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Getting the most from your DVM



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Old 11-18-2006, 06:52 PM   #1
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Getting the most from your DVM

I plan to add to this article as time allows. When it is finished I will ask Rastus if it may be put into the TECH SECTION.

DVM's or digital voltmeters range in price from $5 to several hundred dollars. You can make many checks with a simple $5 meter. Pick up a couple and keep one in your garage and one in your trail rig tool box.
I will include testing and use information for 1st time users and even the most experienced mechanics. If you have a question PLEASE fell free to ask it here or by PM if you do not want it posted. I hope this article helps you out.

*****CAUTION***** As always take care to use this tool properly to avoid personal injury. If you do not know how to do a test ask for help. We want you on the trail in one piece.

1st I am going to bore you with the bare minimum of Terms and definitions:

Resistance measured in OHMS is the restriction of electron flow in a circuit.

Current measured in AMPS is the measure of the volume of electron flow in a circuit.

Electromotive Force in VOLTS is the potential of the energy stored in your battery or if you are talking household use the potential energy available at the wall socket. The main thing to remember is the lower the voltage you are working with the more current it takes to get work done. The higher the voltage you are working with the less current it takes to do the work.

OHM's law: P=IxE or I=P/E or E=I/P The memory trick here is PIE mmm good.
P is measured in WATTS and I is the amount of current in AMPS and E is electromotive force in VOLTS. Now this is not a lot of useless dribble. The main thing you might use the above formula for is figuring just how much current 2 of those 100 WATT off road lights you just installed will draw from the electrical system of your rig. So lets plug that in:

I=200 divided by 12.6 volts so I, the amount of current the lights will draw is 15.87 so lets round that up to 16 amps. You would need to use a 20 AMP fuse also since a 15 would blow.

****CAUTION***** As little as 24 Volts AC or DC at a CURRENT of as little as 20 milliamps could kill you.

A SHORT is just that, the wire or circuit trace has not made it to the intended termination point, it got"shorted" out. Also called a short circuit. This will usually be accompanied by a blown fuse and/ or a melted wire or bundle of wires, not a good thing at all. A short circuit remains "shorted" till repaired.

OPEN or OPEN CIRCUIT is different from a short in that the wire or circuit trace has become disconnected from the circuit. Maybe a loose connection and could also be a loose or poor ground connection all could fall into the OPEN CIRCUIT category. An open circuit could be as simple as a blown fuse. An open circuit is one of the easiest to fix. We will get to how to trace a short and an open in a few paragraphs.

The parts of the DVM are:
Display.
+ test lead connection.
- test lead connection.
COMM is used on some DVM's instead of -
Another connection is available and will be labeled as I or AMPS or CURRENT
Never leave your test lead in this hole unless you are testing the current draw because it is usually limited in the amount of AMPS it can handle and you could blow your meter if you go to check voltage and have forgotten to put the test leads into the COMM and + holes. Also if you were checking the rigs battery voltage for instance it could even melt the leads and give you a nasty burn on your fingers at the very least.

Uses for your meter:

1. The most common use is to check battery voltage. Do not start the engine yet. The best way to check the rigs voltage is to touch the red test lead to the + post and the black test lead to the - post and record the voltage. Next leave the red lead on the + post and touch the black lead to the point where the battery ground cable makes a connection to the vehicle frame. record this voltage and it should be the same as when you were at the battery with both test leads. Next go to the alternator and touch the red test lead to the connection point where the battery charging cable connects to the alternator and find a good clean spot on the frame or better yet the - post of the battery.
*** At this time if you are not getting any voltage or if the voltage is way lower than it was at the battery you have an OPEN. The only way to trace this is to start following this cable back to the battery and find the cut or burnt up place in the cable. Of course you need to 1st be sure it is not a bad or corroded connector***
Next go ahead and start the vehicle and do this test again and notice the voltage has gone from a STATIC (battery at rest fully chrged) 12.6 VDC (volts DC) or maybe a little higher to around 14 VDC if the alternator is working well. The charging voltage will vary with temperature and with the condition of your battery and even the condition of your alternator.
*** If you have a frind you could have them start the vehicle while you had the test leads on the posts of the battery and you would be able to observe the VOLTAGE DROP while cranking. The worse the voltage drops the more AMPS your engine is using to start or your battery may be too small and not have enough CCA (cold cranking amps) to do the job.

2: The next test involves checking to see what the current draw is with the engine off and all doors closed. If you have a light under the hood go ahead and unplug it just for this test. put you meter in the correct position for measuring current in AMPS and set it on the 10 or 20 AMP setting to be safe. ***** CAUTION****** be sure you have everything off at this time, no dash lights, no overhead interior lights. The idea here is to test to see if we have too much current being drawn when the vehicle is NOT running or being used. You will need to loosen and remove the + cable from the battery post and attach your meter to the post with one test lead and use a test clip wire to attach the other test lead to the battery. BE CAREFUL not to let any of these leads touch any other metal, they are "HOT". The meter should not read any more than 100 milliapms and really that is a lot but should not drain your battery overnight. the current you are seeing here is the radio keep alive memory and maybe what the cellphone charger LED is drawing. If your vehicle is drawing much more than this the way to track it down is to start unplugging accessories from the vehicle and you will see the reading on the meter drop. Generally speaking the radio will draw about 10 to 15 milliamps and an LED light on a cellphone chrger will draw .05 milliamps, not very much. You can actually test accessories one by one by unplugging the power wire and putting the meter inline with that wire IN THE AMPS TEST SETTING and see how much current it is drawing. **** Remember when testing and reading AMPS you will never have one lead touching GROUND and the other on a "HOT" wire. SERIOUS damage to your meter and/ or the rigs electrical system will result.
3: The next test is to use the meter to trace an open circuit or wire. You will need the meter in the normal mode witht eh black test lead in the comm hole and the red test lead in the Volts/ Ohms hole. Put the meter on the 20 volt DC scale. You will need to ground the black test lead to the chassis or any good ground point and the red test lead will do the tracing.
Lets check a rock light to see why it is not lighting up. 1st make sure the bulb is good and the ground is secured with a star washer to the grounding spot. You will want to start at the + connection on the inside of the light where the bulb plugs in. Check for voltage and work your way back along the + wire till you find the bad connection or cut wire etc. Remember most electrical problems are due mostly to BAD GROUNDS and secondly to poor connections, maybe the wire did not get pushed far enough into a butt splice before it was crimped etc...

4: Checking for a short to GROUND. YOU MUST Disconnect the + BATTERY CABLE before starting this test. To begin the test, switch your meter to the OHMS position and set it on the 20 or 200 or even 2K scale. You will ground the black test lead to the negative battery post or to the frame or body. [COLOR="Blue"]By the way, at this point you should at least have a cable from the battery - post to the engine block, from the battery - post to the chassis and from the battery - post to the body. You will then begin at the end of the wire in question and probe it with the + test lead. If the meter display changes to all Zeros or if you have a more expensive meter it may read "SHORT" and you will know the wire is shorted to ground. You will have to trace the wire through the bundle till you find the place it shorted either to a system ground wire or to the frame or chassis or melted to the exhaust pipe or got twisted around the driveshaft etc. You get the idea.
*****Checking a speaker to see if it is good?***** You can check a speaker while you have your meter set up in this mode, place a test lead on each terminal and the meter will read 2 Ohms for a 2 Ohm speaker, 4 Ohms for a 4 ohm speaker etc. If it reads no change on the display the speaker is open, throw it away. You can also check a light bulb which will show anywhere from 4 ohms on up to??? If it shows no change on the meter it is a bad or open bulb. CAUTION, do not check an LED light this way, it could get damaged.


More later................
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Last edited by OldGoat; 11-22-2006 at 08:42 PM..
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Old 11-22-2006, 09:01 PM   #2
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!

I'll work some more on this in a few weeks.
Happy trails!!!
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Old 01-31-2008, 05:43 PM   #3
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Bump, dammit. OG we need you back in here laying down the 411.
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Old 05-31-2008, 05:36 PM   #4
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Nice, I was given one a few years back and never bother to learn how to use it.
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Old 05-31-2008, 08:19 PM   #5
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Dang OG. I use mine everydy working on Air Conditioners and never thought to do such a detailed write up. Good work helping those who are not sure how to use them.
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Old 06-01-2008, 09:00 AM   #6
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Thanks folks. I have not taken time to update my write ups or write any new ones in a long time. Maybe in the near future a new one or two will hit me.
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Old 03-19-2009, 06:20 AM   #7
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BUMP -
http://mechatronics.mech.northwester...ultimeter.html
http://www.doctronics.co.uk/meter.htm
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Old 03-19-2009, 10:07 AM   #8
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well

Very nice.
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