HOW TO - using a multimeter and testing for faults

Batteries

Before we get onto current testing I thought we could go over some stuff to do with batteries as the two are sort of related. I’ll keep it simple, as with all things electrical you can go quite in depth if you want to! :

First then its worth noting that our ‘starting’ battery is slightly different to a leisure battery. A leisure battery (some times referred to as a deep cycling battery) is designed to give a constant power for a greater period of time, where as the staring battery gives you bigger power for a short time.
The leisure batteries construction is such that it can be completely discharged and recharged repeatedly with out causing too much damage, they generally have thicker lead plates inside which are less likely to buckle under a complete discharge and re charge cycle.
If you use a leisure battery for starting (or vice versa) then you are likely to cause damage, and premature failure of that battery.

Electrolyte in the form of acid sits around the lead plates inside the battery (hence the name lead acid) and is used to transfer the charge (as it is conductive) between the cells when charging or discharging.

Amp Hours (AH), you might have seen the amp hour (AH) value on the side of your battery but were unsure what it meant. Well, putting it really simply, you can think of amp hours as the ‘size of your fuel tank’ for batteries. So the bigger your fuel tank, the further you can travel, or the longer your battery will last under load potentially.

You can roughly calculate how long your battery should last when being used if you know what current is being drawn from it.

Amp hours are Amps x hours - If we have a 220AH battery, and 10Amps are being drawn from it then

220AH divided by 10Amps = 22Hrs

So our 220AH battery could supply 10A for 22Hrs before going flat.

As I said earlier, there is more to it than that, and that is an oversimplification but should give you some rough figures for fault finding purposes. If you want to find out more Im sure its all available on Wiki some where! ;D


How do I know if my battery is fully charged?

One way is to use the batteries terminal voltage to find out the state of charge, using the following figures as a rough guide:-

Discharged (flat) battery – between 11.6V and 12V

Charged battery – between 12.5V and 12.9V (usually around 12.7V)

Using terminal voltage does have its disadvantages though. Although it can be accurate, you should leave the battery for at least an hour for any surface charge to dissipate first. This means don’t rely on voltage readings within an hour of using or charging the battery or you may get a false reading.

Obviously, when your engine is running the alternator/dynamo will be producing a higher voltage than the voltage figures given for full charge, so if you test your battery with the engine on you are going to be reading 13V or slightly above depending on what the alternator is kicking out.

A much more accurate way of measuring the state of charge is by measuring the specific gravity (SG) of the electrolyte in the battery. You can get SG testers fairly cheaply, the only problem being if you have a sealed for life battery!!! …….I’ve never tested the SG of my vehicles batteries.

Specific gravity values:-

Discharged (flat) battery – 1.285

Half charged battery – 1.20

Fully charged battery – 1.12

………………I just found those figures in the Haynes book of lies ;D


Battery Faults

I guess battery problems could be broken down into 3 catagories.

A drain on the battery – The battery is constantly going flat either due to an electrical item being left on, or by an electrical fault drawing current from the battery, but the battery its self is not faulty

Bad earth connection – The negative/earth of a vehicles circuit some times can become loose or broken. Its not surprising considering it could be exposed to corrosive acid spills from the battery. When the earth from the battery becomes loose, or the connection to the bodywork isn’t the best then you may find you still get quite a lot of your vehicle electrics still working. It isn’t always a catastrophic failure, some times only one or two circuits seem to be affected. Its always worth double checking the earth connection on the battery and body of the bus visually and with a multimeter as part of your fault finding routine if the fault isn’t immediately obvious.

Defective battery – These can some times be difficult to detect when the battery is on its last legs – working but only just. You may find that measuring voltage (after a rest of 1hour) or testing specific gravity shows the battery is fully charged, so you’d expect it to be ok. Unfortunately it doesn’t always work that way and although it seems charged, it wont hold its charge under load and you wont be able to test if a battery is defective using your multimeter. Ideally you’d need to use a discharge tester to see if it is holding charge, its not worth buying one as I think most places like halfords will test batteries for free.

As an example of a battery fault and symptom, a while back I had a problem on my MGB. The radio would occasionally turn off then come back on again, and, when operated, the indicators would start flashing, then just stay on (without flashing ???). This had me puzzled for ages, Id remade any suspect terminals, tested the indicator circuit, replaced indicator relay and it was still the same, everything else was working. It was only whilst doing some other bits to the car I noticed how bad the earth lead was, I got a new earth lead, made a good connection to the body work and I never had the problems again. It was very strange how none of the other circuits seemed to be affected though.

I think that’s probably enough for another night, next time we’ll go into testing for current.

 

Current testing using a multimeter

I’ve saved current testing for last as your likely to use it a lot less than the voltage or continuity when tracing faults. When using the ammeter setting on your multimeter, you are testing to see, if, and how much current is flowing.

What can I use current testing for?

Theres only a couple of things you are likely to use current testing for, testing to see if there is a drain on the battery, finding which circuit a current drain is on and some diagnostics on certain components.

How do I set up the multimeter?

If you have an automatic sensing type meter there will only be the one selection for current testing regardless of AC or DC simply set the meter to the symbol ‘A’ – make a note though if there is a maximum current value it can test, it will usually have some thing like “max 10A” written on the meter or instructions. If you are expecting the current to be higher than the max value you could end up blowing an internal fuse or causing damage to the multimeter.

The automatic AC/DC tester I have been using for the other tests does not test DC current, so I will not be able to use it for testing the bus currents . If you look at the pic below you will see that the switch position for current is only for AC, so its worth double checking your multimeter is capable of DC current testing.



If you have the type of tester you need to plug the leads into, then you could easily be confused at this point as there are several different ways to set the meter up. Firstly, the black lead will still be plugged into COM but the red leads now plug into one of the other sockets we have not yet used.

As mentioned above, testing for current is the only tests you are likely to do which could potentially damage your tester if not carried out correctly, don’t panic though, it wont blow up, and its no harder to do than the tests for continuity or voltage.

When testing current its always useful to have a rough idea of the maximum current you are expecting to see so you can set up the tester accordingly, if in doubt set it up to measure, on the highest values shown on the front of the tester which is where I usually start (hopefully this will be explained as I go along)

On this particular tester you can see there are two sockets on the left, one is shown as 10A and the other is shown as 300mA (milliamps), the values shown are usually the highest current it can measure before the internal fuses blow.



On an unknown current drain I would start off plugging my lead into the 10A socket. The switch then needs to be in the DC current position as shown below.






You are now ready to test. When carrying out current measurements, the tester needs to be connected in series – disconnect the wire of the circuit you are testing, and connect one probe to the terminal you have disconnected the wire from and the other probe to the end of the wire. Ideally the black probe should be towards the battery –VE and the red lead towards the battery +VE.

The reading shown on the display will be in amps (as you have set it up with the 10A range), so if you get 5 on the display it will be 5A - if you get 0.5 it will be half an amp or 500mA which is why I would always start on the higher setting as I would have just blown the internal fuse had I set it on the mA scale in the first place.

If you are not getting any thing showing on the display it could be that the current flow is so small that the 10A range is not sensitive enough. So providing (in the case of this particular tester) you haven’t measured any more than 0.3 on the 10A setting then you can swap the red lead over into the 300mA socket as in the pic below



Now you are on mA scale you should be able to read lower currents much more accurately. Generally when using the mA socket, the tester will auto range and shown milliamps on the display rather than amps, so if you are getting a reading of 150 on the display it will be 150mA or 0.15A

The tester shown below is a little different again to the last one. It still has two sockets for current reading, one for 10A but the second one is slightly higher rated to 400mA (0.4A) as mentioned earlier, always start with the red lead in the 10A socket and swap to the mA socket if needed.



This tester, to confuse things even more has a separate button to toggle between AC or DC which you need to select DC



You now need to select the correct function on the rotary dial, when starting off with the lead in the 10A socket have the rotary switch set to 10A.



providing we have not got a reading of anything higher than 0.4A we can now swap the red lead into the 400mA socket



And move the rotary switch to the 400mA position.



You will notice that there are 3 other rotary switch positions for current reading – 40mA, 4mA and 400micoamps. You can use theses positions with the red lead in the 400mA socket, these other settings for the rotary switch are used if the current is still too small to be picked up on the 400mA setting – they are progressively more sensitive.



So hopefully that has made some sense to you, and you should now be able to set the tester up for measuring current.



Testing for a battery drain

Lets assume that you’re battery has gone flat in a short period of time, this has happened a number of times now and you have even had the battery tested at a garage and they have told you there is nothing wrong with it. This can only mean one thing, something is draining the battery!. >

First then, we need to check that there is a drain (although we probably know there is). Start off with a fully charged battery (obviously we cant check for drains if the battery is flat!), you might have noticed when you re-connected the leads to the battery that there was a small spark as you touched the connector to the battery post? This is a good indicator that something is drawing current.

Double check that there is nothing in the bus which is turned on – ie fridge, interior lights, nothing on charge from a cigarette socket, and ideally with the cab doors shut so that the interior lights are off.

Go to the battery (obviously the one which is going flat – its no use testing the staring battery if it is the leisure battery which is going flat!) and disconnect either, the lead to the bodywork (-VE) or the +VE. It doesn’t really matter which as you will either be testing the current coming out of the +VE, or when it goes back into the battery at the –VE post on the battery.

Remember to set your multimeter to the highest current rating so you don’t blow the internal fuse and select DC current. We are now going to connect the tester in series.

If you have disconnected the earth lead to the battery, then put the black lead on the –VE post of the battery and the red lead to the connector you have taken off of the battery (and vice versa if +VE is disconnected), and look at the display.

For this scenario, we will assume you are getting 2A shown on the screen.

What does this mean? Well it means something is drawing 2A from your battery and if you don’t find out what it is your battery will go flat again .

You could have started your testing at the fuse box, at the front of the bus, there is a reason I didn’t start there though. If you have several drains on different circuits, you might find one when testing at the fuse box, assume that is it, and not spot the other one. By testing at the battery first, you will find the total current that is flowing either from one fault or multiple drains.

…..back to the scenario. We have found that we are getting a 2A drain so now move to the front of the bus and will test each circuit individually.

There are 2 ways of testing at the fuse board, you can either pull the first fuse out and connect the probes to either side of the fuse holder where the fuse was (if you can get them to make contact) you can make something up to fit, if the probes wont reach the fuse holder contacts. This way you don’t have to disconnect any wires, the tester will still be connected in series as the current comes from one of the fuse contacts, through the tester, and back through the second fuse contact. Repeat this on all the fuses until you get 2A showing on the display.

Once you see 2A you then know what circuit the drain is on and can investigate further.

What happens if you get to fuse 2, and you only get 1A shown on the screen? Well, it probably means you have more than one fault draining the battery. Continue testing all the other circuits and chances are you will find another one where you get a 1A drain, you can then investigate that circuit also.

So removing the fuses and testing across the holder is one way. The other way is to disconnect each individual wire from the fuse box, connect the tester in series and see if your getting any current readings. At least doing it this way you can usually test individual circuits which are fed from the same fuse thus narrowing it down further.


So that should now help you track down where a drain on the battery is coming from or at least narrow it down to a particular circuit.


I've got one more bit on current testing I want to do, then I might breifly cover relays and fault finding on circuits containing relays........then I think this thread is done ;D

 

Using current testing for other fault finding.

We have discussed using the current function of the multimeter to detect a battery drain but you can also use the function for a bit of diagnostics on certain other circuits…….it has its limitations though

Let us assume you are having problems with a fuse blowing regularly. Fuses blow when their current rating has been exceeded for a duration of time, so this indicated there is a problem with the circuit the fuse feeds (unless an underrated fuse has been fitted incorrectly)

Lets use the wiper circuit again for this example.

Remember, our wiper circuit draws approx 2.5A on slow speed and 3.5A on fast speed.

Set up the multimeter for DC current measuring, connect up in series in the wiper circuit and take the wiper blades off the screen (as the friction will put extra load on the motor and draw more current)

Turn the wipers onto slow speed and measure current – is it close to 2.5A?

Next measure current on high speed – is it close to 3.5A?

If both measurements are considerably higher it would point to a common fault such as the wiper spindles or mechanism seizing up.

If only one of the speeds is drawing more current (and the other is ok) then this points to a problem with the motor.



You can test any of the circuits on your bus in the same way for problematic fuse blowing. Remember to find out (or calculate) how much current should be flowing in the circuit when it is healthy, measure the actual current and compare the 2 values.


Generally if the circuit contains an electric motor, the problem will probably be friction or something binding causing the motor to work harder and draw more current followed by internal faults with the windings and worn brushes etc



Never put a bigger fuse in than that which has been specified in your owners’ manual, in the event of a fault the wrong fuse will allow too much current to flow with out blowing. This can, in the worst case lead to wires melting and fires starting.



Going back to the battery drains in the previous post, its worth noting that some circuits will always draw current. Your entertainment equipment/head unit is a case in point, it will always be using power to keep the clock going and to memorise radio stations and other settings, this should only be in the milliamps though and not enough to drain the battery in the short term.


Severe battery drains are likely to be either –

a permanent ‘live’ wire which has shorted down to earth (the insulation may have been damaged where it goes through a hole in the body work)

A faulty component (usually an internal short to earth)

Or caused by incorrect wiring after some work has been done on your bus.


…………I think that wraps up current testing.




Relays

I thought that before I finish this post it is worth mentioning relays.

So what is a relay?

A relay is basically a switch ……but with a difference. Normal switches have a set of contacts which join together to make a good electrical connection when you turn the switch on – there is a physical connection between the contacts and the part of the switch you operate to turn it on.

Relays are a bit different, there is no physical connection to the electrical contacts and the switch you use to operate it. The electrical contacts within the relay are operated magnetically by a coil. When you physically turn your switch 'on', in a relay circuit, all it does is energise the coil in the relay (a coil with current flowing through it becomes a magnet) as the coil becomes energised it magnetically pulls the electrical contacts closed thus turning the ‘relay switch’ on.

So why use a relay?

There is no electrical connection between the coil and contacts of a relay, this means you can energise the coil with a tiny current. To energise the coil you need to get current and voltage to it, so you will need a switch. By using a very small current to energise the coil, you don’t have large currents going through your ‘physical’ ‘on’ ’off’ switch, which can cause damage and shorten the life of that switch over time. So using a relay can extend the life of items such as switches.

The electrical contacts in the relay can be rated for much higher currents than the coil, so you can have mA’s operating the coil of the relay but a large current flowing through the contacts.

In effect a relay circuit could be seen as 2 separate circuits with no physical or electrical contact with each other – the control circuit would be the switch and the coil, and the load circuit would be the relay’s contacts and final components – lights etc.

Its worth me pointing out, if you are going to be installing a new relay for additional equipment:-

When a relay is energised, the coil produces a magnetic field which is used to switch the contacts (as mentioned earlier). When you take energy away from the relay, the magnetic field in the coil collapses, as the field collapses it can, momentarily induce a relatively high voltage across the coil. This isn’t generally a problem unless you have electronic components installed in the same circuits - micro chips, transistors etc, as damage can occur to these components. To avoid this damage it is worth either installing a diode across the coil – or more practically, buy a relay with a diode built in as they should only cost a few more pence.


So now you understand that circuits containing relays are almost like 2 separate circuits, you can appreciate that fault finding will require a slightly different approach, whilst still using voltage and continuity tests.


I'm going to have a look for a simple relay circuit which may be fitted on a bus, and then use that as an example of fault finding with relays to finish this thread with.

 

Excellent work have some K for taking the time to type all that up.

 

its never a fluke with a fluke multimeter

Dale this is an awesome how to and may it be the first of many

A pool of knowledge is a valuble thing

 

Thanks guys

its taken a while to type up, its up to 23 pages in the word document I drafted it in ;D

So I'm going to make this my last technical post in this thread (unless any one has any questions) and for the final bit we will fault find on a simple circuit containing a relay.


Fault finding on a circuit containing relays.


I’m using the heater air blower circuit for the final example – fault finding on a circuit containing a relay, as it is quite a simple circuit to understand.

For this example it might be easier to follow using this circuit diagram (some times called a line diagram) – (1973-1975)



The heater blower circuit is shown on the left side of the diagram.

The wiring diagrams were got from:-

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



Firstly an explanation of the circuit, as I mentioned in the last post a circuit containing a relay could be seen as 2 separate circuits with no physical or electrical contact with each other – the control circuit would be the switch and the coil, and the load circuit would be the relay’s contacts and final components – ie. the heater motor

The control circuit gets its 12V feed from the Alternator (C) and is connected to one side of the relay coil (J14 terminal A) via a red wire and a blue wire. The other side of the relay coil (J14 terminal 31) is connected to the heater blower switch (E16) on the dash board via an orange and white wire. And finally the other side of the switch is connected down to earth.

The load circuit gets it’s 12V directly from the battery (A) it then goes through the in line fuse (S25) and connects to the relay contacts (J14 terminal M). When the contacts are closed the current can then flow through them and out at J14 terminals B & J, through the yellow and black wire to the heater air blower motor (V4) and finally out the other side of the heater blower motor (V4) back to earth to complete the circuit.

T8 & T3b are connector plugs which I’m going to omit from the scenario for clarity, its worth checking these points for a good electrical connection if you have tested everything else when fault finding (but as mentioned I’m leaving these out to concentrate on the relay circuit.)

So, when the relay coil is energised, the relay contacts close, current flows from the battery through the relay contacts, through the heater blower motor (making it operate) and back to earth.



Probably the easiest way to test a faulty relay is to replace it with a known good one as they are only cheap. Alternatively, you could energise it by connecting the terminal “31” to earth and terminal “A” to 12V, this should energise the coil and close the contacts, using your multimeter set on continuity should confirm or deny this when you test between terminals M & B or M & J. But for our scenario we will try to eliminate if the relay is faulty by other means.



So the scenario is – the heater blower is not working.

First things first, is the inline fuse (S25) ok? Test it using continuity across the fuse (with it removed from holder) or test you have 12V at both ends of the fuse (remember black lead to body work – earth, and red lead to each end in turn)

The fuse is ok, which means the wiring up to the fuse must also be ok. So we might as well continue along the same wire to terminal “M” of the relay (J14), using voltage testing – is there 12V?

Yes – so we have the supply up to the contacts which means either the heater blower motor is faulty, the wiring or earth to the heater blower motor is faulty, or the relay is not closing the contacts allowing current to flow, so the next test will be to see if the coil of the relay will energise.

Looking back to our circuit diagram, we see that we should have 12V at terminal “A” of J14 whether or not the switch (E16) is closed or open . Using our multimeter set for voltage, do we have 12V at terminal “A”?

Yes – so this means all the wiring up to that point must be ok



The next bit might require a bit of a leap of faith! ???


With the switch (E16) off there should also be 12V on terminal 31 of J14 (assuming the relay coil is ok), test it, is there 12V?

Yes – this means the coil is probably ok but doesn’t necessarily mean the relay isn’t faulty

No – the coil of the relay has a break in it (the coil is just a long length of wire wrapped around an iron core (usually) to form a coil of wire) – relay is faulty, replace with a new one.

We will assume that there is 12V on terminal 31.

So why, if there is 12V on each side of the coil with the switch off does the coil not energise? ???, well, remember in one of the very first posts in this thread I made about voltage, I mentioned potential difference? For current to flow there needs to be a potential difference between 2 points .

For example - If you have your black tester lead connected to earth and you touch a 12V wire along its length, you will get a reading of 12V every time you test it. If you took you black lead and held it on the 12V wire and touched your red lead to another part of that wire, you would read 0V as there is no potential difference between the 2 points of the wire. Where as, there is a potential difference of 12V between the wire and earth (this principle is why birds don’t get electrocuted on power lines, as they are not touching earth at the same time as the wire, there is no potential difference between the birds feet )

So between terminals A and 31 there will be no potential difference (there will be a minimal resistance but Im not going to over complicate things any further) and so no current flow. As soon as the switch (E16) is closed, there will be a direct path to earth so it would be similar to putting an earth on terminal 31 – this would then create the potential difference between terminals A and 31 needed for current to flow and the coil to energise

Hopefully that explained the leap of faith!..............to an extent ;D



o.k. so we have 12V at terminal 31 which means we should have 12V at the switch (E16), do we?

Yes – so the wiring up to this point should all be good, which means either, the problems mentioned earlier (motor etc) or the switch or wiring after it.

We can test the switch, and the wiring from the switch to earth using the continuity function – are they all ok?

Yes – switch is working and wiring from switch to earth is good. So this means that the control side of the relay is all working, so the problem is either blower motor and wiring related or there still may be a problem with the relay.

You could also confirm the control circuit is functioning by using the current testing function of your multimeter. Disconnect a wire (from terminal “A” for example) and connect your multimeter in series. Turn the switch (E16) on and you should get a reading confirming current is flowing so the control circuit is fine.

Don’t forget to reconnect the wire you just disconnected before going any further ;D

Back to the relay.

With the switch (E16) turned on, we will go back to the relay and test for 12V at terminals B & J are we getting 12V?

No – We know the control circuit is working fine, we know we are getting 12V to one side of the contacts (terminal M) but we have now proved that with the switch (E16) turned ‘on’ the voltage has not reached terminals B & J – this can only mean that the contacts are not closing and are probably stuck, this isn’t uncommon if moisture has got in there forminga bit of corrosion. So we’ve confirmed that the relay is the faulty part of the circuit and we can now replace it with a good one.

Don’t forget, its always worth checking that the relays are making good contact with their holders if they are the plug in type, as this could give the same symptoms as above.

If you were getting 12V at terminals B or J then the problem would most probably lie with the heater blower motor or wiring, this can be tested using voltage testing or continuity testing as explained in the earlier posts.

Hopefully that has gone a little way towards helping fault find when there is a relay in the circuit. I’ve tried to go through the testing in a logical manner, but you could short cut the process (this works on any circuit). Instead of going through bit by bit testing at each point, you could go straight to the ‘middle’ of the circuit and test there, which would prove the fault is either before or after the place you are testing which could narrow things down a bit quicker.

So for the heater air blower circuit, you know that with the switch (E16) ‘off’ there will not be 12V at the relay contacts J14, B & J but with the switch (E16) ‘on’ you should get 12V. So testing at B or J first, whilst some one operates the switch (E16), you should be able to say that if you get 12V that the control circuit is fine and the fault will be around the motor part of the circuit. If you get 0V then the problem is most likely with the control circuit side and you have halved the amount of points you need to test.




I think that has just about covered everything I was hoping to, for basic fault finding, I hope you’ve found it useful or its given a bit of confidence to have a go at fault finding.

Remember, look at the circuit logically, eliminate the obvious (fuses, bulbs etc) look for related faults, break the circuit down into smaller ‘chunks’ if it helps, then visually inspect and test and you should get to the bottom of things in no time.


Good luck and happy testing!

 

This saga of a post def deserves a nomination in the 2012 awards.

 

Could I point out that for most non-electricians, the style of the wiring diagram for late bays while logically correct, could be baffling. Some of the components are spread about in pieces. If you're reading this and that resembles you, don't be put off - look also at some earlier diagrams on the link above which are laid out like a bus and very easy to grasp. VW stuck to the same wire colours on all buses and a bus is a bus. There are a few small changes.

 

this thread saved my sanity - thought I was going crazy the charger kept saying the battery was full when I tried using it there was no juice (dim lights, window wipers moving slooooow etc.)

so I bought a multimeter and tried testing it - it was never getting above 8v - checked the multimeter against a battery I new was good 12+ volts

result!! looks like I need to buy a new battery

With my new found electrikery confidence i am going to try testing the various circuits looking for faults as most of the lights don't work or the relays)

cheers diddy

 

always happy to help

 

Thanks again for this post - I keep coming back to it as my understanding grows. Fantastic post.

 

Only recently found this thread.
As a retired physics teacher - May I compliment you on a great set of explanations.
My son is now teaching college kids on motor vehicle maintenance and I have directed him to this thread - many thanks.

 

Great thread. Just another note if you've been measuring current: always put the leads back in the "volts" position when you're done. Easy to forget. If you do, and then go testing voltages, there'll be a bang: a meter in the "current" position looks like a dead short, so you'll be popping fuses if you're testing 12V stuff on the van.

If you're poking around on the mains, there'll likely be an even bigger bang. Cheapo eBay meters aren't well protected against high-energy stuff, and the meter could explode in your hand.