Technical HOW TO USE A MULTI METER/VOLT METER

This picture may help some out. I use a test light and a multi-meter in all my dealing with electricity. The test light is great for knowing when you have 12 volts in any given wire. Also, if you take that wire that you normally ground when looking for 12 volts and hook it to a known 12 volt source, you can double check your ground spots as well. Not all metal in the car acts the same as others. That is what they call ground paths. A circuit ground wire screwed into a metal spot on the body gets back to the battery in a different "path" than say some other accessory.

Meters will tell you when you shot a screw through a wire and shorted it out (ever had that happen?) able to find current draws on a system etc... Great thread keep it up.

 

You would use the ohms setting and attach one probe wire to the wire in question, then simply start probing the other wires at the end of the bundle in which you think the other end is hiding. If you got a spiffy multi meter, it will beep when you have a dead short- meaning you found the other end of the wire. If no beep, it should read all zeros.

 

PS, if it has not been said yet, do NOT use a test light on a newer computer controlled cars. The bulb introduces too much resistance to some circuits and then it could fry the BCM or other modules.

 

Would love to use that picture in class but the kids would have a hey day making up stories about what Volts is doing to Amps....

 

I still giggle every time I see it....

 

+1. Hopefully your probes are long enough to be able to do this! This is also assuming the wire does not go through any components, or circuits, which add resistance. You can use resistance as a way to tell if things are connected by setting your meter to ohms; the resistance will read extremely low if they are connected - say 100 ohms or less. A lot of meters will not beep on this setting though, and must be set to continuity to make a beeping noise for you - which looks like a triangle followed by a line. All continuity means is: are two connected electrically; it would have to be with something conductive though, such as a wire.

Remember for these two tests to work, you need to make sure nothing is powered up. As we know with V=IR, in order to check the resistance you need to know the voltage and the current; so you have to let your meter pump a small voltage through the circuit and it will let you know if both ends are connected. Once again, having a schematic, or even a block diagram of something is a life saver.

 

Use a low scale such as 10 OHMs. Connect one meter lead to a wire then go to the other end of the wire bundle and touch the probe to each wire. The meter will read "infinite" or 10 OHMs until you touch the other end of the wire, it will read "0 OHMs" indicating there is no resistance between the probes.

 

That's pretty funny.

 

Idk about you guys, but I have a set of jumpers so I can get my meter to me no matter where the wires are. I also have a set of dental pics to probe the tiny way too deep plug terminals.

The beeper is a real time saver .

 

OK here is one for you: Tried to start my RV yesterday that has been sitting in a moist forest. Dash lights come on but just a faint click coming from somewhere. So I figure the battery is low (which it should not be), so I change it out, same thing. Clean battery terminals, no change. This has a two battery setup with an electronic switch to charge the accessory battery which is out of the loop during starting. What that means is there are several battery cables and a switch in addition to the typical old Ford starter solenoid on the firewall. And it's all mostly buried in the van front end. I should mention it does not click and go dead the way they often do when you have a bad connection at the battery. Dash lights always come on and just the faint click, not sure if it's the solenoid, starter or that switch. Can't get my ear over there while turning the key.

So I got out a meter and checked resistance on ground cable to frame, positive cable to solenoid, solenoid to starter, etc. all good and same reading. Switched to VDC, 12.5 at battery, down to about 12 with key on (points dist.). Turn to start and only drops another couple tenths.

What I want to know is how to diagnose where the problem lies. Is it possible to have a connection that tests fine (OHMs) with the meter but fails when a higher current (starting) is applied?

 

IMHO anything is POSSIBLE when dealing with electrical systems, but will almost always come back to something pretty straight forward or simple, if you are lucky.
Take a VDC reading at starter cable stud while cranking. Use a jumper wire to meter, if necessary.
If you get full voltage at starter, try shorting solenoid across terminals to jump or "bump" starter. Sometimes this works, but be ready to replace starter or solenoid.

 

Ok, I'll throw some info out there....

First, the tool is only as smart as the guy using it; if you're not sure what your looking at, you can get as much misinformation as information. Now, I prefer to use a meter. Test lights and buzzers/beepers are of limited use in serious troubleshooting as they basically just tell you it's a go/no go situation; the wire/circuit is broken/not broken. Test lights are particularly bad at this, as they can not light under some circumstances (say a high resistance short) that may cause problems. A high resistance, low current short can allow a circuit to work yet have enough 'drain' to discharge your battery. If using a test light, the wattage of the lamp will affect it's 'sensitivity' to detecting problems; a small lamp will do better at showing low voltage/current issues, a larger lamp may not light up. So while a lamp/buzzer can give you a semi-accurate indication, it may not tell the whole story.

For accuracy, you really need a meter. The first thing to realize is the amount of resistance you're looking for will be very small if your not checking a component. The smallest wire you'll find in a vehicle harness (#18) will have a resistance of less than 1 ohm per 1000 feet. So a 20 foot length will have a resistance of only .02 ohm. As wire size goes up, resistance goes down, so larger sizes will be even less. No hand-held meter can resolve that low, generally the lowest they go is down to is .1 ohm. Below that, they'll read it as no resistance (continuity). So for practical purposes, any wire you measure should read as no resistance. But don't think that this 'small' resistance doesn't matter, it can. In a 20 amp circuit, a 'extra' .1 ohm of resistance can cause a voltage drop of almost 2 volts; the higher the current in the circuit, the bigger the voltage drop. For a thorough discussion of voltage drop, go here: http://www.jalopyjournal.com/forum/threads/wiring-101.843579/. Note that wire size affects voltage drop; too small for the load and voltage can drop quickly.

So, does than mean that all wiring will read zero ohms with your meter? In a perfect world, yes. But in reality, you'll see resistances of more than zero, and in almost every case it will be at connections. Either at the meter probe/test item contact, or at any other connection. You want these readings to be as close to zero as you can get them. On older existing wire harness, degraded connections are generally the problem. Cleaning and using 'DeOxit' on the connections will help.

Also note that reading for ohms must be done on a de-energized circuit; checking for ohms with power applied can damage your meter and will not give accurate readings. Always use the lowest ohms scale available on your meter for checking wiring. You may need to use a higher scale for checking some components, but I doubt it. About the only part on a vehicle (no electronics) that will need testing at a high scale will be the secondary side on a ignition coil. These will measure in the 10-40K ohms range (thousand) generally, although you should check the specs for the individual coil.

Remember that grounds are also part of the circuit. Check for resistance between your circuit ground and the grounded battery post. High readings here will cause problems.

If you're checking for a short-circuit or open circuit, a ohmmeter will usually be your best choice.

So you've checked ohms, and it still doesn't work right?

 

If checking for ohms still doesn't give a clear indication of why a circuit doesn't work or works poorly, now you need to check with the circuit powered up. The advantage here is this will give direct readings on circuit quality. While meter limitations in reading low ohm values may or may not show the problem, reading for voltage will.

The best way to check this is by reading between your source voltage and your test point. Use the lowest scale available that's above your source voltage. This check can be between the battery post and the endpoint of the circuit, between the 'power in' and 'power out' terminals on a switch, or even on both sides of a connection. Whatever reading you get is the voltage that's being 'lost'. The absolute maximum loss you want to see is 10% at the end of the circuit; so individual losses at various points cannot add up to more than 10%. This test can also be done on ground paths; again, you want losses at the minimum. I'll note here that too-small wire will cause voltage drop, so if after checking everything you still get a large loss, make sure the wire is big enough for the load.

Now, I'm sure everyone was hoping for a 'magic bullet' method for troubleshooting, but there really isn't one. Start at one end, work your way to the other end in a methodical way, that's how you find problems. Now granted, an experienced tech can find 'shortcuts' but for anyone 'electrically challenged' this is how you'll have to do it. Having an accurate wiring diagram (or a very good understanding of the circuit) is a must. The real key is being methodical and not jumping around; 'clear' each piece before moving on.

 

Thanks Steve for the great info.
Are you saying, for example in testing voltage (previous post), black lead on negative BAT pole and red lead on positive, then move only red lead down the circuit you want to test, finally getting to the end of circuit with only red lead or jumper? If so, you are comparing voltages down the circuit from a known voltage?

 

I'll talk about checking for current draw. Now, not every meter will be able to test for this; the cheaper units may only have DC micro or milliamp positions and these won't be large enough for most tests. The 'better' meters will have a DCA position that can direct-read up to 10 amps, although check your meter for it's capability. DO NOT exceed it's rating, that will likely kill the meter. Connect the meter in series with the load then energize the circuit; the meter will directly read the current draw. If you think that the circuit may draw in excess of 10 amps, then using a 'test light' will work better.

If checking for a 'drain', as was mentioned before connect between the battery ground post and the ground cable. This will show the current being used or light a test light if that's what you're using. Start pulling fuses and hopefully when you remove the right one the drain will disappear. Keep in mind that pulling fuses may or may not clear the drain; if the issue is before a fuse, it won't clear. At this point you'll need to start disconnecting 'power wires' to find it. On older vehicles there can be circuits that aren't fused until they get to the actual component being powered; clocks and radios to name two. You may find some that aren't fused at all. This can get nasty, as you may have to start unwrapping the harness to separate out the wires for testing. This is where having a wiring diagram is almost a requirement.

 

There's more than one way to test for voltage. In the above method, you start out with a 'base' voltage (whatever you read at the battery) then you have to 'do the math' as you move away from the battery, i.e. you started at 13V say, and end at 12V for a loss of 1V.

When checking for voltage loss (which is really what you need to be checking for), I prefer to check between adjoining points. In other words, start at one end and go to the next point, whether that's a junction or device connection. Put one test lead at the voltage source. This could be the battery, or the fuse panel, or the two sides of a switch. Let's do an example.....

Let's say we're checking the brake light circuit. The first thing you'll check if you can is the loss from the source to the end point of the circuit. Put one lead on the battery, the other at the incoming power wire for the brake light (with the brakes applied). If your voltage reading is under 10% of the source voltage as read at the battery (starting voltage is 13V, you read no more than 1.3V), it's probably OK. If your loss is more than that, then you need to figure out where that loss is happening at. Start by putting one lead on the battery post. Put the other lead on the 'hot' fuse terminal. Any voltage shown on the meter is the voltage lost between those points. Now, you may need a really long test lead to check back to the battery from each point; you can check between any two points for voltage drop. So if you test between the two fuse terminals, any reading is the loss between those points. Excessive loss means the fuse is making a poor connection. The same test can be done on switches; excessive loss means the switch contacts are bad. If the device has plug-in connections, check between the device spade and the connector; again, excessive loss is a bad connection. Another thing to remember when testing like this is these losses are additive; if you lose .5V at five different test points, the total will be 2.5V.

If the wire is properly sized for the load, you'll find that losses will almost always be at connections, whether it is the wire connection or switch contacts.

 

An analog meter will also fry modern computer circuits. Digital meter is required.

 

Actually, for 10 gauge copper wire, 1000 feet is slightly less than 1 ohm. 18 gauge would be around 6.25 -7 ohms for 1000 feet. Some of the numbers are slightly off, but otherwise we agree.

 

Lots of good info here, but I will add this; you not only need to know how to read a meter or light, but you need to know what circuit you are testing.
Traditional systems are 6V or 12 V. Almost all old stuff is run BATT power, fuse (if any), switch, load (light, motor, gauge, coil) then ground (earth to Brits) which is connected to BATT ground. Some stuff is switched AFTER the load, like wipers. This is called ground-side controlled.
So with this info, you can test for power from the battery to one side of the switch and make sure it's a complete circuit.
Then test the other side of the switch while turning it on and off. Then test for power at the input side of the load with it on.

Also, a wiring diagram is a huge help in figuring out even a simple harness. I use the analogy of a sewer system. If you have a clog, you don't dig up the whole system, you get out the map and check the manholes along the route. When you find deep sludge (voltage), the problem is further along. When there is no sludge (voltage), the problem is between this manhole and the last one you checked.

 

Here is a tip I have found usefull. When using the meter as an amp meter to measure battery drain the way most people do you put red lead into the amp cavity on the meter and position it between one of the battery cables and its' battery post, put the meter dial to the corresponding amp selection and the current is read. The problem with this method comes when the draw is too much for the meter's internal fuse. Blow the fuse and have to open the meter, good luck having the correct fuse to fit. A better way is to put a 1 ohm 1 watt resistor between the cable and the post, put the volt meter leads in the volt position and read the voltage drop across the resistor. 27 milliamps will be read on the meter as 27 millivolts. 9 amps will be read as 9 volts and will not blow the internal fuse.

The reading on my meter of 26.7 millivolts after doing the math with the 1 ohm resistor is really indicating 26.7 milliamps. Up to 50 milliamps is considered acceptable draw on vehicles with computer memory (in my case it is the radio memory), on a car with no electronics you would expect no reading.

 

Handy hint # 437- Don't forget to put your positive multimeter lead back into the "Volts- Ohms" socket after checking current draw and going on to test voltage, or you will end up with a blown meter fuse. Basically you will put a dead short across your voltage supply.

 

Thanks for the great info and answered questions. We are just getting started, so be sure to include basic info about how to set up and set the meter as you discuss tips like above. We want everybody to benefit from this thread.

 

You're right; I don't know where I got that number.... But even so, that means that a 20' length of 18 wire would still measure out at only .12 ohms, or right at the minimum resolution limit of nearly all hand-held meters.

 

You Tube has excellent info on basic electricity and meters.

 

Thank you rfraze for starting this thread! Info and wisdom like this is why I joined the HAMB. When it comes to electric I might as well be a caveman.

 

The Fluke 233 has a removable display that you can put anywhere you want to.

 

Man, I think I love this one.
Could you please explain each of the symbols. (Even though I just noticed the abbreviated explanation on the right of screen for this one.)

 

Fluke makes a nice meter, but that 223 is overkill for automotive. Looks like it's aimed at HVAC techs...

Functions (clockwise from 'off' with selectable second function listed second) are:
AC volts/frequency, DC volts, AC millivolts/DC millivolts, ohms/continuity, capacitance/diode, temperature, AC amps/AC milliamps, DC amps.

 

The Fluke 223 is a great meter for any one who works on cars boats or any thing else but it will hurt your felling to buy one

 

A bit off topic but while you smart fellers are here:
Batteries on concrete floor, ya or nay?