As a budding electrician or engineer, you may be required to do various testing and troubleshooting tasks with your multimeter.
While measuring electrical quantities like voltage and resistance is a common task, you may also need to use your multimeter to check if a battery is in good condition or whether a fuse is blown or not. And today, we will study the latter: steps on how to test a fuse with a multimeter.
Testing a fuse is a very straightforward and simple task. The basic principle is to check its resistance, which denotes its ability to one, allow the flow of current in a circuit, and two, suppress voltage spikes and surges.
A fuse is a passive component used in electrical and electronic applications prone to voltage spikes. Therefore, an electrician needs to know the basics of fuse testing and replacement.
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Why Test a Fuse?
Most of us have experienced a blown fuse at one time or another and replaced it. Some fuses, though, aren’t cheap, and if a replacement isn’t handy, it means a trip to the auto parts or home improvement store.
As a professional electrician, I have had to test many fuses over the years; it is a standard part of troubleshooting techniques rather than simply replacing fuses that I don’t know for sure are blown.
It is better to test a fuse first to see if it is blown before making a special trip to purchase and install a new one. Testing a fuse to see if it is blown is an effortless task, requiring a minimum of inexpensive tools, and can save both money and time.
A fuse is a small piece of wire inside a special housing designed to burn in half when an electrical overload is present. All we have to do is determine if that wire is still intact.
Some fuses have a small window where the wire can be seen, but the view is generally poor, the wire is often very small, and mistakes can be made. A 30-second test is foolproof and will tell the tale with no possibility of error.
What can cause a fuse to blow?
A fuse protects the circuit from a higher current than the circuit can handle. If a fuse is blown, there is a short somewhere 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 a higher electrical current and possibly pop the fuse. The same can happen if a winding is shorted inside the motor. We noticed a few common problems that cause blown fuses in many cars:
- 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).
- 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.
- A wrong bulb installed in one of the headlights or taillights can also pop the fuse.
- A damaged or rubbed through trailer harness shorts out.
- A wiring harness connected to some component inside the engine bay rubs through and shorts the fuse. For example, in some older Mercedes-Benz cars, it was a fairly common problem when insulation on the engine wiring harness cracks, shorting the wires and popping the fuses.
Testing a Fuse With a Multimeter
A multimeter again has two leads, just like a continuity tester. However, many settings on a multimeter measure amperage, voltage, and resistance in several different ranges. Some multimeters are auto-ranging (no need to choose a range), some are digital, and some are analog meters with a needle to indicate the reading.
With all multimeters, the first step is to set it to measure resistance or Ω. If different ranges are available, choose the lowest range (K means thousand on the dial, so 2K equals 2000) – usually around 200. Like a continuity tester, touch one probe to each contact on a fuse and observe the reading.
A very low reading of 1 ohm or less means the fuse is good; if it is blown, the reading will be infinite, or the maximum the meter will display. An intermediate reading of several ohms probably means you aren’t making good contact; wriggle the probes on the fuse contacts, clean them, and try again.
Find a Multimeter that Can Check the ohms.
You will need a multimeter that has resistance so you can check the ohms. The ohms will tell you if the fuse is open and blown or if it’s closed and working.
Set the Multimeter to 20k
Switch the setting on the multimeter to 20k for the ohms, which measures electrical resistance
Plug-In the Testing Leads
Confirm the testing probes are plugged in to measure the ohms
Test the Testing Probes
Press the tips of the testing lead together to test them. The multimeter should read zero, which means there is little to no resistance. With the tips apart, the multimeter should read 1, which means there is 100% resistance.
Find a Non-Metal Surface
Find a non-metal surface to avoid a false reading.
Remove the Fuse
Remove the fuse from the vehicle so the multimeter doesn’t complete an open electrical circuit and overload.
Inspect the Fuse
Visually check the wire inside the fuse. If the wire is broken, the fuse is blown, and it needs to be replaced. Below is a picture of a usable fuse on the left and a blown a fuse on the right.
Test the Fuse with the Testing Leads
Place a testing probe on each end of the fuse. The multimeter should read close to zero. If it reads 1, the fuse has 100% resistance, is blown, and will need to be replaced. Unless the vehicle is over 30 years old, fuses will either be usable or expendable.
We hope that this simple guide on testing a fuse with a digital multimeter has helped you increase your knowledge about electrical testing. If you have any queries, let us know in the comments below.