I've been revising and editing. How is this looking? (I know the picture of the meter did not show up).Caution: Even on our tractors, conditions are present to deliver a lethal electrical shock, so please be careful above all else.What is a meter/VOM/DVOM/multimeter/voltmeter/ohmmeter?
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 includes volt/ohm/current/capacitance, and continuity 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.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
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 0.000001 farad, and pico-farads – pf 0.000000001 farad are the two most common units you will see). Capacitors can be dangerous depending on size. Even the small ignition capacitor on our tractors can deliver a healthy shock if mishandled.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. Whenever the current flowing through a circuit encounters a device which requires some work, the voltage on the output side of that device is lower than the battery voltage due to the work done. This is called voltage drop, and is entirely normal. Understanding Ohm's law, we can calculate what that drop will be if we know the supply voltage (12 votls for this discussion), and the resistance of the component doing the work (remember our field coil). There are abnormal circumstances which cause a voltage drop, which we will get into in a bit.DC -
means direct current. This is the type of current produced by a battery. The voltage level is always constant with respect to its ground, or common path.AC -
alternating current. Think household current. AC voltage is not constant with respect to a ground or common path, but alternates between a positive peak voltage and a negative peak voltage. For the purpose of this discussion, we won’t go any further than that other than to mention this is the type current produced by the alternator of an alternator equipped tractor.Ground -
also referred to as "common", which is really a more descriptive term. This is almost always the frame or engine block where the negative lead of the battery is connected, and is the common reference point for our voltage measurements. I say almost always because there are some tractors and a boatload of English cars and motorcycles out there which have the positive lead of the battery connected to the frame/engine block. It is important to know if the classic machine upon which you are working is a "positive ground" system.Circuit -
this is the complete path of current flow from the positive to negative leads of the battery (ok, negative to positive for all you Brits, but let's not muddle the issue).Open Circuit, or Open -
This occurs when any part of the path in an circuit is broken, and current cannot flow. Many people will say they have a short when turning a switch on, or pushing a button has no effect, but in reality they have an open circuit.Short Circuit, or Short -
Think blown fuses, smoke, heat, etc. In a short circuit situation, the circuit's path has been prematurely connected to ground or common. Short circuits can occur is a wire is pinched or internal components of a particular device have failed. The reason a short circuit blows fuses and melts insulation off wires is because all or part of the resistance in the circuit has been removed by providing a short path to ground. Remember Ohm's law? Lower the resistance in a circuit, and the current increases. More current produces more heat. Wires can only support a certain amount of current, so when overloaded, they overheat and the insulation starts to melt.Proper and safe connections of a meter -
Let me start off with a couple of safety points.
Batteries produce hydrogen gas. Improper use of a multimeter can create sparks and ruin your day. Oh, gasoline fumes can be ignited by that same spark. We can also ruin our meter, or sensitive electronic devices we are testing by connecting one improperly.
Also, I'll take the opportunity to beat my dead horse about cleanliness being next to Godliness when it comes to electrical connections. You have no idea how many times I have solved my own or someone else's electrical issues by simply CLEANING AND TIGHTENING EVERY DOGGONE ELECTRICAL CONNECTION ON THE TRACTOR.
I'll use my son's Harbor Freight freebie meter for my example, as many might not be able to afford the $200 Samsung I have in my toolbox.
Here she is. We'll refer to this image a lot. Your meter may or may not be similar, but will have some of the common connections and ranges.
Most DVMs will have a rotary range selection knob as seen in the picture. Some may have an off position on the dial, while others may have an on/off switch.
This meter has a legend showing you where to plug the leads for specific measurements. My black lead always goes into the common port, and my red lead goes into the next port up - the Volts/ohms/milliAmp port. It's pretty rare we need to measure current in series, so I won't even go into using the 10 Amp DC
port. There are inductive current meters which are infinitely safer and easier to use.
If your meter was any sort of decent one, it came with a set of leads which allow you to screw an alligator clip end onto one or both leads. If not, go hit up Radio Shack for a pair. Alligator clips let you fasten a lead into place without having to hold it in place.
Connecting your meter to the component to be measured is often difficult to do if you don't have a helper. If the leads are not firmly connected to the work you will not get accurate readings. Also NEVER use your fingers to hold the metal part of the leads against a component to be tested. At best you can get a bad reading, at worst a nasty or even lethal electrical shock.Measuring resistance -
Starting just above the on/off switch on mine, we see the resistance ranges. An old rule of thumb which is a holdout from my younger days with analog meters is to always start your measurements of resistance with the meter on the highest range scale, then work your way down. We see the max range on mine is 2000 Kilo-ohms. If I see a 1. in the readout, I have an open, or infinite resistance, or I have more resistance than that range of my meter can measure. I can test this by not connecting either lead to anything. If I have a 0 in the readout, I have a short or no resistance. I can test this by touching the two leads to each other.
If I initially show a 0 or nearly 0 in the 2000K ohms range, I should gradually reduce my range to the 200K ohms, 20K ohms, etc. in turn until I get a real reading. If I go all the way down to the 200 ohm range and still get 0, I have a no-fooling dead short.
Now of course, if I know about what resistance I am expecting to find, I can go straight to an appropriate range. For example, if I am testing my PTO
clutch and expect 0.5 ohms, I'll just go straight to the 200 ohm range since I know I will be getting either 0 ohms, infinite resistance, or somewhere around my expected 0.5 ohms.
So what can cause my resistance of my PTO
coil to be other than expected? Well, a coil is just that - a coil of wire wrapped round and around.
In the case of low resistance, it's possible the insulation has come off some of the wires, and effectively shortened the length of the current path through the coil. Remember, shorter wire means less resistance. Oh, and let's go back to Ohms's law. Remember what happens if we reduce resistance? More current can flow, right? So, if my PTO
circuit is fused at 25 amps, and my battery is putting out 12 volts, and the resistance of my PTO
coil is no less than 0.5 ohms, it will allow 24 amps of current flow. Now even if my PTO
coil resistance is off by 0.1 ohm and reading 0.4 ohms, then the coil will allow 30 amps to pass. Since our circuit is fused at 25 amps, we would of course expect the fuse to blow.
If my resistance is higher than expected, I might look at dirty connections. Dirt, crud, and corrosion make it harder for electricity to flow (more resistance). If one or more of the wires in my coil are broken, then I would have infinite resistance.Measuring DC voltage -
Working clockwise around free meter's knob, we come to the DC
voltage range. I suppose I should define a few more terms here as there may be a little confusion on them.
You see my DCV range starts at 200 millivolts (a millivolt is 0.001 volts) and ranges up to 1000 volts. For testing most circuits on a tractor, the 20 volt range is just fine. For testing a tractor's generator
output (we'll talk about tractors with alternators later), I would move up to the 200 volt range just to be on the safe side. There are occasions where I might use the millivolt ranges, but they are a little complicated for this discussion.
Again I will preface the section on technique with a safety warning. You can cause sparking and arcing with carelessness. JDBrian noted he prefers to do the majority of his troubleshooting with the battery disconnected utilizing the Ohmmeter function of his meter (when possible). This is sage advice, but won't work for quite everything. If you are tracing why a fuse blows, I would definitely suggest using resistance testing.
When measuring voltage, I like to have an alligator clip on my negative lead, and have it firmly attached to either the battery, frame, or engine block. Many practiced folks will melt some heat shrink tubing over all but the very tip of the positive lead of their meter to prevent accidental short circuits while troubleshooting.So what am I going to test on the DCV settings?
I might check my battery voltage, as this is one indicator of a battery's charge state. 12.6 volts indicates a fully charged battery, but if you measure right after taking a charger off, it will read higher. Battery voltage alone is not definitive of a good battery however. For an education on batteries, please read www.batteryfaq.org. To properly measure battery voltage, one battery lead or the other should be removed to ensure there is nothing in the circuit causing a voltage drop (which I'll get to in a bit).
Oh, and when disconnecting the battery for safety as directed by a test procedure, remove the negative cable first. (If I do a good enough job here, you will be able to figure out why).
You would also use the DCV function to test the output of your generator as I mentioned.
Finally, I would use the DCV range to test for unexpected voltage drops. While this can be done with the resistance function on non-powered equipment, it's a little easier than doing the math associated that method. The classic example of an unexpected voltage drop is when you turn the key to start, and the solenoid clicks. You have verified the battery is good, tightened the cables, cleaned and inspected your connections, but the solenoid just clicks. We measure voltage at the solenoid while holding the key in the start position and observe 9 volts where we expect 12. We do the classic jumper test and the tractor starts. Something in the starter circuit is robbing us of 3 volts. We replace the ignition switch, and all is suddenly well. Long story short, the ignition switch's internal contacts have become corroded over time. Corrosion adds resistance to a circuit. Adding resistance causes voltage drop. What I should have done before replacing my switch was to have measured DCV at the battery connection of the switch and observed 12 volts. Then I should have measured the solenoid lead of the switch (with the solenoid connected and holding the key in the starter position). I would have observed only 9 volts.
Back to our hose example, lowering pressure (voltage) can keep us from getting the work we need done. You can't spray your second story window if there is a kink in your hose, can you?Measuring AC Voltage
voltage, I am going to set my meter initially to the highest range and work down, unless I know the expected range of voltage of my alternator. Typically, the 200 volt range is more than adequate.
About the only component I need to test on the AC scale is the alternator, so I will focus on that.
voltage, I am not going to connect my negative lead to the negative lead of the battery, rather to the “hot” and “neutral” leads of our AC current producing device (alternator). Here I am looking for the voltage to be in specification, low, or non-existent. Other than engine RPM being too high, I can’t think of a good reason for my alternator voltage to be higher than spec.Other measurements
Since I want to keep this simple, I am going to skip the other 6 functions of my meter. Even this free meter can measure current, test frequency, capacitance, continuity, diodes and transistors. You can read the manual which came with your meter, or google the other functions if you want to learn more.Conclusion
Hopefully the reader understands what he or she is actually doing when executing some of the test and diagnostic procedures in their service manuals.
Thank you for reading.
Edited by AverageJoe, September 30, 2012 - 11:10 AM.