I have very limited knowledge of electricity. Instances are becoming more frequent that I have needed to use an ohmmeter, but haven't because I don't know quite how. Do any of you have a link or thread I can study up on to familiarize myself better with an ohm meter and how to use it? For example- I need to test my blower motor that quit on me.
Thanks
Chris

go to radio shack
pick up a pocket digital volt/ohm meter about $25
simple,easy to use,comes with instructions.
i have a big $ fluke that has collected dust for over 10 years.
i bought the radio shack one to put in my track kit and i use it every day at my shop.

You didn't say you didn't already have one but here's a good one to learn on for 4 bucks and it will do anything ever needed working on our old Chevelles.
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=90899

There are a lot of web sites on the subject.
http://www.google.com/search?hl=en&q=how+to+use+an+ohm+meter&aq=0&oq=how+to+use+an+ohm

(Harbor Freight has multimeters that actually work for under $5. )

Rather than use an ohmmeter, you could bench test your motor using a battery and two wire with alligator clips on the ends.

I have an ohm meter, just don't know how to use it.

Ohms are a unit of measure of resistance to the flow of electricity (current).

Ohms law states Voltage (EMF) / Current (flow of electrons) = Resistance (Ohms)

Ex. 10 volts / 1 amp = 10 ohms of resistance.

You can move the formula around to solve for any 1 if you know the other 2.

And you can throw in Power (Watts) to solve for other values

Power (watts) = I^2 (current squared) * R (ohms)
or P = I * E (voltage)

.....confused yet....well ignore all that if you just wanna use the "Ohm Meter" to tests for "shorts" or "opens" in a circuit or wire.

#1 rule when measuring resistance (in Ohms).......TURN THE POWER OFF or you will blow the meter fuse or fry it (usually has a fuse).

Most cheap electronic meters are "auto ranging", meaning if it is set to the 2k (2000) scale, the reading will be (using 3-1/2 digits) ranging from .001 (minimum) to 19.9 or (maximum) or "___" (open, meaning no connection - infinite resistance).

It is usually best to pick a range where the expected resistance will be in the middle of the measurable range where the meter is most accurate. does not much matter if the expected value is minimum (short) or max (open).

An ohm meter may "dance" or fluctuate if your probes/connection is not 100%.

You do not need to worry about polarity when checking resistance, unless you are checking diodes. A normal diode only allows current (this time it's current internal to the meter since power is off) to flow in one direction. When you have minimum resistance, the black lead is the anode, and the red lead is the cathode. Flip the leads and the diode should read "infinite", hence "one way current flow.

Isolate the component you want to test. Place the probes on the (+) and (-) wires on the component
the meter will display how much resistance the component has.


make sure the dial on your meter is set to read ohms. look for a dial setting with this symbol

Ω

There will probably be several dial positions for reading ohms that range from tens of ohm, hundreds of ohm, thousands of ohms. If you get a funny reading on one setting the scale for that setting may be way off for the amount of ohms in your component and you could try a different setting.

Whoah! I'll have to read that later after work. Thanks Chris

Isolate the component you want to test. Place the probes on the (+) and (-) wires on the component
the meter will display how much resistance the component has.


make sure the dial on your meter is set to read ohms. look for a dial setting with this symbol

Ω

There will probably be several dial positions for reading ohms that range from tens of ohm, hundreds of ohm, thousands of ohms. If you get a funny reading on one setting the scale for that setting may be way off for the amount of ohms in your component and you could try a different setting.

So how will I know if the motor is bad by probing the + and -?

A few ways:

1 - the meter will do nothing, indicating that the circuit is "open" (something inside is broken)
2 - the meter will read an incredibly low number or 00.0, etc. This would indicate a dead short/something fried & welded together inside.
3- the meter will read a number in the 100's or 1000's of ohms. This would indicate a potentially good circuit.

In the case of a blower motor, you'd put the + probe on the wiring connector, the - probe on a bare metal part of the housing. The meter readings will tell you if the windings, armature, and brushes are intact - but not tell you the condition.

And an ohm meter or even a simpler circuit tester won't tell you if the motor is any good. It CAN tell if it's bad, but just because the ohm meter gives a reading doesn't mean the motor is good.

Connect a wire to the power connection and another to the body of the motor via allegator clips. Touch the power connection wire to the positive side of a 12v battery and the other wire to the negative side and the motor should run at a high rate, the fan shouldn't wobble and it should run smoothly without any squealing or grinding.

If it were me. Before I did anything with the blower motor, because with Chevelles. Its a pain in the you know what to get at. I would first get a test light and attach the lead to a ground connection and disconnect the wire connecting to the motor. Turn on the blower with the key on and use the your test light to see if you have power at this connection.

Also, if you try Richs suggestion up above. Which is what I do to test a blower motor also. Make sure you hold it tight and make sure your fingers are free from the fan. It spins pretty good and will jump a bit in your hand when you first touch the battery post.

I'll give you a couple of tidbits. Most meters will have a setting usually in the ohms scale range that will give you and audible beep when you have continuity. This setting comes in handy if you are just checking to see if there is a complete circuit and really don't need to know the ohms value. This way you don't have to watch the meter and you can concentrate on getting your connections just right.

Sometimes when testing small components you may want to hold the component on the meter's probe with your fingers. If you touch the metal ends of the probes the meter will read the resistance through your body. If you want to read the resistance through something, keep your fingers off of the metal part of the probes.

When first turning on the meter to the ohms position, touch the test leads together. This checks that the meter is working and that your test leads are connected properly.

If you are using the meter to test voltage, make sure that you set the meter for dc volts or ac volts.
The dc volts like batteries and car voltage is the setting with the V ----- and a straight line. The ac voltage like for the house is a V with a wavy line.

Keep fresh batteries in the meter it will work better.
Squido

I restored the whole car and finished it up late last year. The motor worked fine before I put it in the car but the plastic fan had a couple of broken fins , so I replaced it with a aftermarkte metal fan. This fan must have been a slightly different size becuase it made a screaming noise like it was grinding on something when I turned it on after it was installed. It did this for about 10 seconds then quit altogether. I may have just blown a fuse, but not positive.
I didn't start this thread for my blower motor because I am assuming I have to remove it anyways to replace the fan. I just keep running into predicements where I know knowledge of an ohmmeter would help me out.

A squeal is very typical of bad GM blower motors when the front bearing goes bad.

It is good to know how to use testing equipment :thumbsup:

A squeal is very typical of bad GM blower motors when the front bearing goes bad.

It is good to know how to use testing equipment :thumbsup:

Hmmm bad bearing, good to know. Ill check soon.

To add to Rich & Chris's suggestion....

Anytime you are using a test/jumper lead to +V, PUT A FUSE IN-LINE!! Match the fuse rating of the circuit you're working on, and add an amp or five to the test lead fuse.

Using a non-fused test jumper wire is asking for trouble, why?
1) - If the device is internally shorted, the instant you touch the + post of the battery it'll weld itself, and you'll have a 600amp toaster in your hand.
2) - If you're on an espresso buzz, or a little shaky, you might accidentally ground the test wire, and you'll have a 600amp toaster in your hand.
3) - If there's something else in the circuit that's bad, and you've not isolated it yet, you could potentially fry a good component.
4) - Another troubleshooting aid... if a 10A blows, but not a 15A, and the circuit is factory fused for 10A, you may have a grounding problem, or a worn device.

"4) - Another troubleshooting aid... if a 10A blows, but not a 15A, and the circuit is factory fused for 10A, you may have a grounding problem, or a worn device."

John: Bad grounds increase resistance, which reduces current, so this would seem unlikely... Just check the Ohm's Law post above.

In any case, I'd say that testing a blower motor with an Ohm meter isn't going to help much since its a high current device and the meter is placing almost no current into the motor.

Better to test by applying voltage through a jumper from the battery and possibly measuring current. See the earlier post about using a fuse... Be careful since 10A is typically the max current a meter wants on the unfused current setting and the meter could fry. Chances are the blower motor wants more than 10A.

Doh! Brain fart on the mathematics.... http://www.chevelles.com/showroom/data/500/ohmslaw.jpg

Doh! Brain fart on the mathematics.... http://www.chevelles.com/showroom/data/500/ohmslaw.jpg

What the #$@^%?

That pie chart is the key to the electricity universe.

If you know 2 of any of the 4 common "properties" of an electric device you can find the value of the others by doing simple math equations.
(Even if you only know 1 value, you can "swag" the 2nd and get in the ballpark.)