VOM 101

[Willl]

We need a good VOM teach thread.

Truly wish I had the skills to put my thoughts to text like others, but guess I'd just confuse more then help.

Maybe start with non-harmful single dry cells like D's or C's and work up to what we use in our tractors, 12V crank batts.

So many of our non-crank faults could very quickly be found using a VOM. To many times I see parts bought "just because".

 

[vwnotrunning]

It would need to be two subjects actually.

One for the operation / selection of ohms, volts, amps, etc. How to set the meter up. Many different meter configurations would make this difficult to "textually" express.

Second for when to decide what type of readings to take and expected results.

I think it could be very beneficial if the proper explanation could be provided, but the proper for some would be different than others.

 

[Soundguy]

to do that, proper notation and nomenclature would need to be taught.

IE.. the difference in inline tests and tests across a component.

the difference in open circuits, and circuits with current flow.


many no starts i read about are lack of maintenance and dirty connections

 

[Willl]

many no starts i read about are lack of maintenance and dirty connections

Exactly, and many of those could be diagnosed very quickly with just a knowledge of how to read voltage alone, and where to exactly to put the test probes.

Big difference between 'battery post clamp' and the battery 'post' itself.

 

[weedpharma]

Being an electronics tech for some 35yrs, I have great concern with SOME users. This is not specific to TBN. I have read requests in other forums, for non techs wanting instruction on highly dangerous and technical build or repairs. Many people are not aware of their shortcomings in skill or ability. That is why we refer to forums to be told how to do things.

As long as we do not give advice on repairs that are beyond the ability of the "average" user we should be ok.

I am aware of my lack of knowledge on farming. I have learnt a lot from TBN. The main thing I have learnt is not to try things that have been learnt by others by bad experience!

Weedpharma

 

[Soundguy]

like someone taking a measurement on/in a hot ac circuit, then touching the other meter lead...

 

[jinman]

Willl, since electrical/electronic technical instruction has been almost my whole life, I can tell you that understanding how the circuit is supposed to work is an absolute prerequisite to teaching how to troubleshoot it. Much of troubleshooting with a VOM is intuitive based on how something acts or responds. In many instances, it's experience that drives the intuition. I can't tell you how many times on TBN I've read posts from people who think electricity is voodoo magic. The point you start instruction with someone like that is a whole lot different than with someone like yourself, Willl. Also, I've read threads on TBN that turn into peeing contests between two technical professionals who are saying the same thing, but stating it differently. Once they sufficiently insult each other enough to call it quits, the whole thread is toast. That's why books are so great. When you buy a book about something and read it, you get one person's philosophy and methods. If they have done a good job, you'll understand their methods and learn something useful. If you don't understand, you have the choice of finding another source. Either way, you determine whether you like a learning method or not. It's one of the great things about adult learning. Unfortunately, on TBN, you see threads easily hijacked so that even the most dedicated person gets confused and disappointed by all the "background noise" that often occurs.

I guess, if you were really dedicated and wanted to do something like you suggested, you could develop your training offline and then post it all at once in a thread so that all responses come after your completed descriptions. That's probably the only way to do what you're asking and not get lost in "tangent land." TBN is a great source of ideas, but I just don't know if it is the preferred source for step-by-step training. That especially goes for something both technical in nature and potentially dangerous if misunderstood.:confused3:

 

[texasjohn]

All posts so far sooooo true.

And, just this week, I had the following experience, demonstrating how ornery electrical stuff can be. Note, I have a VOM, know how to use it but didn't have it with me on this occasion.

Went to eat lunch, car starting and running dandy. Got back in car, dead, no starter click, nothing. Battery bought in April this year. Well, I'll check the battery connections...applied lots of hand pressure to cables, no wiggle to them at all...so, must be Ok...lemme try again. Still no start. Well, I'm in parking lot, nothing else to do but repeat connections test. No obvious corrosion on either battery post.

This time, with LOTS of hand pressure, the negative post rotates...ah, good, twisted it back and forth couple of times, stopped when it is in a bind and tight. Try start again...nope, no go. This has gotta be the problem....remove cable from battery post, no obvious corrosion inside, remove leatherman tool from my belt and use it as hammer to pound connection onto post.

Now, car won't start because car alarm goes off because I disconnected the battery. Turn alarm off and use published technique to get around alarm relay battery disconnect. NOW it starts!!

A VOM would have helped, but only if I had tested the battery post AND the battery clamp.

I agree, having and understanding a VOM is handy, as is electrical knowledge, as is diagnostic technique....a noble thought to address this on TBN, but not sure how to do it.....electrical stuff, even DC, is vexing at times.

Conclusion...Leatherman needs to add a VOM to it's many blades:cool2:

 

[BobRip]

I suggest looking at tutorials on YouTube. For example YouTube - THE BEST Multimeter tutorial (HD)
Get a cheap meter and play around with it using the many tutorials. No need to have a thread here.

 

[LD1]

I agree with jinman here that unless you have an understanding of the circuit you are dealing with, knowing how to use the meter is pointless unless you know what and where to check.

And EVERY situation is different. Far too many to cover in one thread. IMO, it is best to handle them case by case to guide the poster and not have too much to remember all at once.

That said, some QUICK pointers:

Usually in all cases, a quick voltage check is step 1. While NOT running, a battery reading 12.0v is dead. 12.2v is 50% charge, 12.4v is 75% and 12.6v and above is 100%. And typically, anything UNDER 12.0v needs a new battery and charging it will be just a temporary fix.

While running, you should see ABOVE 13v, and in some cases as high as 15+v. If you arent seeing this, the battery is not charging. Could either be an alternator/generator failure, or a bad connection. If 13v+ isnt present, it leads into more complicated things to measure and this is where understanding the circuit is important.

Another common complaint is just a starter solenoid "click". MANY times people automatically assume bad solenoid, when in reality, it usually isnt. Back to battery voltages....12.0v is enough to make the solenoid "click", but only enough to make the starter laugh at you. So a dead battery will often cause a "click".

You can also use the meter on ohms to check fuses. DONT rely on just looking at them. I have seen blown fuses that looked PERFECT. Use the ohm meter when in doubt.

Thats about all the basics. It gets much more involved than that. You cannot check resistance (ohms) on a wire that has voltage to it. BUT...you can do voltage drop testing to get the same answers.

Inline amperage testing is more advanced and you better understand the limits of the meter. Most have internal fuses that when they blow, you are done until you replace them. Doing inline testing is excellent for troubleshooting slow draws on the battery as the clamp style is suited for higher amperage and isnt reall accurate when dealing with milliamps.

Again, every situation is different. There really are no "basics". Using a VOM is as easy as using a calculator. But unless you know WHAT you are checking for, knowing how means nothing.