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Technical How to Use a Multi-meter

Discussion in 'The Hokey Ass Message Board' started by Crazy Steve, May 16, 2019.

  1. Here's another 'tutorial' to help the 'electrically challenged' among us. I'll talk here about volt-ohm meters (VOMs) and what all the 'stuff' on them means. I'll go over how to use one in another post. While volt and ohm readings will be basically the same on all meters, if your meter has other features be sure and read your owners manual thoroughly to understand those. No manual? You can find the manuals for most on line.

    For most of us, you'll have or will want to get a hand-held meter. If looking to purchase one, keep in mind that buying a 'better' meter will pay off in the long run, but a cheapie can get you by. I would recommend spending at least $30-40, as you generally get better quality, accuracy, and internal protection against 'operator error' as price goes up, although at certain points they add features that you'll never need. There's all sorts out there, you may have one you've inherited, but there's two basic types; the analog meter and the digital meter. Here's a pic of the two types...


    The analog is on the left, the other two are digital. Both types will work for our purposes, but the analog type takes much better attention to reading it to get reasonable accuracy and usually can't match a good digital for that. One advantage analog has over most digital meters is it will 'read' moving values, while most digital meters take a moment to stabilise before giving a reading. I'll talk more about this further in. I'm not going to go into the analog type heavily due to their rarity these days, but I will point out that an analog meter will have most of the same functions we need as a digital meter but due to the analog aspect getting accurate low-value readings is difficult.

    So how do you 'set' the meter to get readings? Here's pics of the two digital meter selector switches...

    [​IMG] Meter2.jpg

    The top meter is a full-manual meter (and a analog meter will be similar); you have to turn the selector switch to the value you want to measure. You do have to take care here on live circuits, as using a wrong setting can kill the meter, such as selecting a 20 volt scale then trying to measure a voltage above that amount. The lower meter is a 'auto-ranging' type; it will automatically select the correct range, although this model also allows manual selection if desired (a very good feature to have). On the red meter, the test positions you'll be using will be the 20 DCV and the 200 ohm. You may also use the diode position once in a while to check diodes. On the yellow meter, the positions are (clockwise from 'off') AC volts, DC volts, 300 Millivolts, ohms, diode test, and AC then DC amps. Again, you'll be using the DC volts and ohms, and the diode test rarely. The button in the middle of the selector switch is for manually selecting range.

    Ok, What's the reading on the display mean? We'll start with the manual meter...

    In this case, the meter is set to the '200' ohm scale, so you'll get a direct reading in ohms of whatever you're checking as long as it's resistance is under 200 ohms. If it's over that, the display won't change. As you increase the range, the meter won't be able to measure small amounts and a low resistance will show as zero. For 99% of the resistance checks you'll want to do on a vehicle, this is the scale you'll use. You'll get the best accuracy by using the lowest scale (without going under the measured value) for both ohms and voltage.

    Here's the auto-range meter...
    There's more stuff going on here; first, this meter displays the type of measurement you're making, in this case ohms. It's also putting a 'M' in front of it, which means it's set to measure megohms (millions). A 'K' would mean thousands, no letter is straight ohms. The 'OL' it's displaying means 'open line'. Being an auto-range type, it will automatically switch to a lower scale if it measures a low resistance. The other thing to note is the 'bar' at the bottom of the display; this is a 'analog' graph (giving you the same function as an analog meter for reading moving values), extremely useful to have. Voltage values will be read out directly.

    Using the diode setting. You'll rarely use this unless you're checking a diode, unless you have a meter with an audible tone for testing. These can be used as a continuity tester (get a beep, you have continuity) but be aware that the numbers displayed ARE NOT in ohms; that's the pass-through voltage and when using as a circuit tester they'll usually show zero. To check for resistance, you must use a ohms scale. If testing a diode, typical values will be around .7 volt one direction only if the diode is good.

    Meter accuracy. This is where quality shows when buying a meter. You want a meter with the best 'resolution' you can afford, particularly if measuring resistance. Good quality hand-held meters can measure down to .1 ohm accurately, cheaper meters won't do as well, so be sure to check specs before you buy. I'll show why this is important when I explain how to use the meter, but don't let this prevent you from picking one up, as even the cheap ones can be useful. The red meter shown in the pics is a $3 (yep, three) unit from HF that checked very close to my more-expensive Fluke when I first brought it home, but a few months in my damp garage seems to have skewed it's readings some (hey, they warned to keep it dry so my bad), I'll see how it does after it dries out...
    Last edited: May 16, 2019
  2. This is where I'll go over how to use your VOM (volt-ohm meter) and what to look for.

    So, what constitutes a poor connection? How much resistance is 'too much'? So a few facts are in order...

    The resistance of the wire is a usually non-factor; the smallest OEM size you'll find on vehicles (18 gauge) has a standard resistance of about 1 ohm per thousand feet, so a ten foot length would only be .01 ohm. This value is far smaller that you'll be able to measure with any hand-held meter, so for all practical matters it's the same as zero. And as size goes up, resistance goes down, so larger sizes will be even less.

    Now, wire size does enter into this; too small wire size for the load will introduce voltage drop, but assuming the wire is sized right most problems you'll have will be where the wires are connected, either to another wire or to a component. The majority of connections are either a friction type, spring type (most light sockets), or a combination of the two (most plugs, switches). A few high-amp connections are bolted (battery, starter, main ground points, some switches). When dealing with electrical connections, cleanliness is the main key here, as dirty connections don't conduct well, although loose connections can be just as bad. Ideally, the resistance through any connection should be zero but you won't get that, but you want to get as close to that as you can. So, what's a 'good' value?...

    Right here I'm going to drag out a part of Ohm's Law to illustrate this; no, you don't need to know this, no math needed while checking [​IMG]. But this will show just how much a little 'extra' resistance will affect your wiring for the worse. The formula I'm using is for calculating voltage drop across a resistance, any resistance, and it goes like this:

    Vd = I x R

    Where Vd is voltage dropped (or lost), I is current in the circuit, and R is the specific resistance, whether it's a component or a poor connection. So using this formula, let's do a couple of typical calculations that have direct bearing. So for say a turn signal light, you have a .1 ohm 'extra' resistance in the circuit, caused by it's being dirty or loose somewhere. The lamp draws 2.25 amps, so: Vd = 2.25 x .1, or Vd = .225 volts. Not a lot you say, and you're right. But if this is in a circuit that sees current of up to 19 amps, now you're talking about a 1.9 volt loss (almost two volts) and that's enough to affect circuit operation. Increase the resistance another tenth or two and you'll see problems on even the low current circuits. Personally, I try to limit voltage drop to a 5% maximum at any point in the system. Keep in mind that these drops add up, if you have 3% drop between the battery and the fuse panel, and 5% in individual circuits as measured from the fuse panel, you now have 8% total. I'll also note that voltage dropped isn't 'lost'; it's converted to heat, so excessive drops can cause fires. So moving on...

    There's two ways to check circuits; 'cold' (no power) and 'hot' (powered up). Both ways have advantages/disadvantages, and sometimes you won't have a choice like when checking ignition coils or other ignition bits. One big advantage about checking cold is you have no worries about shorting anything out, blowing fuses or burning stuff up. It's also the method you'll probably have to use if the vehicle won't run or if checking the harness before running it. But one disadvantage to a 'cold' check is you're only applying a very, very small current to the circuit; once you put the circuit into use, the higher current can cause heating in a marginal connection and a higher resistance can show up. So even if a connection shows good 'cold', if you're still having issues you'll need to check it 'hot'.

    To check 'cold', simply put the probes at the ends of the piece of harness you want to check. If you're not trying to read through a device (lamps, gauge, whatever the circuit is supplying), you should get a reading of zero, or no more that .1 ohm. You can check switch and NC relay contacts this way also. These should read zero, any value above that and you probably have damaged contacts.

    So for cold checking, you need to know how low your meter will 'resolve' resistance, in other words how small a value it will measure. With a analog meter, it'll be tough to see a one or two tenth ohm value on the scale. Cheaper digital meters may not resolve well either, your better digital meters will be good down to a tenth. So the first thing to do is to see what the meter reads probe-to-probe (touching the two leads together).

    My $100+ Fluke meter will 'zero out', so any reading above that is in the circuit. Note that most digital meters will take up to 15 seconds to zero out, so touch and hold until the reading stops changing. One advantage analog meters have is you can manually zero them, but again, seeing one or two tenth values on the meter face won't be easy.

    The cheap HF meter won't zero out; this is the lowest it will read. All is not lost though; simply subtract this from your actual reading to get the circuit number. So if you see a value of 1.1 ohms, circuit resistance is .2 ohm. If your 'zero number' moves around, get another meter...

    So let's perform some actual 'cold' tests; turn your meter to 'ohms' (on it's smallest scale if a manual meter) and make sure your test leads are making a good contact connection, scraping to clean metal if you have to. If you have an auto-ranging meter, pay attention to what the display range is.

    Here I'm checking the alternator stator coil at the regulator plug on a motorcycle. I'm looking for .4 ohm (+/- 10%) between the white wires. Now, seeing how the meter resolution is only .1 ohm, anything between .4 and .5 will have to be considered good. This is one type of measurement can be done better with an accurate analog meter as it won't 'round' the number up or down.

    Here I'm checking actual internal plug connections; I've inserted the probes on both sides. This is very useful when checking a connection to a component. Get a reading above your zero amount, you have a loose/dirty connection that needs attention.

    Here's a test of the charging system output wiring; insert one probe at the regulator, put the other on the positive battery post (where the other end of this wire's output goes). Again, any number above your zero amount means this needs attention. This is also a useful test for any other circuit positive or negative wire; simply put one lead on the wire to be tested and the other on the respective battery post. If you get a 'bad' reading, then you'll have to start checking the various connections and/or switches between these points to see where the problem is.

    These test were performed on a bike, so the meter leads are long enough. On a car or truck, you'll need to extend at least one lead, I'd recommend #14 wire. Doublecheck to make sure your meter still zeros out when you touch the two leads together.

    Ok, let's do some 'hot' checks. Turn on the vehicle, maybe start it for some tests. Set your meter for the lowest DC voltage range above 15 volts in a manual meter. Auto-range meters will read directly.

    Connecting the leads as shown in the connector pic above, this will give a direct value for voltage loss in the connection. Zero volts would be great, between .1 and .2 volt is still decent, .3 volt or more you have a problem.

    Connecting the leads as shown in pic below that one, this will give you a point-to-point voltage loss reading. Going back to the positive battery cable hopefully will give the same readings as the above paragraph, but you may get a higher reading. I'd personally try for under .5 volt loss, less on higher current circuits like the charging system, ignition switch wiring, headlight, and ignition system supply for sure.

    I'll note here that if your checks at connections, switches, and relays don't account for all of your losses, it's likely that the supply wire is too small for the load. Sad to say, but most voltage losses in automotive harnesses is due to undersized wire.

    You can also use this method to check the ground half of the circuit; perform the above test, and note the amount. Subtract this from the system voltage (as measured at the battery). An example would be a .3 volt loss, 13.9 volts at the battery, which means you should have 13.6 volts at the test point. Check between the test point and your negative battery cable to verify this. Then check between your test point and the component ground point and see if it's the same. If it's lower, you have a poor connection in the ground half of the circuit.

    I'll note here again that ground connections are rarely directly tied together (depending on paths through the frame/body), so checking between the negative battery cable and other ground connections for resistance can be critical for proper system performance.

    You'll never get all the connections perfect, but this will help pinpoint just where your big problems are. Again, the high current systems will show the effects more than low current circuits will, but the better you get everything, the better things will work. This isn't as hard as it sounds, a novice who has done a bit of homework should be able to check all this in a busy weekend. If your harness has been 'POed' with extra splices or otherwise hacked on, you'll probably have to do more checking/tracing to make sure everything is right.
    Last edited: May 16, 2019
  3. WB69
    Joined: Dec 7, 2008
    Posts: 1,857


  4. blowby
    Joined: Dec 27, 2012
    Posts: 8,384

    from Nicasio Ca

    I see them. I recognize the red meter. I have 32 of them. ;)
    loudbang, Rich S. and 56don like this.

  5. Gimme a minute.... sheesh... Ignore the one red X, that's not supposed to be there...
    SDhotrod and loudbang like this.
  6. Bump to get it back to page one...
    loudbang likes this.
  7. 41rodderz
    Joined: Sep 27, 2010
    Posts: 6,315

    from Oregon

    All good here .
    loudbang likes this.
  8. Sporty45
    Joined: Jun 1, 2015
    Posts: 1,038

    from NH Boonies

    Glad you got this going, I'm in! :cool:
    loudbang likes this.
  9. If you tuned in early, you may have caught me in the middle of adding pics/editing and wondered what the hell was going on...
    loudbang likes this.
  10. It's coming through loud and clear here, Steve.
    loudbang likes this.
  11. brad2v
    Joined: Jun 29, 2009
    Posts: 1,650


    Thanks for this
    loudbang likes this.
  12. Boneyard51
    Joined: Dec 10, 2017
    Posts: 5,618


    Steve, maybe this would be a good place to add information on batteries? Batteries are one of the most misunderstood items around.

    loudbang likes this.
  13. TrailerTrashToo
    Joined: Jun 20, 2018
    Posts: 1,149


    Maybe a separate thread. This is good info, adding other topics will confuse the novice.
  14. Chavezk21
    Joined: Jan 3, 2013
    Posts: 694


    Thank you Steve!
    loudbang and dirty old man like this.
  15. Rex_A_Lott
    Joined: Feb 5, 2007
    Posts: 1,019


    Hope this was chapter one. Thanks
    loudbang, dirty old man and 56don like this.
  16. ken bogren
    Joined: Jul 6, 2010
    Posts: 964

    ken bogren

    loudbang likes this.
  17. thanks Steve for taking the time and putting this out there !
    loudbang likes this.
  18. Much appreciated...:)
    loudbang likes this.
  19. Fortunateson
    Joined: Apr 30, 2012
    Posts: 4,582


    Going to go back and re-read; my heads hurts. Really appreciate this.
    loudbang likes this.
  20. dirty old man
    Joined: Feb 2, 2008
    Posts: 8,911

    dirty old man
    Member Emeritus

    Really appreciate you taking the time and effort to post this Steve, and I hope you continue on with the multitude of stuff that can be checked with a VOM multi-meter.
  21. If anybody has any questions, I'll do my best to answer them...

    If you're shopping for a meter, remember that for most of our purposes all you really need is volts, ohms, and maybe the diode test. Unless you're planning on seriously delving into electronics, any other features won't be anything you'll use.

    Amps on these will be of very limited use. The majority of hand-held meters are limited to a 10A maximum if not less and there's very few circuits on a HAMB-era vehicle that will be under 10A when you include current inrush. About the only useful test you could perform would be to track down a small draw that discharges the battery. With the key off, lift the positive battery cable, turn the meter to the 10A position, then put the test leads in series between the cable and the battery post. This will give you a direct reading of the draw. But be cautious; if you draw much of an arc when lifting the cable and the battery goes down fairly quickly, you can smoke the meter.
    Last edited: May 18, 2019
    loudbang likes this.
  22. dirty old man
    Joined: Feb 2, 2008
    Posts: 8,911

    dirty old man
    Member Emeritus

    Steve, I have found that having an ammeter in the tester when you're looking for a current drain that is so low that it won't light up a bulb when you're looking for whatever is causing a vehicle to drain the battery over a couple weeks without driving it.
    However you can no doubt spell the procedure out for the HAMB much better than I, although using a good quality meter I have found drains so small they would,t even glow a small bulb placed in series with the battery post! But just be sure to put a quick blow fuse of a lower amperage than the meter's max rating in series with the meter!
  23. doug3968
    Joined: Sep 13, 2014
    Posts: 38


    What did I miss, why is there a
    distributor cap is at the beginning of this post?

    Sent from my iPad using The H.A.M.B. mobile app
    loudbang likes this.
  24. doug3968
    Joined: Sep 13, 2014
    Posts: 38


    I hate auto correct, it took out Marmon dist. and left a blank space.

    Sent from my iPad using The H.A.M.B. mobile app
    loudbang likes this.
  25. Go easy on the Tequila....

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