HOW TO - using a multimeter and testing for faults

Discussion in 'How To' started by Diddymen, Nov 23, 2011.

  1. Diddymen

    Diddymen Moderator

    I’ve written this in the hopes that it is easy enough for some one with no experience to understand and to give them the confidence to try to find faults or diagnose where a problem lies.
    If it isn’t as clear as I’d hoped then post up, ask questions, pm me (or Im sure Matty would be happy to answer questions or others in the buisness)

    This might get a bit long (like my usual posts! ::)) so I’ll try to break it up over a few posts to make it a bit easier to read. I’ll also be adding some pics at a later date to the original posts.

    My background (if your interested) I served a 4yr apprenticeship in electrical and electronic engineering, I’ve got C&G’s in electrical installation, ONC, HNC and currently completing my HND in electrical/electronic engineering, various other electrical qualifications, I’ve got 20 years experience in electrical systems, Im a senior design engineer and I’ve been a design engineer for 6yrs…..enough of the big headed stuff ::) :-[ :p

    Before going any further it is useful to understand some terminology which I might be using


    Circuit – When I refer to a circuit I am referring to a whole system - the supply +Ve through switches, lights etc and back to –Ve


    Components – the individual bits of a circuit but not including the wires ie switches, bulbs(lamps), heater elements, motors etc


    Parallel – connecting in parallel means connecting something across another component, if you put the leads of your tester across battery terminals, either side of lights or across a fuse (without disconnecting or removing those components) then this is connecting in parallel


    Series – connecting in series means connecting something in line with. So if you disconnect your battery +Ve lead, connect one of your tester leads to the battery post/terminal and the other tester lead to the disconnected +Ve lead of your battery then this is connecting in series. Any current will have to flow through your tester.
     
    Last edited: Oct 2, 2017
    Kruger likes this.
  2. Diddymen

    Diddymen Moderator

    Voltage (symbol V) – imagine a pipe full of water, for the water to flow it would need a pressure behind it. Voltage could be likened to that pressure – this is only an analogy, it isn’t really a pressure.
    Voltage is a potential difference between 2 points, in simple terms, if 2 wires both carry 12V there is no difference in voltage between the wires, so a volt meter connected between the 2 wires will read zero volts. If you now measure one wire against ground (or earth) there will be a difference of 12V between the 2 points (the potential difference). A potential difference is needed at the terminals of a component for current to flow and for the component to work (to an extent.....I'll explain that later).


    Current (symbol A – measured in Amps) – if we use the analogy of a pipe filled with water again, Current can be seen as the flow of water. If the pipe is obstructed or broken some where, water will not flow out of the end of the pipe. A switch or a broken wire will have the same effect, if a switch is in the off position or a wire has a break in it no current will flow. If no current flows through a component such as a lamp, it will not work. A bigger pipe will allow more water to flow than a smaller pipe, the same is true of cables, bigger diameter cable will have less resistance and allow more current to flow.

    At this point it is worth noting that if you are installing new equipment and wiring in your bus that you ensure the current rating of the cabling is suitable for the equipment other wise you could risk causing a fire – ie if your equipment will be pulling 5A then you don’t want 2A rated cabling ……but that’s another topic which may involve a few calculations which if I get round to it may or may not happen! ::)


    Resistance (symbol Ω measured in ohms) resistance is fairly self explanatory, if you have a blockage in a pipe or a small bore pipe, the flow of water you will be expecting at the end will be greatly reduced. Smaller diameter cables will generally have a higher resistance than the same length of a large diameter cable. In electrical circuits high resistance can be caused by poor connections or faulty components, this resistance will not only pull more current from the supply, it can also cause high temperatures at these points (possible risk of fire) and reduced current flow through the final circuit leading to improper functioning. Every where there is a high resistance you will get a voltage drop across it – less voltage and current flow at the component (dim lights etc).


    most of what Im posting is probably available if you goole it, it might also be better explained ;D

    ..........I'm getting to the testing!!
     
    Last edited: Oct 2, 2017
    D5andy likes this.
  3. Diddymen

    Diddymen Moderator

    Using test equipment........

    Firstly knowing how to take measurements and use your multimeter is one thing, knowing what the readings are and how they relate to the circuit is the key to understanding what is going on. A circuit diagram with the colours of the wires shown, and understanding that diagram can be really beneficial in fault finding. Knowing how a circuit is supposed to work and where all the wires come from, are connected and go to will help your fault finding no end :D

    In most circumstances a cheap multimeter should be all you need for fault diagnosis on a vehicle as old as a bay, in [glow=red,2,300]FACT[/glow] you could even get away with using a test lamp for a range of faults (I’ll try to cover that later).

    Heres a selection of my multimeters, I would imagine if you've got a multi meter it will look like one of these

    [​IMG]

    [​IMG]

    [​IMG]

    There are a few different testers on the market, most multimeters will all test the same things, voltage, current and resistance. There may be a few other functions so its always worth reading the instructions which come with your tester (unless you’ve inherited it) but most of them are not needed for testing an old VDub


    Before starting, leads – some testers will have the leads ‘hard wired’ to it like the one below

    [​IMG]


    others you will have to plug the leads into holes/sockets on the tester. Again there are variations to the type with plug in leads, some have 2 holes for a red and black lead (needs no explanation) and some meters will have 3 or 4 different sockets, so ensuring you have the leads connected to your meter correctly will ensure it is working properly.

    below are an example of multimeters with multiple sockets

    [​IMG]

    [​IMG]

    For meters with multiple sockets, one is usually marked common (COM) and is coloured black, the other 3 are usually coloured red and next to each socket should be a symbol for what it is used to test – most of the time one hole will have a V next to it and an ohm symbol.

    When testing for voltage you will need to connect the leads to the COM socket and the V socket as in the pic below.

    [​IMG]

    [​IMG]

    The other sockets will be used for testing current at different ranges, so connect one lead to COM and the other to the current range you are testing as below.

    [​IMG]

    So in summary, connect the black lead to com and the red lead to the socket which corresponds to what type of test you are going to carry out.

    Next - testing voltages
     
    Last edited: Oct 2, 2017
  4. Diddymen

    Diddymen Moderator

    Hopefully this is helping and I've not boared any one or lost any one ;D

    how am I doing? ;D

    ok, I've ammended the previous post with pics....onto some testing

    testing for voltage

    Voltage – what can you achieve by testing voltage? Firstly you can make sure voltage is getting to where you want it, that switches are operating, that wires are connected and not broken, fuses are ok and haven’t blown and you can, to an extent, discount or confirm a component is faulty thus narrowing down your fault finding. I tend to test for voltage first in most instances.


    Measuring Voltage – This is simple enough to do, however, some multimeters have 2 settings for measuring voltage which is worth mentioning here. One for AC and one for DC (no not the rock group!)

    for vehicle electrics you need to set your meter to DC – this is usually shown with a dotted line under a solid line next to the letter V. if you have a V and a squiggly line (sine wave symbol) this is for AC and is not used on vehicles the pics below should explain.

    DC voltage is to the right of the 'off' position (second click/position of switch)

    [​IMG]

    again DC voltage is the second position after 'off'

    [​IMG]

    It is important to make sure you have the right setting.

    If there is only one selection for voltage on your meter, it is probably an automated tester which can sense the difference between AC and DC so just set the switch to the letter V and your ready to start – the DC symbol will usually be shown on the screen when you are testing. see pic below where voltage would be the first position after off

    [​IMG]

    So how do we actually test voltage?

    Firstly set your tester to read DC voltage if necessary. If you now take the 2 wires connected to your meter and connect the black to the negative/ground (-VE) on your battery and the red lead to your positive(+VE) you should get a voltage displayed. (We will go into terminal voltages of batteries and what they mean later).

    If you get a minus voltage showing, it just means you haven’t plugged your leads into your tester the right way round ie black lead in V and red lead in COM - which is the wrong way (if you have that type of meter) or if your colour blind you might not have put the red to positive …..it doesn’t really matter, it wont damage your meter and it will still show that a voltage is present, and that is all we need to know for this type of fault finding.

    Now, for the testing of your circuits for voltage................
     
    Last edited: Oct 2, 2017
  5. Diddymen

    Diddymen Moderator

    ...............But before I do we better go back to some basics ;D

    It may seem obvious to a lot of people that the whole body of the vehicle is negative, but to others this might not be so obvious.

    In all circuits you need a supply and a return path. In your house the supply is the ‘live’ and the return is the ‘neutral’ this is simplifying it somewhat but hopefully serves as an analogy.

    In a vehicle you have the supply to circuits as a ‘live’ or 12v from a wire but the return path (ground or negative – Ve) is the whole body of the vehicle. This is simply achieved by connecting the –Ve of the battery to the bodywork, as the bodywork is metal (and hopefully not 100% filler >:D) it conducts allowing a return path back to the battery for current to flow.
    Doing vehicle wiring this way greatly reduced the amount of wires needed. Imagine having to run an earth back to the battery (or connect into other earths) for every circuit! This way you can take a local connection from the body work for your earth.

    I will probably be referring to testing for 12v quite frequently, by this I don’t mean 12v exactly I just mean the battery voltage. A charged battery can be around 12.7V but Im not going to keep talking about the .7V for testing purposes.

    So now we know this, we do not need to connect our tester back to the battery –Ve each time, we can instead connect the black lead to the body work (or earth terminal) remember if connecting to the body work it will have to be to a piece of bare metal – it wont work through paint! :(

    With the black lead now connected to earth you only need to use the red lead to probe at different points or terminals to test for voltage.

    This has taken a while and Im getting kicked off the PC soon!

    So next I will continue with testing for voltage for fault diagnosis, then we’ll cover testing for current and battery drains….followed by resistance, continuity and other testing ;D
     
    Last edited: Oct 2, 2017
    Kruger likes this.
  6. Excellent Diddy. I'd judge from Techenders people are now starting to yawn a bit - tell them how to test something they will want to test. :thumbsup:
     
  7. Looking good get ready for the questions
     
  8. I've been reading all this about how to use a multi meter - really useful and pitched right at my level. Is there any more? Showing my ignornace about how to use the site but I can't find where it goes. Anyway thnaks for the bit I did read - it was great. John
     
  9. Diddymen

    Diddymen Moderator

     
    Last edited: Oct 2, 2017
  10. good stuff this might be able to make my lights work at the weekend lol k+++
     
  11. Diddymen

    Diddymen Moderator

    Thanks Clowie :D .....good luck with the fault finding, my next post may possibly help a little :D

    Ok, lets get on with some examples,

    I’m going to cover some general fault finding processes and use the voltage function on the multimeter to narrow down or pin point the fault.

    As mentioned earlier, its very useful to have a circuit diagram to help you understand how a circuit works. If you haven’t got one then check out the link below to download or view the right diagram for your year of bus

    http://www.vintagebus.com/wiring/

    for this I will be talking you through the diagram ‘VW type 2 - from August 1970 (1971 models)-shown below …..only because it seems most relevant to mine and the file fits onto one page. A lot of it should still be applicable to a lot of models

    Sorry the pic is a bit small but you can down load it from the link above

    [​IMG]


    We’ll start off with an easy to understand circuit, the rear interior light.

    You may or may not have this fitted but it’s a nice simple circuit to understand and test, you may also have had other items fitted by previous owners which may confuse things but for this we will assume yours is standard as per the diagram.

    I will talk through it several times, each time the fault will be somewhere else in the circuit.

    A quick description of the circuit – the rear inner light (W1 on the circuit) comes on when you flip the switch (E6 on the circuit) to the ‘on’ position, the switch is a separate component (which may be mounted on the dash board) the light (W1) will illuminate if the ignition is ‘on’ or ‘off’ as it is fed by a permanent 12V supply comes which comes from fuse 9.
     
    Last edited: Oct 2, 2017
  12. Diddymen

    Diddymen Moderator

    Scenario 1

    Well the problem is obvious, the light doesn’t work. Before we use the testers its worth having a look at the ‘bigger picture’.

    Firstly have I been an idiot and not put the switch to ‘on’? – a quick double check of the switch – no its defiantly not coming on with the switch in either position

    Do we know if there is anything else that has stopped working at the same time? – not that we know of, we do know that the ignition key warning buzzer (H5 on the circuit) (if fitted) comes from the same fuse, is that still working? Yes buzzer is working so that rules out the fuse being blown.

    You don’t have to start reaching for the circuit diagram straight away to see what’s connected to the same fuse, I’m just using this example to sort of prove a point, if you know some thing else is connected to the same fuse off the top of your head, its just a quick indicator that the fuse is ok.

    Its always worth checking the fuse at the start of fault finding (especially if there are no other circuits fed from it) as it’s a quick fix if its blown and will save you taking stuff apart if you don’t really need to. Also check the fuse is making good connection to the fuse box – the fuse may not have blown but it may be loose or making a poor connection possibly through a bit of corrosion/oxidisation.

    So we know that the fuse is good (by testing or elimination), and that the switch is in the ‘on’ position……..and its still not working.

    At this point I would go straight to the light. First we need to remove the cover. Inside we should see a brown wire (earth) and a red wire (supply/12v)

    We know how to set our tester up for voltage so now we are going to use it at last!! ;D

    First find a good earth, don’t always rely on the earth that is in the thing you’re testing as this may be what is faulty. With your meter turned on, leads plugged in and set to voltage (dc) connect the black lead to a good earth (bare metal of the body, a bolt or screw head or a known good earth wire)

    Take the red lead from your tester and touch it to the terminal the red wire is connected to in the light fitting.

    Do we have 12v (or there abouts)? – yes!! :D

    So what does that tell us? Well, we now know that the wiring is all good up to the light, and that the switch is fine so the problem is most probably either the light, lamp(bulb) or the earth

    We have cut out a lot of things we now dont need to test and narrowed the fault right down.

    Next I would check that the bulb hasn’t blown (which I should have done before testing! ::))

    Bulb looks fine, so that means either the bulb isn’t making a good connection to the fitting or the earth is bad (either poor connection, not connected or broken) I’ll go through how I would test for these when I cover ‘testing for continuity’

    So it should be easy enough to fix the problem now we know where it is.




    Scenario 2


    Lets go back a few steps, and the fault is now somewhere else.

    We’ve tested the fuses, switch in the right position etc.

    We are now at the light fitting and we test for voltage at the terminal the red wire is connected to. Is there 12v? - NO! :(

    So what does that tell us this time? Well it means that the light fitting, bulb and earth at the light are probably all good. So the fault must be either the switch or the wiring.

    I would now go to the switch. You may have to remove the switch from the dash board if you cant get to the back where the terminals are to test them, leave the wires connected and take off the switch knob and unscrew the little silver retaining disk (if you have that type of switch)

    Hopefully you can bring the switch out from under the dash with some slack on the cable so you can test.

    Testing for voltage again - Find a good earth and connect the black lead to it. Touch the red lead to one side of the switch, are you getting 12v? – NO

    Try touching the red lead to the other wire on the switch are you getting 12V? – Yes

    What does this tell us? Well we know that we are getting a supply to the switch but its not getting ‘out the other side’ so it is most probable that the switch is broken and we will need a new one.
    Touch the lead back on the other wire (where there was no 12V) and operate the switch just to double check – still no voltage? – no – defiantly the switch is the problem (or poor connections from the wire) – I’ll go through testing switches under ‘continuity testing’

    As a double check, you can take both wires off of the back of the switch and touch the wires together. You are now bypassing the switch and the light should come on.....thus proving the switch is faulty



    Scenario 3

    We’ve done the usual checks, fuses, switch in the right position etc. we’ve tested for voltage at the light and we are not getting anything.

    We’ve now gone to the switch and we are getting 12V on each terminal in this scenario, what does that tell us?

    We know the switch doesn’t seem to be the problem (as we are getting 12v in and out) and we are not getting voltage to the light so the most obvious thing must be that there is a break in the wire from the switch to the light fitting (if its joined some where it may have come out of its connector.)

    Bummer, that will mean I might have to run a new cable!! :mad:


    If you were getting no 12v at all at the switch, but the fuse is fine then you most probably have a break in the wire (or poor connection) between fuse box and switch.


    Hopefully talking through that with a fault in a different place each time will help to understand how we can use the voltage setting on the multimeter to find out where a fault lies. You can use the same principle on just about any circuit. :D

    That’s all for now as its getting late………..
     
    Last edited: Oct 2, 2017
  13. Diddymen

    Diddymen Moderator

    thanks Chris :D

    Right we’ve covered the basics with voltage testing, we will move on to continuity and resistance in a bit.

    Before I do ::), its worth pointing out a few things regarding ‘live’ wires and general faults. There are basically 3 types of ‘live’ wires,


    permanent ‘live’ – there are a number of ‘live’ wires on the bus which are permanently live. By this I mean if the ignition is off or on (and as long as the battery is connected) there will always be 12V – such as the one in the example above for the rear interior light (the wire from the fuse to the switch). If you disconnect these for testing make sure they don’t touch any body work as this will blow the fuse.

    Ignition ‘live’ – these wires will always have 12V as long as the ignition is on (not necessarily with the engine running) as above, don’t let them touch the body work whilst the ignition is on or you will blow a fuse.

    Switched ‘live’ – these wires will not have 12v in them until a switch is turned ‘on’, this would be the wire from the switch to the light fitting in our rear internal light circuit, again, don’t touch it to the body whilst it is live.



    Generally, faults usually fall within one of 4 categories

    Faulty Component – such as blown lamps or fuse, faulty relays, broken switches etc. The fix is simple enough once diagnosed, simply replace the faulty part.


    Broken wires
    – faults in wires can be a break in the conductor (the metal in the wire), you might not see it if the insulation is intact, symptoms are usually intermittent and can be difficult to trace – you cant find the fault if it suddenly starts working :mad:!

    Generally though, most faults with wires are usually confined to the ends where they are crimped. Over time as metal is repeatedly flexed it becomes brittle and will eventually snap. You can some times see this at connectors where there are only a few strands of the wire in the crimp, the rest are broken off - I bet you’ve all seen similar wires on your busses :p! Admit it ;D!.
    These faults can at times be difficult to find and intermittent in nature, they will eventually break and the circuit will stop working – you have been warned!
    Another word of warning regarding this sort of damage – you can occasionally get a heat build up at these points due to the higher resistance. If they look suspect, cut the crimps off, strip the ends of the wires and put a new crimp on………it will avoid you doing it at the road side in the dark!


    Bad connections
    – we have touched on that a little in the paragraph above, If the wire is still intact and the crimp looks good, it maybe that the crimp is not making good contact on the terminal it is connected to. You can get oxidisation over time due to dampness, or it could be through build up of dirt – just think of the terminals on the back of your horn under the bus (if you haven’t got rubber boots over them). If it is a bad connection, then pulling the crimp off and cleaning the terminal it connects to, with some fine wet and dry sandpaper until its bright should make things better. If the crimp is loose then try squeezing it slightly closed with some pliers to make it fit tighter on the terminal. As above, you can get a heat build up at bad connections.


    Bad earths
    – these could be classed as bad connections or wire faults but I thought its worth mentioning them separately. An example of an earth fault would be where the earth on a rear light cluster has broken or rotted away, you may notice the break light flashes slightly when the indicator goes on, or the indicators may glow slightly when the breaks are applied – this is usually where current from one circuit is passing through another trying to find a path back to earth……or the light cluster may not work at all!
    Some times earth faults can cause strange symptoms. Its always worth checking the earths on a faulty circuit visually and with your tester.

    Let me know if any of you are getting board ;D, or if I need to be a bit clearer on anything.

    And finally before I get on to continuity, remember………..just because you have found a fault, doesn’t mean it’s the only
    fault on the circuit! >:D
     
    Last edited: Oct 2, 2017
  14. this great i ve finally managed to fix my main beam
    good times
     
    Last edited by a moderator: Oct 2, 2017
  15. Diddymen

    Diddymen Moderator

    nice one mate :thumbsup:

    did any of my previous posts help you track down the problem??
     
  16. Having read this I'll have to get you up to do any electricals!
     
  17. Diddymen

    Diddymen Moderator

    I'd hope by the time I've finished this thread you should be able to fault find with confidence your self :D ......if you get stuck though, you know where I am :D

    another long post comming up


    Continuity and Resistance

    I mentioned resistance earlier but what is continuity?.

    Continuity simply means continuous, if we are talking about testing the continuity of a wire, we are basically saying is there a break in the wire or is it continuous? …….or is there a good electrical connection between 2 points.

    So what can you achieve by testing continuity or resistance?

    Well, we can check that a wire doesn’t have any breaks in it, check fuses aren’t blown, bulbs aren’t blown and that switches operate. If we test the resistance of certain components, it can, in some circumstances, tell us if the component is faulty. So using continuity testing along side testing for voltage can narrow down where the fault is even more.

    Its worth pointing out that you should not test for continuity on live circuits as the voltage from the bus may interfere with the meter giving false results. It shouldn’t damage your tester, but it may be worth removing the fuse to the circuit you are testing.

    So how do we test for continuity?

    Its just as easy as testing for voltage. Firstly we need to set up our multimeter as before. If you have the type where you have to plug the leads into the meter, the connections should be exactly the same as for voltage testing – Black lead into COM and the red lead into the socket marked V which usually has an Ω symbol next to it.

    Next we need to select continuity on the meter. Some multi meters have a separate position for continuity and resistance (they do the same thing) others only have one setting. For testing continuity I use the setting with an audible tone, the switch position will have a symbol of sound being emitted – bit hard to explain so see the pic below!

    [​IMG]

    Others may have a second symbol next to it, but will still show the ‘sound’ symbol

    [​IMG]

    And if you only have the combined function of resistance and continuity it will have the ‘sound’ symbol and an Ω symbol

    [​IMG]


    Once the meter is set up, touch the two probe ends on the meter leads together and you should ge a continuous ‘beeeep’ sound, if you have we’re ready to go

    Firstly then we will test if a fuse is blown


    Pull the suspect fuse out of its holder, if it’s a blade type, hold one probe on one blade and the other probe on the second blade. If it’s a cartridge fuse, hold the probes against the fuse at each end.

    If you hear the ‘beeeep’ then you have good continuity and the fuse is ok. No ‘beeeep’ and the fuse is blown.



    Testing wires
    is done exactly the same, disconnect the suspected faulty wire at each end. Hold one probe on one end and the other probe on the opposite end – if you hear ‘beeeeep’ the wire is good.

    You might find that your tester leads don’t reach each end of the wire so how do we test it then?? – easy, we sort of bring the other end of the wire closer!
    Ok so we’re going to cheat a bit. Connect one end of the wire to a good earth on the vehicle body. Now you only need to test at one end. Hold one probe on the opposite end the wire you have shorted to earth, and the other probe touch to the vehicles body. The multimeter will then test continuity through the wire and all the way back to the meter via the body work – cleaver eh?! 8)

    Testing lamps/bulbs is the same (not LED’s or fluorescents), this time though you may find you don’t get a ‘beep’. This is because the lamps have resistance. In this case you might find you get some figures on the meter display, this is fine, it doesn’t mean the bulb is blown. The figure on your meter is unlikely to read greater than 10 Ω. If you see O/L or >200M Ω then it means there is no continuity and the bulb is blown.

    So what about testing switches?

    Again, exactly the same way, Touch the probe to one terminal, and the other probe to the other terminal. In the ‘off’ position you wont get a ‘beep’ but you should in the ‘on’ position. If you are not getting any thing in either position then it is most likely the switch is faulty.

    On some switches you have multiple terminals. Its worth finding out which terminals should contact together before testing, either by working it out from the colours of the wires and referencing your circuit diagram, or by testing a good switch. That way you should know what to expect when you turn the switch ‘on’


    If you have a good switch that you want to use and you are unsure how to wire it up, then you can use the continuity tester to find out what terminals connect when the switch is on or off.

    Similar method, choose one terminal to start with, and with the switch in the off position touch one probe to the terminal. With the other probe, touch each terminal in turn and make a note of which ones give a ‘beep’ as the are linked together. Next turn the switch to ‘on’. Repeat the process starting on the same terminal, and with the probe go through and test all the other terminals. This time you should get ‘beeps’ on terminals which weren’t connected before. You can then work out which terminals do what and connect your wires accordingly.


    Testing earths


    Testing earths is done in the same way as testing wires for continuity. Simply hold one probe against a known good earth, metal body work or bare screw/bolt head and touch the other probe to the earth connection in/at the component you are testing. Before I touch the second probe to the component earth, I will always touch it first to a separate exposed bit of body work just to make sure the first probe is making a good contact – if it is you will get a ‘beeeep’ and you can then take the second probe and touch it to the component earth.

    If you don’t get a ‘beeep’ then the earth is either broken or making a very poor connection to the component. To double check, if you can get to both ends of the earth, test it for continuity the same way you would testing a wire.

    On some components (mostly motors of one type or another) you can get a short to earth, this is usually coupled to fuses blowing as soon as the item is switched on.

    To test a component for a short to earth
    , disconnect all the other wires connected to it and note where they go for reconnection, also note which is the earth terminal. Now touch one probe on either the earth terminal of the component, and the other probe to one of the ‘live’ connection terminal. If you get a ‘beep’ between the live terminals and the case/earth then you have a short to earth and will need to replace component or repair it.

    That’s about all you need to know for continuity testing. I will do a little bit on resistance perhaps tomorrow, and then use the original scenarios, this time using voltage testing and continuity test.
     
    Last edited: Oct 2, 2017
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    Last edited by a moderator: Oct 2, 2017
  19. Diddymen

    Diddymen Moderator

    Really glad you managed to sort it clowie :D

    heres a bit more on testing......................................

    Resistance

    What can I use the resistance function on my tester for?

    All the same things that you did with the continuity function, this time though you wont get the ‘beeep’ but will get a value shown on the display.
    If some thing is ‘open circuit’ – such as a wire with a break in it, the display on your tester will probably say O/L or >200MΩ if the display shows readings of 0Ω or close to zero - 0.03, 0.1, 0.5Ω etc. then the wire or component doesn’t have a break in it.

    The other thing you can use resistance testing for is, on some occasions, diagnose a fault with in a component.

    If you have the type of tester you need to plug the leads into, they should be connected exactly the same as the other tests we have performed - black to COM etc.

    the switch position (if you have resistance and continuty as a separate function) will be to the Ω symbol. see pics

    [​IMG]

    [​IMG]

    You can test the resistance of the coil – the Bosch blue coil is approx 3Ω cold, if the resistance is much higher then the coil may be beginning to go ‘open circuit’. If the resistance is much lower then the coil may have started to break down. Having said that, coils have a tendency to break down under load or when hot, so testing the resistance of the coil has its limitations.

    Testing electric motors- such as fans, wipers and starters etc.


    If you know the current which a component draws, then using a bit of maths and the resistance function can tell you if there is a fault with a component.

    Voltage, current and resistance are linked by a mathematical formula commonly known as ohms law (V = I x R). If you know 2 values, such as voltage and current, you should be able to calculate what the resistance should be.

    The formula for resistance is:-

    Resistance (R) = voltage (V) / Current (I) - voltage divided by current

    So for an example – the wiper motor on high speed draws 3.5A

    Using the formula, we know we usually have approx 12V and 3.5A (current) for the wipers so the calculation would be:-

    R = 12/3.5 = 3.4Ω

    Use 3.4Ω as rough figure, don’t be too concerned if it isn’t exactly that, a few ohms plus or minus should be fine.

    So if we have tested the rest of the wiper circuit , and cant find any faults then using the resistance function on the high speed terminals of the wiper we should expect to get 3.4Ω on a healthy motor. Anything much higher or lower points to the motor being faulty.

    I think that’s the basics of continuity and resistance covered for now, so lets combine them with our original scenarios to see how we can use voltage and continuity/resistance functions together to pin point faults.


    Scenario 1


    Well the problem is obvious, the light doesn’t work. Before we use the testers its worth having a look at the ‘bigger picture’.

    Firstly have I been an idiot and not put the switch to ‘on’? – a quick double check of the switch – no its defiantly not coming on with the switch in either position

    Do we know if there is anything else that has stopped working at the same time? – not that we know of, so at this point we will test the fuse.

    Set the multimeter to continuity, pull the fuse out and test it as we discussed earlier. Do we get a ‘beeeeep’ – Yes :), so the fuse is ok :), and the switch is in the ‘on’ position……..and its still not working :(.

    We then go straight to the light. First we need to remove the cover. Inside we should see a brown wire (earth) and a red wire (supply/12v)

    Set up the tester to measure voltage now.

    Find a good earth, remember, don’t always rely on the earth that is in the thing you’re testing as this may be what is faulty. With your meter turned on, leads plugged in and set to voltage (dc) connect the black lead to a good earth (bare metal of the body, a bolt or screw head or a known good earth wire)

    Take the red lead from your tester and touch it to the terminal the red wire is connected to.

    Do we have 12v (or there abouts)? – yes!! :D

    So what does that tell us? Well, we now know that the wiring is all good up to the light, and that the switch is fine so the problem is most probably either the light, lamp(bulb) or the earth

    We have cut out a lot of things we now need to test for and narrowed the fault right down.

    Next I would check that the bulb hasn’t blow (which I should have done before testing!) – so, take the bulb out, set the tester to ‘continuity’ and place the probes on each of the bulb terminals.

    Do we get a ‘beeeep’ or a reading on the display? – yes, then the bulb is fine and hasn’t blown. If we didn’t get the ‘beeeep’ then the bulb has blown, put a new bulb in and hopefully it should all work now.

    So that means the problem is either the bulb isn’t making a good connection to the fitting or the earth is bad (either poor connection, not connected or broken) Clean up the ends of the bulb and bulb holders, does it work now? – No. then the only thing it can be is a bad earth terminal or wire.

    Set the multimeter to ‘continuity’ again. Find a good earth away from the light and hold one probe on it (remember to double check the first probe is making good connection by touching the second probe to a couple of different earth points to hear a beep)
    Leaving the first probe on the good earth, take the second probe and touch it to where the light unit’s earth connects to the body work. It should be fine here and you should get a ‘beeeep’

    So that must mean the problem is the earth wire. You don’t have to disconnect the earth wire for this, just touch one probe to one end, and the other probe to the other end of the earth. If you get a ‘beeeep’ the earth wire is good – in this scenario though, the fault was a broken earth wire so we wont get a ‘beep’. So now we know what the fault is, we can make another bit of earth wire, reconnect it to the light, and the light should now work



    Scenario 2

    Lets go back to scenario 2, and the fault is now somewhere else.

    We’ve tested the fuses using continuity, switch in the right position etc.

    We are now at the light fitting and we test for voltage at the terminal the red wire is connected to. Is there 12v? - NO! :(

    So what does that tell us? Well it means that the light fitting, bulb and earth at the light are probably all good. So the fault must be either the switch or the wiring.

    I would now go to the switch. You may have to remove the switch from the dash board if you cant get to the back where the terminals are to test them, leave the wires connected and pull off the switch knob and unscrew the little silver retaining disk (if you have that type of switch)

    Hopefully you can bring the switch out from under the dash with some slack on the cable so you can test.

    Testing for voltage again - Find a good earth and connect the black lead to it. Touch the red lead to one side of the switch, are you getting 12v? – NO :(

    Try touching the red lead to the other wire on the switch are you getting 12V? – Yes ???

    What does this tell us? Well we know that we are getting a supply to the switch but its not getting ‘out the other side’ so it is most probable that the switch isbroken and we will need a new one.

    To double check this time we are going to use continuity testing for the switch. Set the multimeter to continuity, take the wires off of the switch and hold a probe on each terminal.

    Now operate the switch whilst testing for continuity. If the switch was good, then, with the switch in the ‘on’ position you would get a ‘beeep’ with the switch off – no ‘beeep’

    In this scenario it was the switch which was faulty so you would not get a ‘beeep’ from either switch position confirming the switch was the problem. :(

    Scenario 3

    This time, we’ve tested the fuse, the switch is in the right position etc. we’ve tested for voltage at the light and we are not getting anything.

    We’ve now gone to the switch and we are getting 12V on each terminal in this scenario, what does that tell us?

    We know the switch doesn’t seem to be the problem (you can double check by using the continuity function as in scenario 2) and we are not getting voltage to the light so the most obvious thing must be that there is a break in the wire (if its joined some where it may have come out of its connector.)

    To confirm this, set your multimeter to continuity. Place a probe on each end of the wire – switch end and light fitting end (with the wire disconnected) remember if your tester leads aren’t long enough short one end to earth and test from the other end.
    If you get a ‘beeeep’ then the wire is fine – in this scenario though we are not getting a ‘beeep’ which confirms there is a break in the wire somewhere.

    Bummer, that will mean I might have to run a new cable!! :mad:


    So there you have it! :D That is how I track down electrical faults 90% of the time, you can use those tests and methods on all the circuits on your bus to help you fault find. :D

    Hopefully you should now have a bit more confidence using your multimeter and have a better understanding of how to track down faults.

    So to recap, for fault finding, familiarise your self with your busses wiring with the aid of a circuit diagram. Look at the bigger picture to see if there are faults on any other circuits which happened at the same time, and try to narrow the problem down to a common point between circuits. Then use voltage, continuity or resistance testing to pin point the fault – then fix fault

    I’ve still got current testing and a few other bits to post up but I think that will do for now! ;D
     
    Last edited: Oct 2, 2017

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