garfunkle24
Well-known member
Enough said. Do you, and if so how big of a cable would you be able to check with just an ohmeter?
Thanks
Thanks
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Does anyone hdere check ohms of a cable
- Thread starter garfunkle24
- Start date
Enough said. Do you, and if so how big of a cable would you be able to check with just an ohmeter?
Thanks
Yes.
Any size you want. All cables have some resistence.
OP
garfunkle24
Well-known member
yes they do, but a multimeter ohms test is useless
yes they do, but a multimeter ohms test is useless
Can you explain why?
OP
garfunkle24
Well-known member
Sure I can. Your multimeter checks resistance by pushing a tiny, tiny, current. On a 4ga cable you only need one good strand and your meter will read perfect resistance, Try and put the cranking amps through it.....no dice. What you're needing is a voltage drop test.
I've been surprised as to how many don't know about or use this simple test.
I've been surprised as to how many don't know about or use this simple test.
tonydanzah
Well-known member
Depends on the circuit, you can often use a multimeter and wiggle the wire if you dont have simpson handy.
Measuring the difference between 10 feet of 16 gauge wire vs 10 feet of 12 gauge wire with a typical dmm isn't likely. Things like contact resistance, or the resistance of the wires in the probe leads is larger than the resistance of the wires to be tested.
Very low resistance is measured if I remember correctly by a bridge made to just for low ohms.
Very low resistance is measured if I remember correctly by a bridge made to just for low ohms.
TheGrooveking
Well-known member
For checking internal corrosion a good test is capacitance test, that will tell you if the internal strands are in contact and how much.
TheGrooveking
TheGrooveking
jimexcursion
Well-known member
So how do you perform a voltage drop test or a capacitance test?
Mickey O
Well-known member
You can check any size wire for continuity with an ohm meter, if you want to test the insulation for damage you use a megger. I don't see a strand breaking and causing problems, it would arc and reattach or damage the insulation and that would be found with a megger.
TheGrooveking
Well-known member
Arcing within a cable will cause heat, to get the cable hot enough to reattach you would melt the insulation.
TheGrooveking
TheGrooveking
Mickey O
Well-known member
Arcing within a cable will cause heat, to get the cable hot enough to reattach you would melt the insulation.
TheGrooveking
Exactly, if there is a break in a wire in a cable that is bad enough to cause a severe voltage drop it will arc and either reattach or damage the insulation and that's where a megger will come in.
Here's a cheap way to test to the milli-ohm level:
http://www.matronics.com/aeroelectric/articles/grnding.pdf
http://www.matronics.com/aeroelectric/articles/grnding.pdf
speed bump
Well-known member
Sure I can. Your multimeter checks resistance by pushing a tiny, tiny, current. On a 4ga cable you only need one good strand and your meter will read perfect resistance, Try and put the cranking amps through it.....no dice. What you're needing is a voltage drop test.
I've been surprised as to how many don't know about or use this simple test.
???? you have a voltage drop because v=IR and actually if you have a good multimeter (generally an analog one) your meter will read a difference. All you are saying is because my cheapy digital meter cant read very small changes in resistance I need to throw a lot of current at something so I can possibly burn it up (p=I^2R) to overcome my resolution problem.
Teken
Well-known member
Ideally if a person was to measure resistance both ends of the cable would be terminated with a pin, plug jack, etc. When you're measuring a bare cable and there are lots of strands in the cable this only tells you that you have continuity on one or more of the strands.
The end result would show continuity, this does not address resistance, or capable current load on said wire.
I have worked on vehicles that have been at least 20-30 years old and have found battery terminals, to wiring harness completely green with rust. Measuring the resistance in this case would determine that there is, and would be a large voltage drop.
Now, if you're measuring complex circuits say with a relay, many of a technician have been fooled only measuring resistance, continuity, but not the actual voltage drop while the circuit is live . . .
That is why often times relays are misdiagnosed as being a problem, and test out OK, but under load their resistance increases . . .
Just in case someone new is reading this some basic's should be stated.
Continuity = Measures a wire from A-B point, if you do not receive confirmation of continuity you either have a separation in the wire, or it is on the edge of just breaking away, but it is being held in place by the insulation. But, when there is movement or vibration it causes wire to flex then separate thus causing a *open condition*
Resistance = Can only be measured with out voltage applied. Setting your meter to the correct values is paramount assuming your meter even reads to the lowest levels say milli ohms . . . When measuring resistance its always best practice to zero out your meter and verify that your test leads are 100% and clean. Setting it for a beeping tone for continuity while watching the display should result in a beep, and display 0 ohms.
There are times you will see it does not show zero when you do this, this is OK. That is the whole point of you doing this test, and determining what tolerance your meter is capable of resolving.
If the said meter always displays 1 ohm, then that is your reference to subtract that difference from the measured device. When you're working on delicate electronics that is the difference in a good to go, or no go situation as all makers have tolerances.
Current: Unless you have clamp on meter which does not require physical separation of the circuit. You will have to turn off the power to the device. Configure your DMM to measure ampere's, break open the circuit under test, and insert your test leads *observing correct polarity* and the limits of the DMM, and then turn back on the power, then activate the said device and measure the current passing through the live circuit.
I have to eat lunch now . . .
The end result would show continuity, this does not address resistance, or capable current load on said wire.
I have worked on vehicles that have been at least 20-30 years old and have found battery terminals, to wiring harness completely green with rust. Measuring the resistance in this case would determine that there is, and would be a large voltage drop.
Now, if you're measuring complex circuits say with a relay, many of a technician have been fooled only measuring resistance, continuity, but not the actual voltage drop while the circuit is live . . .
That is why often times relays are misdiagnosed as being a problem, and test out OK, but under load their resistance increases . . .
Just in case someone new is reading this some basic's should be stated.
Continuity = Measures a wire from A-B point, if you do not receive confirmation of continuity you either have a separation in the wire, or it is on the edge of just breaking away, but it is being held in place by the insulation. But, when there is movement or vibration it causes wire to flex then separate thus causing a *open condition*
Resistance = Can only be measured with out voltage applied. Setting your meter to the correct values is paramount assuming your meter even reads to the lowest levels say milli ohms . . . When measuring resistance its always best practice to zero out your meter and verify that your test leads are 100% and clean. Setting it for a beeping tone for continuity while watching the display should result in a beep, and display 0 ohms.
There are times you will see it does not show zero when you do this, this is OK. That is the whole point of you doing this test, and determining what tolerance your meter is capable of resolving.
If the said meter always displays 1 ohm, then that is your reference to subtract that difference from the measured device. When you're working on delicate electronics that is the difference in a good to go, or no go situation as all makers have tolerances.
Current: Unless you have clamp on meter which does not require physical separation of the circuit. You will have to turn off the power to the device. Configure your DMM to measure ampere's, break open the circuit under test, and insert your test leads *observing correct polarity* and the limits of the DMM, and then turn back on the power, then activate the said device and measure the current passing through the live circuit.
I have to eat lunch now . . .