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Reading An Ohm Meter


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#1 KennyP ONLINE  

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Posted September 29, 2012 - 10:34 AM

I'm no guru on this, but thought if those in the KNOW could offer up some advice on this, we could help others learn to use one. Might be good sticky material, so let's keep this on subject.
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#2 ducky ONLINE  

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Posted September 29, 2012 - 10:43 AM

Kenny, are you just concerned with Ohm Portion of a dvom?

#3 AverageJoe OFFLINE  

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Posted September 29, 2012 - 10:50 AM

It's a good idea, Kenny, but indeed a very broad subject.

Then of course there are analog and digital meters, with different operating characteristics.

Maybe the sticky should start out with a list of the scope of things you want to learn (i.e. specific tasks as related to GTs).

For instance:
Measuring continuity, versus measuring resistance, voltage, voltage drop, currrent draw, and understanding their relationships and effects on each other.

It should probably start out with the basics of electricity and electronics.

Voltage -

Current -

Resistance -

Capacitance -

Watts law -

Ohms Law -

Voltage drop -

Proper and safe connections of a meter -

Settings and ranges of a meter -

Measuring voltage (AC and DC, and knowing the difference)

Measuring current - (safely)

Measuring resistance -

Measuring continuity - (and knowing when and why you should probably measure resistance instead)


How does that sound for a start?

Edited by AverageJoe, September 29, 2012 - 11:03 AM.

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#4 KennyP ONLINE  

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Posted September 29, 2012 - 10:57 AM

It's a good idea, Kenny, but indeed a very broad subject.

Then of course there are analog and digital meters, with different operating characteristics.

Maybe the sticky should start out with a list of the scope of things you want to learn (i.e. specific tasks as related to GTs).

Kenny, are you just concerned with Ohm Portion of a dvom?

Most meters are digital that I have seen lately and using one to fix our GT's is mainly what I am after. Ryan313 is having issues with a charging system and didn't understand what the manual meant by 'measuring resistance'. So I thought if we had a thread that would help explain some of this, folks could learn more of what their meter could do for them. Plus, the info could be left here to link others to if needed. If you guys want to break it into different tasks, that is fine with me. I can test something, but I don't always know what the numbers are telling me.
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#5 AverageJoe OFFLINE  

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Posted September 29, 2012 - 11:07 AM

I agree that the vast majority of us probably have digital meters as they are cheap and easy with no periodic calibration required. But we oldtimers who cut our teeth on analogs actually prefer them for most of our classic GT related tasks. I own both a VERY expensive (at the time) Simpson 260 analog meter, as well as a cheapy anaolg from Radio Shack. I prefer either to my moderately expensive Samsung digital for working on tractors.

But, we can focus on digitals if you like.
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#6 AverageJoe OFFLINE  

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Posted September 29, 2012 - 11:28 AM

How about this for a first post:

What is a meter/VOM/DVOM/multimeter/voltmeter/ohmmeter? (I'll leave VTVMs off the list).

When diagnosing electrical problems on our GTs, a measuring device of some sort is required. Often a simple test light to measure presence of some sort of voltage is adequate, as is a simple variation of a test light, often called a continuity tester. This is nothing more than a test light with a battery in series with the bulb, so we can see if a circuit is good end to end.

But most troubleshooting texts will tell us to use a volt meter, amp meter, ohm meter to test certain things. A multimeter is a test device which typically include volt/ohm/current/capacitance, and contuinity test functions. Two basic types are the analog (meter with a needle) and digital (LED or LCD readout). Fewer and fewer folks with less than a half century on this planet know what an analog meter even looks like, so we'll focus on digital meters.

Digital meters range in price from free (I have 3 from Harbor Freight coupons) to a thousand dollars. Expect to pay around $100-$150 for a decent one, though those Harbor Freight freebies will help you get a lot of work done.

Next post, we'll talk about some terms you need to know
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#7 AverageJoe OFFLINE  

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Posted September 29, 2012 - 12:19 PM

What do I need to know before using a Multi-meter?

Some terms we need to be clear on before using our meter include:

Voltage - Why of course everyone knows a volt is a unit of electromotive force, the difference of potential that would carry one ampere of current against one ohm resistance. But in plain English, let's liken a volt to the pressure of water flowing through a garden hose. HIgher pressure is analogous to higher voltage. Pretty simple if we look at it like this.

Current - Measure in amperes (amps), and mill-amps (0.001 amps) most frequently. The ampere is a measure of the amount of electric charge passing a point in an electric circuit per unit time with 6.241 × 1018 electrons, or one coulomb per second constituting one ampere. Alles klar? Or maybe we liken it to the amount of water in gallons per minute flowing through our garden hose. How abou that? Obviously a lrager diameter garden hose can pass more gallons per minute at the same pressure (voltage) than a smaller hose. Hey, that really makes sense, because higher electrical cables which are used in higher current applications are fatter, just like our water hose.

Resistance - When electrons flow through a bulb or another conductor, the conductor does offers some obstruction to the current. This obstruction is called electrical resistance.
  • The longer the conductor higher the resistance.
  • The smaller its area the higher its resistance
Every material has an electrical resistance and it is the reason that the conductor give out heat when the current passes through it. Resistance is measured in units called ohms.

Even with our garden hose example, there is resistance caused by the water running through the hose, rubbing against the sides. If we captured our water coming out of the hose, and pumped it back through, it would eventually become warm. Resistance causes heat. Don't forget this.

Capacitance - Capacitance is the ability of a body to store an electrical charge. Any body or structure that is capable of being charged, either with static electricity or by an electric current exhibits capacitance. I can't think of a really good garden hose example here, but devices on our GTs which act as capacitors include the ignition condensor. Capacitance is measured in farads (micro farads - uf, pico-farads - pf are the two most common units you will see).

Watts law - I learned years ago, Watts law is as easy as PIE (P=IE). A watt is an amount of work done, or the amount of energy required to do a job. Going back ot my hose example, let's say our job is to fill a bucket of water. The total work to be done is to fill a 5 gallon bucket, so the pressure (voltage) times the gallons per minute (current) equals the watts, or amount of work done. As I mentioned earlier, it's as easy as P=IE, or power equals current (I is the standard symbol) times voltage (E is the standard symbol for electromotive force). we can do a little arithmetic and solve I given P and E or solve E given P and I. These come in handy as we progress in our troubleshooting abilities.

In Europe, a car's horsepower is often stated in Kilowatts, or KW.

Ohms Law - Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points. Introducing the constant of proportionality, the resistance. The basic formula for Ohms law is I (current) equals V (voltage) divided by R (resistance).

Remember our garden hose example. Ohms law saws that the current (flow of water in gallons per minute) is equal to the Voltage (pressure of the water going through the hose) divided by the resistance (diameter of the hose). So, basically, if we want to fill a bucket faster, we need more current. We get more current by increasing the pressure (or voltage), or by lowering the resistance of the circuit (using a thicker hose or larger electrical cable).

Voltage drop - The length of our garden hose, height to which we raise the water, etc. can all cause a pressure loss. Same with electricity - the more load or resistance we put on a circuit, the more voltage we lose. This is important to remember for later.

Edited by AverageJoe, September 29, 2012 - 12:42 PM.

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#8 HowardsMF155 OFFLINE  

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Posted September 29, 2012 - 12:24 PM

Personally, I think this is one of those subject that might be better served up on Youtube. Much faster to show, rather than try to tell. I might even volunteer, but I haven't replaced the camera that was ruined by the rain this summer.
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#9 John@Reliable OFFLINE  

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Posted September 29, 2012 - 12:45 PM

I agree really hard to write "how to" use a meter with out being 20 pages long.
I use a couple of them daily for HVAC work, the cheap one was around 200 bucks,and I would be hard pressed to write how it works, and what it can tell you. But with that said, a test light and a basic meter (under 20 bucks) and a few u-tube videos will give plenty of information on how to use one for most gt problems.
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#10 AverageJoe OFFLINE  

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Posted September 29, 2012 - 12:55 PM

I agree really hard to write "how to" use a meter with out being 20 pages long.
I use a couple of them daily for HVAC work, the cheap one was around 200 bucks,and I would be hard pressed to write how it works, and what it can tell you. But with that said, a test light and a basic meter (under 20 bucks) and a few u-tube videos will give plenty of information on how to use one for most gt problems.


I agree a video might be a better way to "get-r-done" but I think it does help to know the what and why of what you are testing. Why for example is it such a bad thing to see a reading of 0.3 ohms when the spec is 0.5 ohms? Two tenths of an ohm shouldn't really matter, right? How do we know what is tolerable unless the book states it? How many lights can I put on my tractor without killing my battery? Why does my battery measure 13 volts after I charge it, but still won't start the tractor? These are the good to know things.
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#11 KennyP ONLINE  

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Posted September 29, 2012 - 01:06 PM

Joe, you are getting across what a video won't. Learning a few basic things can make you understand what that video is saying. Most videos I have seen on this still leave me trying to figure out 'what did they just do'.
Learning to read a meter is one thing. What's that reading is telling you is another. For those of us without the background, we are in the dark.
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#12 DH1 OFFLINE  

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Posted September 29, 2012 - 01:12 PM

I'm no guru on this, but thought if those in the KNOW could offer up some advice on this, we could help others learn to use one. Might be good sticky material, so let's keep this on subject.


I think for using an Ohm meter or Multi meter we only need to be concerned with Ohms and Volts. Keep it simple.

If the manual tells you to measure resistance and you should get ???ohms. Then you need to have the meter hooked up right and be on the right scale to get the right reading.

If your checking voltage then again you need to have the meter hooked up right, AC or DC and be on the right scale.

With using these 2, Ohms and Volts you can pretty much find out all you need to know to test or fix your tractor.
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#13 KennyP ONLINE  

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Posted September 29, 2012 - 01:15 PM

Doug, I understand what you say! But without some basic knowledge of what that is, how do you know what you are reading?

#14 DH1 OFFLINE  

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Posted September 29, 2012 - 01:53 PM

In a nutshell

OHMs is resistance, a length of wire will have practically no resistance, a good connection will have practically no resistance.
Both these will measure 0 ohms you might see a reading on the better more accurate meters but it is practically 0 ohms.
A bad connection will show some resistance.
If your measuring a coil, stator or ignition, and the manual says for example you should get 400 ohms. If you get 378 ohms on you meter it's practically 400 ohms. If you measure 80 ohms or 2,500 ohms then there's something wrong with the part.
Everything has tolerances, nothing is perfect.
Short circuit is very little Ohms,
Open circuit is very high Ohms,

Volts we have AC and DC, need to be on the right settings and know what your looking for, either an AC voltage or an DC voltage.
example, if you testing Battery voltage you need to measure DC voltage and be on the right scale.
12 volt battery need to use a scale that can measure 12volts DC.
Use a 0 to 25 volt scale not a 0 to 250 volt scale.
If your checking for stator output voltage then you need to use an AC volts scale as you looking for about 30 volts AC
Some meters only have 1 DC volts scale, same for AC volts and Ohms

Polarity
DC - has polarity + positive or - negative
AC - has no polarity
Ohms - has no polarity but if you measure resistance of a Diode hooking the meter up 1 way will show very little Ohms, reverse the leads it will show high Ohms.

There is a lot more to it than what I just said but again I think if things are kept simple there easier to understand and use, at least for me.
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#15 coldone OFFLINE  

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Posted September 29, 2012 - 01:54 PM

Measuring resistance. Sounds simple and is simple. Understanding what you are doing and what the numbers mean can be simple.

When you measure resistance your meter uses its battery to put electricity through the object your are measuring. The meter makes one lead +dc (red) and the other -dc(black). When you put the leads across the wire (for example) The meter measures the volts it sends and the volts it recieves to calculate resistance. The current is set by the meter so it is a constant in the equation.

Now that you know what the meter is doing, lets look at what is happening. You want to measure the resistance of a starter coil so you locate Ign and the Ground terminals on the coil. You put one lead on the ign and the other on ground. The meter reads 2.0 ohms. You go online and check to see what that particular coil is supposed to read in resistance. You find it and see that you are within spec. That means the coil is good.

Same senario different reading. You find that the meter dosent show any reading, not 0 (zero) but no reading at all. That means that the coil is open (a wire is broken and current cannot flow) this is a bad coil.

When measuring resistance with GTs most of teh time you will be looking to see either it reads some resistance or if it gives you a "no reading". This is basically a continuity test, it either passes current or it dosent. (GO/NOGO) reading.

With Gts we are mostly seeing if a wire is good or a switch is good, or a coil, or points. At the most basic level these are nothing more than continuity. A coil or stator is where you start to really get into resistance. Both a coil and a stator or nothing more than really long pieces of wire wrapped around something. The wrapps are called windings (like winding a clock). The wire is coated with an insulator (shellac as a generic term). So even though they are stacked on one another they act as a single piece of small wire instead of becoming a giant piece of conductor. When coils/stators overheat the shellac breaks down and allows current to pass from one winding to the next. It dosent have to take the long way around the rest of the windings to get to this point it took a short cut (sound familiar "a short") Because the current took a short cut and skipped abunch of wire in the windings, the resisance reading changed. SO now when you measure the resistance it will not be within spec.
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