continuity test: Remove the fuse and check for continuity from end to end. Set your DMM to measure resistance, Ω, and probe both ends of the fuse. A good fuse will measure near 0 Ω, while a bad fuse will measure OL Ω or ∞ Ω.
- Test light: A high-impedance LED test light is one of the easiest ways to check automotive fuses. Connect the test light’s clips to power and ground, such as in the power outlet or directly to the battery terminals. Then, probe both sides of the fuse. Both sides of a good fuse will show power, usually indicated by a red LED. If the fuse is blown, one side will show power, while the other side will show ground, usually a green LED.
A note on impedance: Impedance is resistance to electrical flow. Auto parts stores still sell old-style incandescent test lights, a simple form of test light with a single ground wire and a positive probe. The light illuminates when current flows through it, which is a good indicator of available voltage. Similarly, old-style analog multimeters can measure voltage well enough, but allow much current to flow through them. In either case, low-impedance electrical test tools allow for high current, which can damage sensitive circuits, and even set off the airbags if you’re probing something you shouldn’t.
- Fuse tester: Auto parts stores may sell automotive fuse testers, alone or part of a fuse assortment kit, which is basically a small voltage drop tester. Some work on live circuits, while others work on unpowered circuits. With this tester, simply touch the probe pins to the back of the fuse, touching the fuse contact points, and look for an LED indicating “good” or “bad.”
- Visual check: We save this one for last because it isn’t really a test, and it isn’t always accurate. To check a fuse visually, remove it from the fuse holder and hold up to the light. Inspect to see that the wire from end to end is intact. Sometimes, blown automotive fuses will be obviously blown, sometimes even burnt. Other times, it may look intact or the wire might be too small to see well. Electrical tests are the best method for checking automotive fuses.
How Do You Replace an Automotive Fuse?
Replacing an automotive fuse is one of the easiest automotive repairs you can do. Some fuses can simply be pulled out by hand, while others might require a fuse puller because they’re in tight spaces.
- Modern blade-type fuses are very common, such as APM, APR, ATC, ATO, ATS and APX, and can be removed with a fuse puller, usually close by in the fuse box.
- Older glass-tube type and Bosch type fuses are round and can be removed with a fuse puller designed for them.
- Needle-nose pliers can be used, but carefully, as you could inadvertently cause a short circuit, which is why fuse pullers are made of non-conducting plastic.
After removing the burnt-out automotive fuse, match it up with a new fuse and install it. Be sure it matches both size and amperage rating, or else it either won’t fit or might cause damage. Don’t use non-fuse items, such as foil, paper clips, or jumper wires, in place of a proper automotive fuse. Using a lower-rated fuse, such as a 5 A instead of 10 A, may work, but might blow under normal load. Using a higher-rated fuse, such as a 15 A instead of a 10 A, will work, but may not sufficiently protect the wiring in case of overloading. If your fuses keep blowing, you have an electrical problem that requires further investigation.