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Technical HOW TO USE A MULTI METER/VOLT METER

Discussion in 'The Hokey Ass Message Board' started by rfraze, Jan 25, 2015.

  1. The battery doesn't care what it's on, it's an old wives tale, they sit on cold metal battery trays all the time. Just don't drop it and you will be fine!


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  2. rfraze
    Joined: May 23, 2012
    Posts: 2,003

    rfraze
    Member

    I may be suspicious, but I never leave one on concrete under a ladder.
     
  3. rfraze
    Joined: May 23, 2012
    Posts: 2,003

    rfraze
    Member

    Truth, I never leave one on concrete,
    but now that I am learning how to use a meter, maybe I will do a scientific test.
     
  4. That was semi-true years ago, but not now. Way back in the day, battery housings were made with a lot of carbon black for tint. Under the right conditions and depending on what other materials were used to make the housing, the case could be slightly conductive and slowly drain the battery if in contact with 'earth'.
     
    Last edited: Feb 1, 2015
  5. blowby
    Joined: Dec 27, 2012
    Posts: 6,392

    blowby
    Member
    from Nicasio Ca

    Thanks guys, that's what I thought. But I guarantee you, if four guys are standing around my garage and I put a battery on the floor, one of them will say not to leave it there. :)
     
    kidcampbell71 likes this.
  6. Crazy Steve hit it on the head....not a worry anymore but someone always has to say they know for sure but have no idea why!


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  7. blowby
    Joined: Dec 27, 2012
    Posts: 6,392

    blowby
    Member
    from Nicasio Ca

    How about this: The batteries in my mill saw and tractor are not well protected. They are often wet on top and may sit that way for days. Any significant drain?
     
  8. blowby
    Joined: Dec 27, 2012
    Posts: 6,392

    blowby
    Member
    from Nicasio Ca

    Is there an Idiot of the Week award around here? 'Cause if there is, I'm the winner. Went out to the woods yesterday with spare cables, meters, etc.. Then it hit me...

    Wiggled the shift lever (automatic). Fired right up. In my defense, it was in Park, just needed the wiggle.
     
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  9. LAROKE
    Joined: Sep 5, 2007
    Posts: 1,630

    LAROKE
    Member

    Been there, done that, have the "Idiot Award" tee shirt.

    Several years ago, I had a computer apart down to the motherboard to find out why it wouldn't boot up when I discovered a co-worker had turned off the circuit at the electrical panel.
     
    blowby likes this.
  10. rfraze
    Joined: May 23, 2012
    Posts: 2,003

    rfraze
    Member

    Let the great meter use tips continue.
     
  11. Gasser_Dave
    Joined: Aug 18, 2013
    Posts: 63

    Gasser_Dave
    Member

    One thing to remember when trying to determine what a particular wire does, is to try different things to make it change its state. For example, if you have a wire that is reading ground in or around your dash, try pulling on your headlights to see if that wire suddenly turns hot. Some switches rest at ground when off. Same for interior lights and door jamb switches. when open they may rad 12 volts, but when you push the pin in, it goes to ground or vice-versa. Then if you still do not know what circuit it belongs to, start pulling fuses until the readings drop off.
     
  12. If you need to get into a circuit but can't get to a connector to check whats happening while it's running, raid your missus sewing box and get a couple of pins. These can be forced through the insulation of the wire and connected up to with alligtor leads.
    If your'e working under the dash, first find yourself a real good ground. Pick a nice and shiny bolt, screw, etc, and attach an alligator lead to that. This leaves you withboth hands free to goon around prodding at terminals, moving wires, etc.
    If you want to check that it IS a good ground, first extend one or both of the leads (with alligator leads again),so that your meter can measure between your battery negative and your ground point you are testing.
    Then set to the lowest resistance scale. Short the ends of the test leads together, and note the reading. It will be around 0.5 to 1 ohm. Note this down. This is your "lead resistance".
    Now measure between your battery negative and your ground point. Subtract your "lead resistance" from this reading and you will get your resistance reading for that point.
    It should be as close as possible to zero, but up to 0.7 ohm would be OK for most stuff, but not for high current draws like a starter.
    If you are trying to measure voltage near your engine and it's running, your digital meter may look like the display is going nuts, EG,not stable. You have 2 options- 1- extend the leads (our good friends the allicgator leads again), so that the meter can be used acouple of yards away from the motor or 2: use an analogue (needle type ) meter.
    The digital meters will pick up stray High voltage from your ignition leads and it will play havoc with the display.
     
  13. rfraze
    Joined: May 23, 2012
    Posts: 2,003

    rfraze
    Member

    Great info! Please remember to be basic. Beginner users may not be sure about lowest resistance scale when setting dial.
     
  14. The lowest resistance scale usually measures 0-200 ohms, ideal for most auto type problems. The higher up the scales you go , the higher the resistance you can read. The "beeper" type of continuity meters are good too, as you can "hear" a closed (Or short) circuit. The continuity buzzer will usually stop beeping when the resistance measured goes above about 50 ohms.
     
  15. rfraze
    Joined: May 23, 2012
    Posts: 2,003

    rfraze
    Member

    Today is a good day to check or change the battery in your volt ohm meter. Put a new one in and check the voltage of the one you just removed.
     
  16. JonF
    Joined: Mar 6, 2010
    Posts: 169

    JonF
    Member

    Here is one I found out about the hard way. When you take the battery out of your highboy, never set it down on the tire. Bad things happen when you jack the car up and the battery hits the concrete. As me how I know.
     
  17. rfraze
    Joined: May 23, 2012
    Posts: 2,003

    rfraze
    Member

    Can a volt meter measure voltage at the output side of a ballast resistor without a connection to the coil? Would it be a different reading if connected to coil?
    Discussion here: http://www.jalopyjournal.com/forum/threads/chevy-ignition-question-ballast-resistor-or-not.965760/ brings up the question.
    If you can measure voltage at the end of a wire that is not connected to anything, why would you not be able to measure voltage at the output side of a ballast resistor without it being connected to anything but the multi meter and it's grounded black lead?

    School us, please.
     
  18. Yes, you'll get different readings connected/disconnected, with a lower reading connected. The reason for this is a voltage meter presents an extremely small load (very high resistance). As an example, most ballast resistors are designed to reduce voltage by about 1/2 when the key is in the 'running' position. So assuming a 3 ohm coil, a 3 ohm resistor would present the same voltage drop as the coil (in a series circuit, the sum of the voltage drops across each resistance equals the applied voltage). So with the coil in the circuit, you'll drop six volts across each. Now remove the coil and connect the meter. Now your resistances are 3 ohms for the resistor and 2K ohms for the meter (best guess) for a total of 2003 ohms. Current drops to .oo6 amps. Voltage dropped at each resistance is E = I x R, so plugging in the current and each resistance we get .00018 volts dropped at the resistor and 11.9998 dropped in the meter. Most meters won't resolve that low, so you see full voltage at the resistor if it isn't connected to the coil.

    You can actually get three different readings when reading at the resistor with the coil connected (points ignition); about 6 volts with the points closed and the motor not running, 8-9 volts with the motor running, and 12 volts with the motor not running and the points open (coil disconnected).
     
  19. rfraze
    Joined: May 23, 2012
    Posts: 2,003

    rfraze
    Member

    Live and learn. Thanks Steve, Squirrel, and Ol Buzzard.
     
  20. Gasser_Dave
    Joined: Aug 18, 2013
    Posts: 63

    Gasser_Dave
    Member

    I know a lot of you don't run stereos or anything, but a voltmeter can help out finding which wires are which if you cannot see the back of the speaker. After you use your meter to find ground, hot and switched 12 volts, (don't forget your illumination wire) clip your negative lead onto a left over wire in the bunch and probe the others. Once you find a speaker, it will read around 3.6 ohms or a little higher if it is the stock factory speakers. Now, twist those wires together and find the rest of the speakers. Next you have to figure out the polarity of the speakers. Easy peasy- take a AA battery and tape a wire on each end, (I use red on the positive terminal and black on the negative) then take a pair of wires you found and touch the two battery wires to the 2 speaker wires. you should hear an audible pop from that speaker. Next, have a helper watch the speaker cone while you are "popping" it. if the cone comes out, the wire your positive wire from the battery is hooked to is the positive terminal of the speaker. Keeping speakers in phase will give you the correct sound. if wired out of phase, you will have an extremely tinny noise coming from the system.
     
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  21. rfraze
    Joined: May 23, 2012
    Posts: 2,003

    rfraze
    Member

    Great tips, Gasser Dave. Did not know how to check speakers.
     
  22. rfraze
    Joined: May 23, 2012
    Posts: 2,003

    rfraze
    Member

    Bump for Multi Meter tips.
     
  23. i.rant
    Joined: Nov 23, 2009
    Posts: 2,751

    i.rant
    ALLIANCE MEMBER
    from Illinois
    1. 1940 Ford

    Missed this thread first time around, mucho knowledge to be had here, keep the tips coming.
     
  24. The37Kid
    Joined: Apr 30, 2004
    Posts: 26,675

    The37Kid
    Member

    Who sells the correct Multi Meter/Volt Meter that does the above mentioned tests? Buying the correct one saves money and the never ending raft of shit you'll read about buying the wrong one. Bob
     
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  25. rfraze
    Joined: May 23, 2012
    Posts: 2,003

    rfraze
    Member

    One thing you should look for is an audio continuity test ability. Put switch in the right position, touch leads, and get a beeping sound. Makes things easier when testing wires for continuity.
    I would also suggest a low battery indicator. All kinds of confusing info come from a meter w/low battery.
     
    DBruce likes this.
  26. Remember that any meter I've seen with a audible 'beep' for testing is for checking diodes, not necessarily for continuity. They can be used that way, but you have to remember that the display isn't showing ohms but rather 'pass through' voltage. If you want to measure ohms, you have to use the ohms scale...
     
  27. rfraze
    Joined: May 23, 2012
    Posts: 2,003

    rfraze
    Member

    Thanks again Steve, for your input and for being easy to understand.
    So, you might get enough voltage thru a damaged wire to beep, but not to carry the voltage?
    1. What is the "best" way to test for continuity in a long wire? First, test for full continuity in your jumper wire, then continuity thru the wire to be tested and the jumper wire back to the meter probe?
    2. Which way does the current travel between red and black probes? Diodes only flow one way, right?
     
    DBruce likes this.
  28. A continuity test is a limited test; while very useful, it's only telling you that voltage can flow between two points. It doesn't tell you the current capabilities between those points and it's current that does the 'work'. So checking for continuity may not tell you how the circuit will actually work under load. A meter uses an extremely small current for testing, and voltage drop (voltage 'lost' in a circuit) is a function of resistance times actual circuit current (Vd = R x I). As current goes up, so does voltage drop. Note that circuit voltage has NOTHING to do with voltage drop, it's strictly about resistance and circuit load. Now, one of the problems with the 12V circuits we're dealing with is voltage drop can easily be a significant percentage of the available voltage. So for an example, a circuit that has a .5 amp load and drops 1 volt due to circuit resistance will drop 10 volts if the load is increased to 5 amps, all else being equal. A straight continuity test (using ohms) won't show this unless you do the math and your measurements are within the resolution limits of your meter (which may or may not be true). So from this standpoint, there is no 'best' way to check for continuity.

    Now I'm not saying 'don't use the diode test position' to check for continuity (I use it all the time), just be aware that all it's really telling you is the wire does go from point A to point B. The meter display is only useful if you're checking a diode. If the circuit still doesn't operate right, different tests will be needed to pinpoint the cause.
     
  29. rfraze
    Joined: May 23, 2012
    Posts: 2,003

    rfraze
    Member

    I'm back at a test, having to refer to this great info.
     
  30. H380
    Joined: Sep 20, 2015
    Posts: 428

    H380
    Member
    from Louisiana

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