Testing Auto Fuses with Ohm Meter

[seagull369]

I was wondering if I could use the (Ohm) continuity buzzer on my multi meter to accurately test for blown automotive ATC blade-style fuses while they're still plugged in the fuse box. I know some use a volt meter to test for voltage present on either end of the nubs on those fuses, but using the buzzer would be faster and give me an audible alert so I wouldn't have to keep looking at the meter every time.

 

[gte718p]

Yes .

 

[APEowner]

No, you'll get false positives on any circuit with low resistance. For example a circuit with a short. A 12v test light is the ideal tool for checking fuses.

 

[exranger06]

I agree with APEowner. A test light is the best tool to use. I highly recommend you buy a test light if you don't already have one; it is by far the most useful tool to troubleshoot electrical problems on a car.

 

[Falcon67]

If you are going to test fuses, pull them. Testing in place can give false positives.

 

[wyliesdiesels]

No, you'll get false positives on any circuit with low resistance. For example a circuit with a short. A 12v test light is the ideal tool for checking fuses.

Huh

If the fuse is blown/open, then there would be no continuity. What does a low resistant circuit have to do with an open fuse?

 

[Sevenhills1952]

Huh



If the fuse is blown/open, then there would be no continuity. What does a low resistant circuit have to do with an open fuse?

There are two problems checking fuses in circuit with an ohmmeter. First is meter damage if there is voltage on fuse under test. Disconnecting car battery will help...unless there's a charged capacitor in that circuit.
Second is you're measuring across that fuse as well as whatever else parallels it. So a fuse could be open, but your meter reads low resistance. You assume fuse is good but you're reading something else.
#1 best pull the fuse, ohms test Rx1.
#2 is test light either side, power on.
I'm Gary, retired electronic technician 43 years.

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[seagull369]

Thanks everyone for the quick replies. The possibility of getting false positives when testing for resistance did cross my mind and thats what made me ask,

I have a test light or 2 up in the attic, which I'll take out of retirement and use. Just one last question: Are all fuses typically energized in the fuse box? I imagine it would be helpful to have the key in the on position before I started checking.

 

[rlitman]

Huh

If the fuse is blown/open, then there would be no continuity. What does a low resistant circuit have to do with an open fuse?

There will be no continuity across the fuse OUT of circuit. IN circuit is a different story.

There are two problems checking fuses in circuit with an ohmmeter. First is meter damage if there is voltage on fuse under test. Disconnecting car battery will help...unless there's a charged capacitor in that circuit.
Second is you're measuring across that fuse as well as whatever else parallels it. So a fuse could be open, but your meter reads low resistance. You assume fuse is good but you're reading something else.
#1 best pull the fuse, ohms test Rx1.
#2 is test light either side, power on.
I'm Gary, retired electronic technician 43 years.

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Best answer here! Yes, voltage across the blown fuse can damage an ohmmeter. If there is potentially voltage on the circuit, you should be measuring voltage.

...Just one last question: Are all fuses typically energized in the fuse box? I imagine it would be helpful to have the key in the on position before I started checking.

That depends on the vehicle. Sometimes an interior fuse box is downstream of an ignition powered relay, and sometimes just a single fuse or breaker is fed by a relay. But more often than not, the fuses are always energized.

 

[APEowner]

...Are all fuses typically energized in the fuse box? I imagine it would be helpful to have the key in the on position before I started checking.

Typically you'll need to have the key on to energize most of the fuses and, in the days of incandescent dash lights, you might need to have the parking lights on and the dash lights turned up all the way to energize the dash light fuse.

 

[MattT]

No, you'll get false positives on any circuit with low resistance. For example a circuit with a short. A 12v test light is the ideal tool for checking fuses.

A 'scope on a rope will also give a false positive on a circuit with a short across a blown fuse.

Just one last question: Are all fuses typically energized in the fuse box? I imagine it would be helpful to have the key in the on position before I started checking.

Newer stuff with blade fuses you'll probably be dealing with a mix of always energized and switched with the key in ACC and RUN.

 

[Citation]

Any decent multimeter should be able to handle voltage across the ohm setting. The freebe HF meters, no but most anything else yes. You won't get a good reading but it won't damage the meter.

 

[Sevenhills1952]

Thanks everyone for the quick replies. The possibility of getting false positives when testing for resistance did cross my mind and thats what made me ask,

I have a test light or 2 up in the attic, which I'll take out of retirement and use. Just one last question: Are all fuses typically energized in the fuse box? I imagine it would be helpful to have the key in the on position before I started checking.

To know what fuses have voltage on them you'd have to see a schematic, know what it's for.

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[Sevenhills1952]

Any decent multimeter should be able to handle voltage across the ohm setting. The freebe HF meters, no but most anything else yes. You won't get a good reading but it won't damage the meter.

If in doubt always, always use voltage scale. Continuity/ohms/resistance testing you never check if there's power. Even disconnecting the battery is not 100% since there may be a charged capacitor somewhere.
Fuses check on lowest (Rx1) scale. Higher scale readings on anything like if you're checking resistor plug wire resistance you don't want fingers touching probes since skin has resistance and gives false reading.
The older analog and some even newer digital meters on Rx1 scale can produce enough current to actually blow a good fuse under test...but that's typically fuses less than 1/4A (250mA) or so.
The beauty of a test light is this...when using any test equipment test IT first! A test light put across battery, be sure it lights. The light itself is a good thing if it's incandescent because it draws current, not much but 1/4 to 1/2 amp. So...it's possible (and I speak from experience) to have a fuse that's not open...but high resistance. A test light loads circuit a little, it's fast and visual. So light one side fuse... It's bright, other side it's same brightness, fuse is good (99+%). Bright one side, dim the other...bad fuse.
Typically automotive I doubt you'd see a less than 1 amp fuse.
Oh...always ohms scale on any meter you should zero it. Short leads together, reading should be zero ohms. If you can adjust it. If no adjustment, on Rx1 short leads and it reads say 0.5 ohms...not to worry...that's the resistance of the leads.
You measure a wire or fuse and it's 0.8 ohms, subtract the 0.5 lead resistance and you know it's 0.3 ohms (fuse, wire, etc. under test).
Hope this is helpful information.

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[mark#3]

the ohms selection of the meter is not made for use on a hot(energized) circuit.I always pulled the fuse out of the circuit, then used the ohm meter to measure continuity.As mentioned the test light is a good way of testing the circuit.Ya, I am just repeating what has already been said.

 

[rlitman]

Any decent multimeter should be able to handle voltage across the ohm setting. The freebe HF meters, no but most anything else yes. You won't get a good reading but it won't damage the meter.

I disagree. The voltage probes on any good meter go through the low current fuse. On a good meter, that will be an expensive ceramic fuse you blow when you make the mistake of trying to read resistance on a circuit under power.

 

[joel63]

#1 best pull the fuse, ohms test Rx1. And be done with it.

 

[grounded-b]

Huh

If the fuse is blown/open, then there would be no continuity. What does a low resistant circuit have to do with an open fuse?

Parallel paths.

If the fuse blows because the load side is shorted to ground, and you put your ohm-meter from the line side to the load side of the fuse, you are now measuring the total resistance of ALL loads connected from +12 to ground.

The more paths, the lower the total resistance.

Plus, you may damage your meter, if it doesn't have voltage protection on the resistance range. There will be 12VDC across a blown fuse which is shorted to ground, if there is power on.

Steve
JW

 

[Wrench97]

Incandescent not a led/lcd Test light is the best way, if it lights on both sides of the fuse it's good. If it lights on only one side it's blown, if it does not light on either side there is no power to it this is when you need to know the circuit design to know if it should or should not have power. A wiring diagram will help BBB industries has most of them for free with a sign up. >https://www.bbbind.com/tsb-wiring-diagrams/

 

[Citation]

I disagree. The voltage probes on any good meter go through the low current fuse. On a good meter, that will be an expensive ceramic fuse you blow when you make the mistake of trying to read resistance on a circuit under power.

So what about meters that have no current measurement?
None of my meters, included the HF freebees test impedance through the low current fuse. Most cannot since they low current fuse is only available to the low current input jack. I'll try to find a video about this later. It shows a Fluke meter set to impedance showing no issues when subjected to the full voltage rating of the meter. The HF meter in the same demo pops... with flames.

 

[PassnThru]

I need more clarification on this. If the key is off - what difference does it make?
No power to the fuse - can it go from point A to point B?
How would a short affect it if I'm not testing against a ground but between the fuse terminals?

 

[DHCrocks]

does your meter have a auto hold function? on my fluke I can put it in DC volts, turn on auto hold and it will give an audible beep when it reads. you can use this to to quickly go through the fuses. if you hear a beep you got voltage.

 

[Citation]

This is a video from a Fluke tour (not me). At about 5:30 into the video they demostrate high voltage on the impedance circuit. A CAT rated meter should be able to safely withstand full CAT voltage on any input mode/jack combination. It is understood that the fuse may blow if that voltage results in excess current in amp mode.

 

[Sevenhills1952]

I'll try to help.

I need more clarification on this. If the key is off - what difference does it make?...Some circuits are energized all the time, even with key off.

No power to the fuse - can it go from point A to point B?... I don't understand question (A to B), however, if a fuse happens to be open then reading ohms or voltage while open fuse is still in place means nothing. This is because you will read ohms (resistance) of whatever parallels that fuse. A simple circuit not so important, a complex circuit there may be multiple paths. Voltage you're reading from ground to either side of fuse. Again, if you're using a high impedance meter it may read 12 volts on both sides of an open fuse. A test light, not being high impedance would be more reliable.

How would a short affect it if I'm not testing against a ground but between the fuse terminals?...again, that depends on the circuit. An example would be an open fuse. You put your meter on ohms and measure 1 ohm resistance. You naturally assume fuse is good. However! One side of fuse has a dead short to ground (a pinched wire say). Other side of fuse goes to power (which is why fuse blew). Key is off, so no power...however...that line goes to things supplied with power when key is on. Low resistance things all in parallel, so one side (pinched wire, no insulation, it's grounded) is 0.1 ohms to ground. Other side has 20 things all in parallel so very low resistance to ground. Your reading means nothing.
Once again...#1...pull fuse ohm it.
#2...incandescent test light across fuse with power on.

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[Sevenhills1952]

This is a video from a Fluke tour (not me). At about 5:30 into the video they demostrate high voltage on the impedance circuit. A CAT rated meter should be able to safely withstand full CAT voltage on any input mode/jack combination. It is understood that the fuse may blow if that voltage results in excess current in amp mode.

Do whatever you're comfortable with. I have Fluke meters. I would never and have absolutely no reason to do that. Key word with " A CAT rated meter should be able to" is "should". There is never ever a logical reason to test resistance/ ohms/ continuity in a live or potentially live circuit.
Ohms test is for testing a wire, fuse, light bulb, relay coil, switch, etc. all by itself.
If I suspect I have a bad circuit breaker in my panel box, I'm removing it FIRST, then check it. I would never ever measure ohms in circuit, even with main breaker thrown.


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[PassnThru]

I'll try to help.

I understand what you are saying - but I think you are digging into the 1% at best that few will encounter.
For the other 99% what's the problem?

 

[marineman]

Testing fuses is 99% more complicated than it needs to be in this thread. They cost 10 cents, when in doubt throw it out.

 

[Citation]

Do whatever you're comfortable with. I have Fluke meters. I would never and have absolutely no reason to do that. Key word with " A CAT rated meter should be able to" is "should". There is never ever a logical reason to test resistance/ ohms/ continuity in a live or potentially live circuit.
Ohms test is for testing a wire, fuse, light bulb, relay coil, switch, etc. all by itself.
If I suspect I have a bad circuit breaker in my panel box, I'm removing it FIRST, then check it. I would never ever measure ohms in circuit, even with main breaker thrown.


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Should as it the meter is not meeting its CAT rating if it can't. So is it going to damage the meter? No. Will it give you a useful reading? No. To be clear, I wasn't suggesting that in circuit testing of the fuse was reliable, only that it wouldn't damage a decent meter.

 

[rlitman]

This is a video from a Fluke tour (not me). At about 5:30 into the video they demostrate high voltage on the impedance circuit. A CAT rated meter should be able to safely withstand full CAT voltage on any input mode/jack combination. It is understood that the fuse may blow if that voltage results in excess current in amp mode.

The video raises some questions for me. Like did the fuse in his meter blow after testing in ohms. We don't see the display after that test. And the units he stated for the good fuse were dead wrong, though I could see that just being a slip of the tongue...

Anyway, just because a meter CAN survive such abuse doesn't mean that it should. More to the point, giving such advice is outright dangerous, and I'm sure there are many people reading this thread, who then pull out a freebie HF meter. That's not likely to blow up in your hands when only 12V is presented to it set to ohms (which is why I wouldn't look down on someone using a freebie HF meter for automotive work), but if you want to use your coupon at HF and report back to us, I'm willing to listen to see what happens. Afterwards, make sure that not only does the meter still appear to work, but that it also still works "accurately" in all modes.

I could liken your advice to advice to use a tablesaw in an unsafe manner, because you shouldn't be using a tablesaw if you don't have a modern SawStop. It just doesn't make sense.

Should as it the meter is not meeting its CAT rating if it can't. So is it going to damage the meter? No. Will it give you a useful reading? No. To be clear, I wasn't suggesting that in circuit testing of the fuse was reliable, only that it wouldn't damage a decent meter.

The category rating is ONLY there to provide for operator safety. Safe (acceptable) failure includes blowing the fuse. And as I see it, practices that could blow the fuse should be avoided through the use of proper test procedures (i.e. proper training).

 

[Citation]

The video raises some questions for me. Like did the fuse in his meter blow after testing in ohms. We don't see the display after that test. And the units he stated for the good fuse were dead wrong, though I could see that just being a slip of the tongue...

Anyway, just because a meter CAN survive such abuse doesn't mean that it should. More to the point, giving such advice is outright dangerous, and I'm sure there are many people reading this thread, who then pull out a freebie HF meter. That's not likely to blow up in your hands when only 12V is presented to it set to ohms (which is why I wouldn't look down on someone using a freebie HF meter for automotive work), but if you want to use your coupon at HF and report back to us, I'm willing to listen to see what happens. Afterwards, make sure that not only does the meter still appear to work, but that it also still works "accurately" in all modes.

I could liken your advice to advice to use a tablesaw in an unsafe manner, because you shouldn't be using a tablesaw if you don't have a modern SawStop. It just doesn't make sense.



The category rating is ONLY there to provide for operator safety. Safe (acceptable) failure includes blowing the fuse. And as I see it, practices that could blow the fuse should be avoided through the use of proper test procedures (i.e. proper training).

The Fluke 113 had no fuses as it had no current range. Which units did he get wrong? BTW, no one is saying that you should try to test impedance in circuit. I'm saying that if your meter is CAT rated it shouldn't harm the meter and, at least with respect to the meter causing harm, is not dangerous. The only reason why that might blow a fuse is if you connect it to the circuit then switch through the low current circuit. That could result in connecting the mA circuit across the open fuse. That could blow the fuse. Not an issue if your meter has dedicated current inputs.

I did test one of my HF freebees in impedance rage while connected to 120V. It kills there meter but no drama. The display fades and it doesn't work again.

You are correct that CAT ratings don't actually say the meter can survive, only that it will not become unsafe. However, any meter mfr worth anything will make the meter so the impedance setting is protected from an input voltage within the stated CAT voltage.

Again I'm correcting the claim that this will damage a meter, not if the reading is useful.

 

[Sevenhills1952]

My point is it's a whole lot easier to get in the habit of safe proper habits than to do something potentially dangerous for the equipment and yourself.
As my late Dad used to say "Err on the side of caution".
Learn the proper technique when doing things, never ever assume a safety device will work 100% of the time. Hospitals are full of injured people who say "that wasn't supposed to happen".


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[alfredeneuman]

This tester https://www.autozone.com/test-scan-...12-volt-pocket-size-circuit-tester/532119_0_0 is ideal for testing fuses without removing them. $3.69
Save the meters for the jobs that need them

 

[Citation]

My point is it's a whole lot easier to get in the habit of safe proper habits than to do something potentially dangerous for the equipment and yourself.
As my late Dad used to say "Err on the side of caution".
Learn the proper technique when doing things, never ever assume a safety device will work 100% of the time. Hospitals are full of injured people who say "that wasn't supposed to happen".


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Nothing wrong with that advice. It doesn't conflict with what I have said.