testing fuses

[JeepsAreBuilt]

Ok, so every once in a while I gotta trouble shoot a electrical problem, first thing I usally do is check the fuse ! Easy as that huh ? Well, not really.. alot of cars fuse boxes do not label their fuses or are clear on which circuit the fuse is for. Owners manual dose not always clarify this either. So.. I want a time saving method to check fuses quickly and find that "blown" one. How about this gadget ? http://www.tooltopia.com/electronic-specialties-303b.aspx plug it in my voltage meter, set it to ohms and test the fuses one by one. Theres no better way is there ? It even has a overload fuse on the side of this device, so say.. a circuit is shorted, it will blow the fuse and protect the circuit and the gadget will not continue to "jump" the circuit. Only problem is if I put in a 30 amp protector fuse and I jump a 5 amp shorted circuit, will I have a problem here ?

 

[sweetk30]

also simple test light.

clip to ground . turn key on. tip of test light to each pin. if only 1 light out of 2 pins then fuse blown. if no lights out of 2 pin then no power.

 

[usdemt]

Im confused on what you are trying to do here. The link you provided is a lead to measure the amp draw on a circuit. You pull the fuse, replace with the end of that test lead and read the amp output when you set your multimeter to the right setting.

The easiest way I have found to check for the bad fuse is a good flashlight to point through the small windows and look. Or you could always replace your fuses with these http://www.allspectrum.com/store/product_info.php?products_id=555

They arent too spendy and they will provide a small light on the bad fuse.

As far as testing the fuses that tool would work like you said but you would have to pull the fuse to put each contact against the contacts on the test lead and I have only come across one fuse of the thousands I have dealt with that I could not visibly see that it was blown. That fuse wasnt blown but rather a factory defect that did not complete the circuit.

 

[srmofo]

Yup thats a fuse buddy. I opted for the ones with a led screen (around $25). But they test all the way to 30 amps. Most dvoms or dmms only go to 10 amps.

If you pop that meter fuse on anything but a fluke they can be hard to find.

Ive had some fuses that can be difficult to see due to the box location and the tight configuration so I start with a quick visual then just test them all with a test light. It only takes a few secs and if you have ever dug through a car only to find that the fuse really was blown you'll never make that mistake again.

 

[caper]

also simple test light.

clip to ground . turn key on. tip of test light to each pin. if only 1 light out of 2 pins then fuse blown. if no lights out of 2 pin then no power.

Bingo! Takes about thirty seconds to check every fuse in the box.You'll often find more than one blown fuse and make your customer happy when you replace the fuse for something that hasn't been working for awhile that they forgot about.

 

[JeepsAreBuilt]

Ok good tip on the simple test light. However.. if the wire is broken somewhere in the circuit, the fuse could still be good. Also this method requires the key on to have voltage sent though the circuit, and perhaps maybe the circuit is not "on" yet by a switch.

The gadget I posted, I am thinking you can place it right on top of the fuse(while it's still plugged in) and hold it there while you check the multimeter.

For the visual fuse inspection - I guess my eye sight is not good enough for those mini fuses. Also cramped passenger compartments are not easy to get your head up close enough.

 

[caper]

Well you asked about testing fuses not testing the circuit.If you put that device directly on top of a blown fuse it's going to show amps flowing through the circuit like a good fuse because it's going to act like a jumper wire bypassing the fuse.

 

[AutoTech]

The fuse buddy you posted for testing amperage draw not for checking fuses or voltage. The way it works is let's say you keep blowing fuses for your blower motor you plug in the fuse buddy where the fuse goes turn on blower motor and see if it' s pulling to many amps.

 

[caper]

If you hold that device directly on the fuse with the meter on Ohms you run a big chance of blowing the internal fuse in your meter.You said you weren't sure which fuse is which so there's a good chance you'll be trying to check Ohms on a live circuit,meters don't like that.

 

[usdemt]

The device you listed replaces the fuse spot, it is not intended to go on top of the fuse to check it. Like they said earlier odds are you will find at least one fuse that has 12v + even with the ignition off and blow the fuse in your tester.

 

[JeepsAreBuilt]

Thanks for the input.. I have thought of this device in the wrong way. I was thinking the ends of the test fuse act like a test probe and go straight to the meter, and not jump like you guys pointed out.

One other question about using a multimeter for testing fuses at ohm setting. Lets say I test a 30 amp fuse with a 10 amp rated meter. This will only blow my fuse in the multi meter if the fuse that I am testing is blown and that the circuit is on as in fan mower switch is on, correct ?

 

[DrkMtnDew]

to find a blown fuse i like to use the test light method. after i have found it the fuse buddy is nice so you don't blow through a box and a half of fuses.

 

[nissan_crawler]

I don't get what you're trying to do completely, but if you're that worried about it, pull the fuse, and check it with an ohmmeter. You're guaranteed not to screw it up that way.

 

[Ae86 sr5]

also simple test light.

clip to ground . turn key on. tip of test light to each pin. if only 1 light out of 2 pins then fuse blown. if no lights out of 2 pin then no power.

or a power probe

 

[usdemt]

Thanks for the input.. I have thought of this device in the wrong way. I was thinking the ends of the test fuse act like a test probe and go straight to the meter, and not jump like you guys pointed out.

One other question about using a multimeter for testing fuses at ohm setting. Lets say I test a 30 amp fuse with a 10 amp rated meter. This will only blow my fuse in the multi meter if the fuse that I am testing is blown and that the circuit is on as in fan mower switch is on, correct ?

Still not following what your asking. The ohms and amps are completely different. If you are testing a fuse you would put one lead to one side of the fuse and one lead to the other side of the fuse (with the fuse pulled) with your meter on the ohm setting, this tells you if there is continuity between one post to the other. The amperage of the fuse and the meter doesnt matter at all in this situation. Now say you pull the fuse and put the leads to the fuse recipticals or put the leads on the top posts of the fuse and try and measure the resistance (ohms) across the fuse you still may blow your meter as you would receive 12volts from either one side if blown or both if not blown.

Step back and look at what your talking about. Ohms tests resistance, used to check for continuity if the fuse it pulled. Or you could test for the presence of +12v by using a test light with the fuses still in place. But doing either the opposite way is useless.

 

[MrMark]

Now say you pull the fuse and put the leads to the fuse recipticals or put the leads on the top posts of the fuse and try and measure the resistance (ohms) across the fuse you still may blow your meter as you would receive 12volts from either one side if blown or both if not blown.

That isn't going to kill his meter unless current is flowing. If it's an open circuit even with B+ present to ground it isn't going to show anything or harm anything either.

 

[Danglerb]

HF sells a CenTech version for about $13.

 

[usdemt]

That isn't going to kill his meter unless current is flowing. If it's an open circuit even with B+ present to ground it isn't going to show anything or harm anything either.

I know it wont kill his meter but the possibility of blowing the fuse is a possibility. If he was checking from one post of the fuse to the other and the fuse is blown it could flow current through the meter.

 

[MrMark]

I know it wont kill his meter but the possibility of blowing the fuse is a possibility. If he was checking from one post of the fuse to the other and the fuse is blown it could flow current through the meter.

How? If the circuit is open it isn't going to flow any current. I agree that if the load is connected and he puts that meter accross the fuse, whether it is blown or not, and the meter will lose its fuse. You are confusing current and voltage. There's no current in the circuit unless the load is switched on.

 

[larry_g]

How? If the circuit is open it isn't going to flow any current. I agree that if the load is connected and he puts that meter accross the fuse, whether it is blown or not, and the meter will lose its fuse. You are confusing current and voltage. There's no current in the circuit unless the load is switched on.

I have to disagree that puting a meter across a GOOD fuse will blow a meter.

lg
no neat sig line

 

[Lotek]

How? If the circuit is open it isn't going to flow any current. I agree that if the load is connected and he puts that meter across the fuse, whether it is blown or not, and the meter will lose its fuse. You are confusing current and voltage. There's no current in the circuit unless the load is switched on.

Unless there is a short to ground before the switch, or there is an unswitched leg on the circuit. Without a good look at the wiring diagram, not a good idea. Meter fuses can be pricey, I prefer to not use my Fluke as a smoke meter. Quicky preliminary fuse check is with a basic test light, more thorough is with a powerprobe, green light on one side no light on the other means a blown fuse on a circuit that is switched off. You can also power up the circuit to test it. Another advantage of the powerprobe3 is that you can tell if there is a voltage drop in or before the fuseblock, 8 volts will light a testlight, but cause computers to go nuts.

 

[caper]

How? If the circuit is open it isn't going to flow any current. I agree that if the load is connected and he puts that meter accross the fuse, whether it is blown or not, and the meter will lose its fuse. You are confusing current and voltage. There's no current in the circuit unless the load is switched on.

Your right in that nothing will flow until switched on but the OP was saying he didn't know for sure which fuse was for what so he was looking for a quick way to test them.Chances are very good he would end up touching on a fuse which has constant power,even with the key off,which would likely pop the fuse in his meter since when it's on ohms it's like a jumper lead.The owners manual of every meter I've ever owned has said not to try to measure ohms on a live circuit as meter damage is possible.

 

[usdemt]

How? If the circuit is open it isn't going to flow any current. I agree that if the load is connected and he puts that meter accross the fuse, whether it is blown or not, and the meter will lose its fuse. You are confusing current and voltage. There's no current in the circuit unless the load is switched on.

Your right the load would have to be switched on but if the OP doesnt know what fuse is what he may come across one that is on and the fuse is blown, then only one side of the fuse will have power by then using your meter from one terminal to the other you would complete that circuit through your meter. I know what your saying about current and voltage, and I understand both of them very clearly, however he is not sure what is what therefore he might end up finding a fuse that would be drawing current had the fuse not blown.

I think we are both thinking the same thing just not typing our thoughts into the right words.

 

[kylefitz]

This sounds like a simple way to do what you want. Place it across the top if the fuse and it shows continuity.
http://www.amazon.com/dp/B001BXMSMI/?tag=atomicindus08-20
We use them at work and they work great.

 

[MattT]

This sounds like a simple way to do what you want. Place it across the top if the fuse and it shows continuity.
http://www.amazon.com/dp/B001BXMSMI/?tag=atomicindus08-20
We use them at work and they work great.

Does that tester work on live circuits, dead circuits or both? Haven't been able to find a description of how it works anywhere.

 

[MrMark]

Your right the load would have to be switched on but if the OP doesnt know what fuse is what he may come across one that is on and the fuse is blown, then only one side of the fuse will have power by then using your meter from one terminal to the other you would complete that circuit through your meter. I know what your saying about current and voltage, and I understand both of them very clearly, however he is not sure what is what therefore he might end up finding a fuse that would be drawing current had the fuse not blown.

I think we are both thinking the same thing just not typing our thoughts into the right words.

Caper and usdemt, I agree with you that it's a bad idea because he doesn't know what is powered and what isn't. I was just pointing out that it will only blow the fuse when a load is switched on (or there is a downstream short as was pointed out).

Larry G the ohm meter forms a current divider with the fuse when placed in parallel with it. The current divides according to the parallel resistances. In the ohm setting the meter is going to see substantial current that it did not put out when so connected.

 

[MrMark]

Unless there is a short to ground before the switch, or there is an unswitched leg on the circuit. Without a good look at the wiring diagram, not a good idea. Meter fuses can be pricey, I prefer to not use my Fluke as a smoke meter. Quicky preliminary fuse check is with a basic test light, more thorough is with a powerprobe, green light on one side no light on the other means a blown fuse on a circuit that is switched off. You can also power up the circuit to test it. Another advantage of the powerprobe3 is that you can tell if there is a voltage drop in or before the fuseblock, 8 volts will light a testlight, but cause computers to go nuts.

I don't see the distinction between a powerprobe and a test light in this test. The powerprobe IS a test light in this application. If you did not have a test light or a powerprobe you could do the same exact test with a multimeter in V DC setting.

With the multimeter, you would be doing a voltage drop test (the same as the test light or powerprobe is doing) With the multimeter between ground and the fuse pins (on top of the fuse) you check the voltage. Voltage between one pin and ground but not the other shows the discontinuity.

The high imput impedance of the multimeter in the V DC setting allows this test.

 

[srmofo]

power probe 3 has a voltmeter built in. so you can see it only has 8 volts instead of a full 12/14.5. I personally can tell when my light isnt as bright as it should be and will reach for my meter when I smell something fishy.

I dont believe the first 2 pp had the meters. I know the first one didnt.

 

[djb2]

The easiest way I have found to check for the bad fuse is a good flashlight to point through the small windows and look. Or you could always replace your fuses with these http://www.allspectrum.com/store/product_info.php?products_id=555

They arent too spendy and they will provide a small light on the bad fuse.

Those are evil devices.

They put a lamp or LED in parallel with the fuse. When the fuse blows they are supposed to light up.

If the fuse blows the lamp/LED will let just enough current through to make some circuits appear good, yet fail when they need to do something. And without a real (resistive) load, the lamp/LED won't light.

The result is that you get 12V when you measure, and thus not suspect the fuse. And enough current gets through to e.g. light a power-on/backlight LED, so you think the module has failed. On modern cars, it's just enough power for the module to charge its capacitors, answer a few messages on the CAN bus, and then reset from lack of power. And start the cycle over again.

 

[usdemt]

Those are evil devices.

They put a lamp or LED in parallel with the fuse. When the fuse blows they are supposed to light up.

If the fuse blows the lamp/LED will let just enough current through to make some circuits appear good, yet fail when they need to do something. And without a real (resistive) load, the lamp/LED won't light.

The result is that you get 12V when you measure, and thus not suspect the fuse. And enough current gets through to e.g. light a power-on/backlight LED, so you think the module has failed. On modern cars, it's just enough power for the module to charge its capacitors, answer a few messages on the CAN bus, and then reset from lack of power. And start the cycle over again.

Good to know, never used them myself.

 

[larry_g]

Larry G the ohm meter forms a current divider with the fuse when placed in parallel with it. The current divides according to the parallel resistances.
.

And when the fuse is good it is zero ohms and there fore carries all the current.

lg
no neat sig line

 

[MrMark]

And when the fuse is good it is zero ohms and there fore carries all the current.

lg
no neat sig line

No. Google current divider. The meter is not a high impedance parallel load when in Ohm mode.

 

[MrMark]

I'm not sure now after thinking more. The meter is protected between common and V/Ohm's by the high impedance input. In the amps setting is protected by the fuse and the meter would look to be a short circuit in parallel with another short circuit. Current would split equally here.

You may be right that the meter would present high impedance in parallel with essentially a zero resistance fuse and therefore essentially all the current would flow through the fuse.

 

[larry_g]

No. Google current divider. The meter is not a high impedance parallel load when in Ohm mode.

I did go googling and confirmed what I said. The ohm meter will be an infinately larger resistance than the zero ohms of a good fuse. So the current in the circuit will continue to flow the path of least resistance, the fuse, and not through the meter.

lg
no neat sig line

 

[kylefitz]

Does that tester work on live circuits, dead circuits or both? Haven't been able to find a description of how it works anywhere.

It works on live and dead circus. It has a small battery that lights up a LED when the circuit is completed between the two pins on the end.

 

[MrMark]

I did go googling and confirmed what I said. The ohm meter will be an infinately larger resistance than the zero ohms of a good fuse. So the current in the circuit will continue to flow the path of least resistance, the fuse, and not through the meter.

lg
no neat sig line

Current doesn't flow just through the path of least resistance, it flows through ALL available paths, and the meter is certainly not infinitely higher than the fuse, which is not zero, either.

Hooking the meter up in Ohms mode to a live circuit is not recommended by any meter manufacturer and is dangerous. It may be that here, where there would be no potential difference across that fuse (dead short) that the meter would be OK. No voltage across the meter at that connection.

 

[Lotek]

power probe 3 has a voltmeter built in. so you can see it only has 8 volts instead of a full 12/14.5. I personally can tell when my light isnt as bright as it should be and will reach for my meter when I smell something fishy.

I dont believe the first 2 pp had the meters. I know the first one didnt.

Sorry I wasn't clear, pp3 is the only one with the meter, I can tell the difference with a test light too, but then you have to get the voltmeter out to confirm, and you can use the pp to squirt battery voltage or ground into the circuit to see if things start working right. Very handy tool. Nobody borrows my Fluke meter, it is a precision tool and can be damaged by misuse. Powerprobe is more idiot resistant.