How to use a voltmeter?

First, get a good meter. I have seen some good used Fluke meters in a local pawn shop. I actually got my Dad a nice DVM (Digital Volt Meter) and made him give me his $3 meter. In the trash it went. Once you get used to it, and learn what it will do, you will wonder how you lived without it.
Make sure the one you get has a function that will sound a tone when the test leads are touched together (continuity test). Like checking a fuse (out of the circuit of course) as one example. So you really only need very basic functions. No need to get a bunch of fancy features and functions you will hardly if ever, use.

I will look around the net for a primer on basic meter use. I'll post back later.

You can get a lot of milage out of a good test light too. Again, get a good one. Not a cheap one. The cheap ones arn't rugged enough and will break pretty quick.

OK, this not the greatest, but it's a start.
http://www.allaboutcircuits.com/vol_6/chpt_2/1.html

Went back and looked around some more. That link is actally volume 6. Volume 1 is Basic DC (Direct Current) Principles. If your really interested (unable to sleep) this will give you a really good basic understanding of why 12 volts does what it does in our cars.

Keep this in mind first of all. The power in our cars is DC current. The power in your house is AC (Alternating Current) Most all meters will need to be selected for the type of power you will be working with. You will be using DC volts and Ohms (resistance) on yor car.

Technically, the Alternater puts out "Alternating" current. The diode pack ( or rectifier bridge) converts it to DC, and the voltage regulater controls the level or amount that reaches the battery, based on demand or load.

Take it from there and ask some questions as you need.

for wiring a car, you can almost get away with just a simple test light. Runs off 12v and it you have power to it, it comes on.

To use one, you just hook the clip to a common ground and start probing wires to see if they are hot (light on) or not.

A multimeter is pretty easy to use. While some of the more expensive ones will automatically switch to whatever you are detecting, its not mandatory.

Ok...so you can test for voltage...thats basically power. Set the multimeter for "DC" (Direct current voltage) and you need to select a range. You want the max voltage to be that number. We know cars use a 12v system, though you may see close to 14v from the alternator. So you would want to select the 20 volt setting. Its always best to start high if you dont know. You dont want to be on 1volt and pass 200 through it. Start high and move down. Basically voltage will be spit back at you in a number. This is one way to test if a wire is good or getting power. Same can be done with a test light. Attach the black (ground) lead to a common ground and the red to a source. If you see voltage, you its a hot wire.

Another thing to test for is ohms. This is resistance. So with your multimeter you select ohms. It might look like a goofy upside down "U". With this, you put one lead on one side of the wire, and the other lead on the other side. This give you a numerical display of the resistance of the wire. When sending power down a wire, you always lose some of the juice. To compare, you are pouring a qt of oil down a funnel...not 100% of that oil will make it out of the bottle and through the funnel...you will lose some. Same happens with electricity. Usually the thicker the wire, the less loss. The electricity flows over the surface area of the wire, not the center. This is why you will see "MONSTER cable" for home theater. Its not the diameter thats important...its the surface are of the conducting wire.

Amperage is also something you will run into on a car. If you look at a battery, the battery will show its amperage. The easiest way to describe amperage is the amount of balls it has How can we compare this to an engine.... Say a 302 has 300hp... the hp could be compared to amperage. You can have another 302 that cranks out 400hp. The motor is the same size (voltage) but the 2nd one has more power. So again, back to using your multimeter. You can check for amperage by selecting it on the meter. Then you connect to a hot and ground source. It will spit out a number. A car battery is a good source IF your meter will support readings and has the capability of reading over 400amps. This can fry your meter. Just like the motor comparison...a stock 302 motor isnt going to hold 1000hp...its gonna die. you will use this setting to test an alternator. A stock alternator should put out 60-80amps. If you are putting out less, its not able to fully charge the battery and run the electrical system of the car.

The other main option you will see is AC voltage. That is alternating current voltage. With AC voltage, typically you will have higher amperage ratings. AC voltage is electricity is the electricity in your house. Same stuff applies with detecting DC voltage and amperage. I dont suggest EVER trying to read amperage in a house with cheap multimeter. Your typical house circuit will will have 20-30 amps max capability and that is determined by the circuit breaker. The main power to your house is HUNDREDS of amps at 110+ volts.

Nice post, xoxbxfx

Very good treatise there, xoxbxfx! The only thing I would change is the descriptions of "Voltage", "Amperage", and "Power".

(Using the "water-tank analogy"; look at it this way):

"Voltage" is the amount of water in the tank.

"Amperage" is the size of the drain-pipe - therefore the amount of water coming out per second, which is the "velocity" of the water (or electrons) at the voltage.

"Power" (wattage) is the weight pressing down on the water; which causes the current (water or electrons) to squirt out the end of the pipe.

You have to also remember that this "definition" comes from the same field (electronics) that tells you: "All electronic equipment - even your computer- runs on smoke!" If you don't believe me, do something that lets all the smoke out of whatever amplifier/radio/computer you are playing with. Once all the smoke gets out; I'll betcha it doesn't run any more!

i have two 'meters floating around the shop, a digital, which gets the most use (i like the audable feature of the ohms function) and a cheap analog, or "swing" meter... the swinger is handy when you need to know the full range of voltage, such as the voltage regulator behind the instrument cluster. most digital meters only update every second or so, and you will only see a constantly changing set of numbers...

I forgot about Sears, I got a cheap little digital meter for the shop when they went on sale for $10. The audible tone is pretty weak, but it works. And they are accurate enough for automotive work. You may want something better later on. But a meter like that will serve you well.

It seemed like everyones test light broke the same week in the shop. We bought a really nice one off the MAC tools truck for around $25. Had an extra long cord on it and heavy duty alligater clip. THe cheap ones always seem to bend and distort after awhile. You will find that the test light will get used more often. So much faster. But there are times when you really need a meter. You really need both.

XOXBXFX, nice write up. I didn't have the energy to write it all down. THis would be a nice addition to the FAQ. It comes up enough and is probably one of the more challanging parts of dealing with a classic car.

BarnStang said:

I forgot about Sears, I got a cheap little digital meter for the shop when they went on sale for $10. The audible tone is pretty weak, but it works. And they are accurate enough for automotive work. You may want something better later on. But a meter like that will serve you well.

It seemed like everyones test light broke the same week in the shop. We bought a really nice one off the MAC tools truck for around $25. Had an extra long cord on it and heavy duty alligater clip. THe cheap ones always seem to bend and distort after awhile. You will find that the test light will get used more often. So much faster. But there are times when you really need a meter. You really need both.

XOXBXFX, nice write up. I didn't have the energy to write it all down. THis would be a nice addition to the FAQ. It comes up enough and is probably one of the more challanging parts of dealing with a classic car.

Click to expand...

I hate doing electrical stuff... If you guys want to edit it for better word usage and comparisons, go right ahead. Im to lazy to. A basic relay write up would be good to.

edit... I did one http://forums.stangnet.com/showthread.php?p=7141762#post7141762

and if you have a cheap multimeter, i wouldn't use it for ANY amperage testing. Typically it only measures milliamps (mA) and you'll end up blowing the fuse on the multimeter if you test anything. Rarely will you find something using mA, especially on a car. This is more intended for circuit boards say for a radio or computer.

you don't need a digital vom for an old car. Go to a pawn shop and find a good Simpson 260. Simply the best analog bench vom ever made. They go for about $35.00 and when new cost around $350.00 (in 1969). I have one and also a fluke digital but the digital is just too sinsitive for 12vdc old car stuff.