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I have confined this guide to use of a multimeter on 12V systems to measure current and voltage. Whilst there is no danger of electrocution from such a low voltage there are still dangers. Your vehicle battery, leisure battery and alternator are capable of delivering currents high enough to burn you, set fire to the vehicle, explode your battery and at the least ruin your multimeter and expensive systems,
Some precautions before working on 12V systems
Take off your watch especially if it has a metal strap.
Remove rings or tape them over.
Do not disconnect cables whilst they are passing current.
Be especially careful not to cause sparks by such disconnections near the battery.
When isolating or removing a battery always disconnect the negative lead first.
Finally if you have any doubt leave it to a vehicle electrician or have someone qualified to show you what to do.
Measuring current
Current flows through the wires so to measure current the circuit needs to be broken and remade with the multimeter. Note that a multimeter will have a maximum current rating and often this is coupled with a time limit
ie 20 Amps for 1 minute - and that is a good quality meter.
So for example you can't use this type of meter to measure the current drawn by your starter motor or the total current being put into the vehicle battery by your alternator or even the current being put into your leisure battery by most split charging systems. Also the dc feeds to invertors are often high current.
Also note that as well as a switch to select current or Amps the positive test lead will most probably have a seperate current (amp) socket to connect to rather than the one used for voltage readings. In any case its always good practice to start on the highest amp range to avoid overloading your meter.
So as an example to measure how much current your habitation lights are using.
Identify which fuse supplies the lights. Turn off all non essential lights. Break into the circuit at any convenient point. In this example removing the fuse would do but any point in a series circuit carries the same current so breaking the circuit anywhere will do as well
Insert your meter set on the current range, in series with the fuse holder or circuit, and operate the switch to turn the lights on, note the reading.
Note that one other way would be to bridge the switch with your test probes in that case the light should come on without the switch being operated and the current would be indicated. As said above anywhere will do as long as its part of the same series circuit.
Problems you might encounter.
First of all its not always easy to tell what circuits branch off if the junction is hidden, for example on my van if you turn off all the lights the same supply feeds my ceiling fan and that draws a tiny current with all switched off.
Secondly if you don't know how much current a circuit should be drawing then how do you know whether your meter can take it. Well most meters will show an over range display and be quite safe for a few seconds and all I have encountered also have an internal fuse on the high current range.
In general though current measurements are less likely to lead you astray than voltage measurements.
Measuring Voltage.
Voltage exists at a point relative to another point in the same circuit. As normally all the negative connections from any battery are connected to chassis then measurements from the negative terminal of a battery to any point in any of the vehicle 12V systems are valid so taking our simple circuit how can we measure voltage.
First of all get into the routine of checking that you haven't left the test lead in the current socket or the meter switched to Amps. Then put your meter across the circuit the negative lead on an earth point and the positive lead where you want to measure as illustrated below.
So in the above case if the meter reads 12.5V measured near the fuse and 0V near the lamp then probably the switch is faulty or a wire has become detached.
Problems you might encounter.
Digital meters have one feature that can lead to confusion. They are designed to draw only a very tiny current. This can lead to problems of interpretation
Take the reversing light on your motorhome. Say one day you notice the lamp doesn't light you look at the bulb it seems OK you put your meter on the contact with bulb out of its socket and you get 12V, you put it back in and measure say 3V.
The most likely fault is the reversing light switch (on the gearbox) with a high resistance contact when closed. Can't be that! you say I have measured 12V with the bulb out.
Well if you measure voltage in a circuit with no current flowing (and no current can be flowing as you have removed the bulb) it doesn't matter much how bad the circuit is you will measure the full battery voltage more or less anywhere that is connected (however badly) to the battery. Now with a bit of Ohms law and some sums I could prove why but for the sake of this just trust me - its a feature
Of more interest is what the voltages are when you put the good bulb back in.
As you can see with the bulb in the voltage either side of the bulb is 3V (point F) and 0.1V (point G) so across this 12V bulb there is only 2.9 volts and not surprisingly it hardly glows.
Starting at the battery with 12.7V (point A) a little bit of the voltage is dropped by the wire on the way to the fuse (point B) a bit more is dropped by the fuse (point C) a bit more by the wire that goes to the reversing light switch (point D) but then 9.4v is dropped by the switch (point E). Of course you might have found that the voltage was dropped over the fuse and fuseholder or over one of the connectors or the base of the lampholder.
I hope this gives you an insight into the use of a multimeter in fault finding. Of course its other use is putting figures on to what you say on forum posts about your battery condition and/or whether things are charging or not.
This diagram reproduced here from the 12V Systems members guide to save cross referencing:
Features of a simple digital multimeter
Here is a typical digital meter selling for less than Ģ15 with a 10A dc range.
Most cheaper meters have a range switch which as well as selecting the range also selects the function say voltage or current measuring. They come complete with test leads and probably a holder or pouch.
Even the cheapest will have several resistance ranges useful for testing fuses and bulbs out of circuit or with the power off and may also have an audio beep which is useful for continuity, again when all power is off. A special feature may test diodes. The one illustrated also tests transistors.
As well as the obvious features it has an internal battery (can give really odd results if low on the cheaper meters if no warning is given) and at the very least the high current socket will have an internal fuse.
Last edited by sallytrafic on Fri Feb 26, 2010 8:02 pm; edited 8 times in total ______________________________________________________________ Regards Frank - - please follow me on twitter @FrankieBryant
Denn wir haben nichts in die Welt gebracht; darum offenbar ist, wir werden auch nichts hinausbringen.
RIP Peter and Fiona Our thoughts are with you Chris and Graham. Fighting with Mavis and Ray
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Great start Frank and nice diagrams. A really useful addition to the fount of knowledge in my view.
Given their popularity and the number of queries / misunderstandings they invoke, might it be worth including the input current of (most) inverters in your list of things not to try and measure?
capable of delivering currents high enough to burn you, set fire to the vehicle, explode your battery and at the least ruin your multimeter and expensive systems.....................
ThePrisoner You had better believe it I can probably find some pictures
Now up for comment
______________________________________________________________ Regards Frank - - please follow me on twitter @FrankieBryant
Denn wir haben nichts in die Welt gebracht; darum offenbar ist, wir werden auch nichts hinausbringen.
RIP Peter and Fiona Our thoughts are with you Chris and Graham. Fighting with Mavis and Ray
Plusnet, Safari 5, G5 PowerPC iMac running OSX 10.5.8, Salisbury UK : Get behind early - it gives you more time to catch up.
I have added a voltage table and tidied up a diagram
______________________________________________________________ Regards Frank - - please follow me on twitter @FrankieBryant
Denn wir haben nichts in die Welt gebracht; darum offenbar ist, wir werden auch nichts hinausbringen.
RIP Peter and Fiona Our thoughts are with you Chris and Graham. Fighting with Mavis and Ray
Plusnet, Safari 5, G5 PowerPC iMac running OSX 10.5.8, Salisbury UK : Get behind early - it gives you more time to catch up.
Very good Frank. You might add that when measuring current, start off with the meter set to the highest measurement range and reduce range until a reading is obtained. Don't know how digital meters behave, I don't particularly like them, but I have bent analogue pointers before and it certainly cannot be good for the meter to be on too low a range.
peedee
______________________________________________________________ The best journeys are not always in straight lines
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______________________________________________________________ Regards Frank - - please follow me on twitter @FrankieBryant
Denn wir haben nichts in die Welt gebracht; darum offenbar ist, wir werden auch nichts hinausbringen.
RIP Peter and Fiona Our thoughts are with you Chris and Graham. Fighting with Mavis and Ray
Plusnet, Safari 5, G5 PowerPC iMac running OSX 10.5.8, Salisbury UK : Get behind early - it gives you more time to catch up.
Thanks ever so much Frank, I have just finished copying your useful post and shall keep it in my mhome together with my analogue meter, now that I understand better how to use it !!!
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France
Our thoughts are with you Chris and Graham. Fighting with Mavis and Ray 
