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Cowasaki's electrical tutorial part 2

Discussion in 'Common Faults and Questions' started by cowasaki, Jul 5, 2015.

By cowasaki on Jul 5, 2015 at 8:24 PM
  1. cowasaki

    cowasaki Well-Known Member

    Jun 17, 2013
    Likes Received:
    North West England
    Cowasaki's electrical tutorial part 2

    If you have not seen part 1 of the tutorial then have a look at it but here is the long promised follow up with specific instructions for tests and checks.....


    Using test equipment

    Test equipment is only really useful when you know what you are testing. This relates to both how the device should be reacting and what it is actually doing.

    Using a device such as a multimeter we are able to measure voltage (v), current (amps/a/i) and resistance (r) or impedance. Using simple equations from part 1 of the tutorial you can calculate power (watts/w) which is important when calculating what rating of wire, fuse or relay to use.

    A multimeter will usually just give clues as to problems rather than spell them out so it is important to understand the reasons and how things interact. A low voltage at a bulb doesn't tell you what the fault is but it tells you where to start looking. It could be due to voltage drop because of power connection along the wire, a low battery or poor ground. Using the multimeter and information from the circuit diagram we can see how the bulb interacts with other components and then test resistance and voltage at other locations and work out where the fault lies. A lot of it is experience but hopefully with this tutorial and others I have planned you will be able to fix things without pulling out your hair.
    Last edited: Aug 21, 2015
    Coffeelandy likes this.
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Discussion in 'Common Faults and Questions' started by cowasaki, Jul 5, 2015.

    1. cowasaki

      This tutorial will cover using the following electrical test equipment:

      • Bulb probe (or bulb tester)
      • Multimeter
      • Fuse current tester
      • Other devices
      Last edited: Aug 21, 2015
    2. cowasaki
      Bulb tester/probe

      These are VERY cheap but very useful. It's true that a multimeter can do the same task (and a lot more) but a lit bulb is a quick and simple test and it doesn't have batteries which can go flat.


      Using the bulb probe:

      This is the simplest device and is basically a simple bulb circuit like the top half of the following circuit.

      All the following example also apply to a multimeter in which case the black probe is the ground and the red probe is &#8216;the probe&#8217;. You can replace the black probe with a crocodile clip probe wire in which case everything will be the same except that you will see 12v on the screen instead of a bulb lighting up.


      The bottom half of the circuit above i.e. below the switch represents the circuit being tested.

      If you take the clip and attach it to a ground point then the action of touching it against a live connection is effectively the switch. This is useful for quick tests and has the advantage over a multimeter in that it doesn&#8217;t use batteries.

      You could use this device to test quite a few electrical circuits and a few examples are shown below:

      For these examples you must attach the ground clip to a decent ground point.

      Wires (to see if they are permanently live/ignition live/side light live etc)

      One of the most useful places to use a bulb probe is whilst sorting out a dash board. Clip the ground wire to a decent ground point and then you can use the probe to test individual wires to see what they are.

      Permanent live - touch a wire with the ignition switched OFF. If the bulb lights you have a permanent live. If the bulb doesn&#8217;t light it isn&#8217;t

      Ignition live &#8211; touch a wire with the ignition switched OFF. The bulb should not light. Now switch on the ignition and the light should come on. Switch it off again and it should go off.

      Side light, main beam, high beam and accessory live &#8211; Treat these as ignition live but the check is different ie for accessory live switch the ignition to accessory, for side light live switch on the side light, for main beam live turn on main beam and for high beam pull back the high beam switch.

      Switches (that are switching a 12v feed)

      Touch the probe to the INPUT of the switch which should light the probe no matter what position the switch is in. If it is not lighting the probe then you have a failure of the circuit prior to the switch. This could be that the input only becomes live under certain circumstance should as with the ignition on (so rectify this and try again) or it could be a blown fuse or broken wire. Now to test the action of the switch touch the output. This should light up only when the switch is ON. You can have broken switches that are permanently on or off ie then will not switch off or they will not switch on. In either case remove and replace the switch.


      Look at the fuses. One end of each fuse should be live so first test which end than is by touching fuses till you find one that is live on both ends. Now remove the fuse and touch but contacts. Only one end should now be live (the live side) whilst the other is the FUSED side. Remember this and re-insert the fuse.

      Now to test individual fuses.

      Glass cylinder fuses are obvious you can test these in situ by touching the fused side and if it lights then you have a working fuse. Blade fuses can also be tested this way. Take a close look at one and you can see that the metal contacts protrude from the plastic. Touch the metal contact of the fused side and again this will test the fuse in situ.

      If you want to test a fuse that is not is situ then you can look at it and often see that it is blown BUT occasionally you will look at a fuse and either cannot tell or worse still it will look ok but it is not. For this test we can either insert it into a space on the fuse board and test it in situ (making sure it is the same value as the one taken out) or test it&#8217;s continuity. Testing it&#8217;s continuity is best done with a multimeter but you can touch a live point such as a battery live with one side of the fuse and the probe against the other side of the fuse which will light up for a working fuse.



      As you can see, we have a relay in the above diagram (taken from part 1 of the tutorials). Now using the knowledge you have gained so far in this tutorial you should be able to work out how to test the relay :D

      Test the output first&#8230;. So treat the output (87) as a wire like in the section above on testing for an ignition live. With the relay&#8217;s activation switch switched off the bulb probe should be off. With it on the bulb should be on. Simple&#8230;.

      Relays can fail, like switches, in both the ON and OFF position ie they are permanently on or permanently off.

      If the above works the relay is working. If it doesn&#8217;t it might be due to the relay or wiring being at fault. To test this you could stick a replacement relay in and use that to check it or you could remove the relay and check the wiring directly. So lets remove the relay and check the wiring.

      30 &#8211; This is the live feed. Just touch the connector with the bulb probe and it should be lighting up.

      85 &#8211; This is the switched feed. Just touch the connector with the bulb probe and it should only light when the switch is on.

      86 &#8211; This is the ground connection (see later re ground connections)

      87 &#8211; This is the circuit you are activating. If all the other connections are ok and the relay works then the fault lies inside this part of the circuit. Either in the final ground, the device or the wiring between the device and the relay. These are best checked with a multi meter but we can check the entire rest of the circuit by treating it as a ground check (see later)

      Checking GROUND.

      The bulb tester is a simple bulb circuit where you connect the clip to ground and touch the probe against a live connection to test it. If the connection is live the bulb lights up and if it isn&#8217;t the bulb doesn&#8217;t. Well bulbs are NOT directional devices (LEDs are) so we can actually use the bulb test to test for ground connections! Clip the bulb probe&#8217;s clip to a live connection and now touch the probe against a ground. It will light up BUT you need to be aware that the bulb in a bulb tester will light using a very small voltage/current. This means that the thing you touch does not need to be a good ground. If you touch the probe to the live side of a lighting circuit which isn&#8217;t lit the bulb will be acting as a resistance to ground through it&#8217;s filament and the bulb in the tester will light up. This is how people end up with some very strange faults whereby they will fit a dashboard and find that everything works till they turn on the fog lights or something daft like that. Checking grounds, resistance and continuity really require the use of something a little more intelligent and this is where we bring in the multimeter&#8230;.
      Last edited: Jul 5, 2015
    3. cowasaki

      These are cheap but very useful too. You can get one for about £5 but I would pay a little more (mine cost about £80 but most features are not relevant for working on a car such as 4.5 digits etc). Get something half decent and it should last years and help save hours of work


      <Change the image above to better image(s) of multimeters>

      Whilst you are at it you can buy or make additional probes and these are very useful for test purposes.

      The probes plug into the multi meter using &#8220;banana pins&#8221; and these are my go to test leads.

      <Insert banana plug image>

      Set A &#8211; Get two heavy duty crocodile clips, two 50cm lengths of red and black &#8220;equipment&#8221; cable and two banana plugs then use these to make up a black and red set of test leads. You can use these to replace the probes without having to hold them onto a location.

      Set B &#8211; Get a set of very cheap jump leads (the one with cheapo clips and thin wire. Chop them in half and attach a banana clip to each. You can use these on the battery and they are ideal as battery test leads for testing charging and trickle current etc.

      Set C &#8211; Get 2 x 5m lengths of equipment wire and crimp a female crimp on one end and banana plug on the other. These are use for testing continuity from one end of a car to the other. You can then make up several short leads that go from male crimp to something else for specific test such as male crimp to crocodile clip, male crimp to male crimp, male crimp to ring crimp etc. These can now be used with sets D-G.

      Set D &#8211; Get two 10cm lengths of red and black wire and attach male spade crimps at either end.

      Set E &#8211; Get two 10cm lengths of red and black wire and attach male spade crimps at one end and ring crimps at the other.

      Set F &#8211; Get two 10cm lengths of red and black wire and attach male spade crimps at one end and bullet crimps at the other.

      Set G &#8211; Get two 10cm lengths of red and black wire and attach male spade crimps at one end and crocodile clips at the other.

      Set H - Get two 10cm lengths of red and black wire and attach male spade crimps at one end and female spade crimps at the other.

      Set I - Get two 10cm lengths of red and black wire and attach crocodile clips at both ends.

      <Insert images of the probe test leads>

      Testing using a multi meter

      A bulb tester only really tests for a voltage between about 6v and 18v. It can only really test continuity by including the test subject in the circuit. A multi meter has a number of additional tricks.

      A basic multi meter will have

      1 - AC voltage
      2 - DC voltage
      3 - Resistance
      4 - Diode/continuity test
      5 - Current (amps)

      Additional things such as frequency and capacitance are of little use on cars although occasionally might help solve a problem. In the main the above features are what you need. You might find some multi meters aims specifically at automotive use which include automotive functions such as dwell angle etc. I will not be mentioning these functions here.

      So what can you use a multi meter for? Well if you follow the bulb probe section you can use a multi meter to replace the bulb probe in all the above functions by setting it to DC voltage and looking for 0v or 12-14v we can however do so much more.

      Testing the standby battery voltage.

      So you are having problems with your car starting. Connect the multi meter so that the black lead attaches to the negative on the battery and the red lead attaches to the positive.

      [I would use my test leads B for this and DC voltage covering a range of up to 20v]

      You will probably get a reading of around 12-12.7 volts. This is the usual standby voltage of a lead acid car battery. 12.6/7 volts means it is fully charged and the general rule of thumb is about 15% charge for every 0.1 volt below this. If your battery is reading 11.8 volts or less it is probably dying and you should remove it, fully charge it and then take it to a battery dealer for a drop test.

      Testing the alternator/charge circuit.

      Now we have tested the standby voltage we can check that your car&#8217;s alternator is working! Connect the leads as above for standby voltage and check the voltage as above. Now make sure the multi meter is in a safe location and start the engine. Give it a short rev and read the voltage. The voltage should read 13.5-14.5 volts when idling. If it does then your alternator is probably doing it&#8217;s job. Any less than 13.5 volts and I would suggest you need to get your alternator checks. Any more than 14.8 volts and I would also get it checked.


      Continuity is basically checking that an electrical current can run between two points. So a new piece of wire for example should have continuity between one end and the other whereas a piece of plastic shouldn't. For continuity you could read conductivity or just "conducts". If you have a piece of wire and there is no continuity between one end and the other this is likely due to an internal break in the wire. This can sometimes happen when the wire runs over a sharp point or where the wire has something closing on it over and over again or is bent back and forth.

      To test for continuity:

      [Set the meter for resistance of about 2K or more and use either the probes or a combination of probes and crocodile clips]

      Now touch the probes to two locations and the reading should drop to zero. Some testers have a continuity setting and this will often beep given you an audible notification (this is just the meter beeping if the resistance is below a pre-determined value).

      Testing a wire or a fuse

      This is the easy one. Touch the two probes together and you will see the display change from infinity to zero. This shows that there is continuity between the probes (I always do this to show the probes are working before going further). Now if you use attach one probe to one side of a fuse or piece of wire and the other to there to the other side it should show exactly the same ie continuity along that piece of wire/fuse. If the display shows anything other than zero if might be that the ends of the fuse/wire are dirty or oxidized. Try cleaning them and doing the test again.

      Testing resistance

      This is exactly the same as testing continuity BUT you need to set the meter to a range that covers the relevant resistance. In a manual you will often be told that the resistance of the windings will be maybe 12 ohms + or &#8211; 1 ohm (or something similar). Set the meter to the smallest range that contains this value such as 0-100 ohms and touch the meter across the connections. You should get a value of 11-13 ohms. If the value is outside of this range then the motor looks faulty.

      Other items will have a resistance such as the ballast resistor in the charge circuit. You can test this using the multimeter in the same way.

      Testing senders

      This is basically the same as testing any other resistance but you do need to know what you are testing the resistance against. Any sender which has a metal body, screws into a metal block and has one terminal on it can only be a resistance to ground device. Examples of this are:

      • Coolant temperature sender
      • Fuel sender
      • Oil temperature sender
      To test these devices attach the black probe to ground (best using a crocodile clip lead) and then touching the metal terminal (or using a crocodile clip to the terminal). If have been know to have the multimeter on the seat with one crocodile clip attached to the handbrake brake (a good ground) and the red lead extended into the engine compartment and attached to the water temperature sender! You can use this to test if the device is changing in relation to it's task.

      Testing circuits on the car

      Right this is the big one!

      You will need a decent circuit diagram (note the Haynes defender manual has at least 5 faults in it which I pointed out to Haynes and they have still not sorted!) There are better ones on the internet.

      <insert links to Landrover circuit diagrams>

      So we will use an example of a faulty rear side light, you&#8217;ve checked the fuse and changing the bulb does not fix it&#8230;..

      We can first check the voltage as described earlier. Connect two standard probes and carefully touch the red probe to the connector inside the bulb holder (note that there are two in the brake/side light holder check each one and if one is working it should be the side light as nobody is pressing the brake) and the other to the shell. The voltage should show approx. 12v. If it does not then try scratching the connectors with the sharp end of the probes. If you get 12v then you have a dirty/corroded bulb holder which you need to clean or replace. If you get nothing from either one of them then you have a faulty 12v connection. Attach a clip to the black port of the multimeter and attach this to ground. Now touch to red probe to both of the pads inside the bulb socket in turn. If it still reads zero try moving the negative clip and if you still get zero it appears you have a faulty live connection. You can check this by removing another bulb and touching the red probe to that. If it goes to 12v then it proves that the ground is good. Now move the ground back to the shell of the bulb you are testing and check again. A 12v reading proves that the shell of the bulb is grounded and confirms the faulty 12v. You now have to work out where the fault lies. First job is to follow the wires from the bulb holder back along the loom where you will find it often connects to the loom with a male and female bullet crimp. Undo this crimp and check the loom side with the meter scratching the surface with the probe if you do not get a good 12v reading. If you do get 12v then here is your fault. You need to clean and reconnect the two bullets. If you still do not get a reading then try and follow this wire back along the loom (sometimes this is very difficult to do). We now need to see if there is a break in the wire. Check your diagram as to where this feed comes from which is fuse 11 on the defender 1994. Turn off the lights. So we need to check continuity from fuse 11. This is where a long wire is useful and I would use one probe C as described above. Remove the fuse and push a male to male spade adapter [set D] into the end of the long black probe lead&#8217;s female crimp. Insert this into the fuse holder and move the multimeter to the read of the car. Now insert a normal probe into the red side and touch this against the bulb contacts. If this reads zero again there is a break in the wire and you will need to locate this or pull another wire through.

      Checking voltage drop

      The theory

      A wire is a conductor and we use conductors to pass a current from one location to another. On most vehicles (as explained in part 1 of the tutorial) the negative side of the battery is connected to the chassis whilst the positive passes through insulated wires.

      The voltage (also known as potential difference) is shown as V and on most vehicles this is nominally 12v DC (direct current).

      Voltage drop is where the current travels along a wire or other conductor and drops due to it's interaction/resistance with that conductor. The voltage of a fully charged lead acid battery should be 12.6V and using the start motor as an example it will require maybe 12v to start the vehicle.

      So IF the voltage drop is enough to make the voltage say 10v the starter motor will not start the car. A good example is to try and start a car using 4 sets of cheap jump leads hooked together in a line to make two 8 metre jump leads. If we were to use the same length jump lead from my garage which is made from a fork lift truck charge cable and rated at 800amps !! There would be far less voltage drop and the car would start.

      The practice

      If the car actually worked at one point voltage drop shouldn&#8217;t be an issue but other things can increase the resistance and cause a voltage drop. These can be things like a partially broken cable or a badly corroded connection. We can test for voltage drop by placing our probes across a section of the circuit where we want to check. So if we have a corroded connector and we wanted to check that connector we simply attach the probes of the multimeter [set at 20v DC voltage] either side of the connector and test the DC voltage. It the connection is good this should read 0v but anything over say 0.1v shows a connector that maybe needs a clean over 0.3v could be a real issue.

      You can test the starter circuit cables fully by removing the fuel cut off solenoid cable or alternator live feed cable then:

      Whilst turning the engine over note the voltage reading between POS and NEG at the battery - A
      Do the same testing the voltage from the battery POS to the metal body of the starter motor - B
      Do the same testing the voltage from the battery POS to the battery NEG terminal - C

      If B is within 1v of A all is good
      If C is less than about 0.5-0.6v all is good.

      If C is excessively high you can test where the fault is using the NEG closer to the battery till you find the fault or just remove the cable clean and replace after cleaning the terminals.
      Last edited: Sep 7, 2016
      farriermatt likes this.
    4. cowasaki

      Testing the trickle current.

      Disconnect one side of the battery and with the ignition off attach lead set B between the battery post and the disconnected battery lead.


      Now select current (amps) and read the meter. This will show how much current is being used by the car whilst the ignition is off (alarms and certain lights if switched on etc)

      Now turn the ignition to accessory and this will show what the accessory circuits are drawing eg stereo/cb
      Now turn the ignition to on (DO NOT START THE ENGINE) and this will now show what current everything bar the starter motor is using. The starter motor will use FAR more current than a cheap multimeter will handle and if you try and start the car it WILL blow the multimeters fuse or destroy it.

      Testing the current in a circuit.

      There are cheap devices on ebay (for about £10-16) which do this (check you get the one for the right sized fuse or like the picture shown capable of either size) and I would recommend buying one but if you don&#8217;t have one you can use a combination of leads to do the same function.


      <Insert image of Defender blade fuse box with labels to live in / post fuse out>

      Ok pull out a fuse from your fuse box (for this test use the side lights) then:

      If you have a fuse box with cylinder fuses attach a crocodile clip to crocodile clip lead to each end [set I]

      If you have a fuse box with spade fuse attach a male spade to crocodile clip lead [set G] into each side of the fuse box

      So we now have two crocodile clips sticking out. If you attach these clips to the fuse everything should work as before but what we want to do is include our meter into this circuit. For voltage the meter simply needs to connect to positive and negative but for current it needs to be connected within the circuit.

      Change your meter to read current (amps) with the maximum setting.

      Detach the fuse and attach a set of probes with crocodile clips to the meter and attach one of the clips to one of the clips coming from the fuse box and the other one to the fuse then attach the free crocodile clip from the fuse box to the free crocodile clip from the meter. It doesn&#8217;t matter which colour probes you use for current.

      So now the current comes out of the fuse box and through the fuse then through the meter and back into the fuse box&#8230;..

      Now looking at the reading it should say zero now turn on the side lights. The value should now change to 0.1 amps or whatever the side light circuit draws. This is a useful test to see how much current a circuit is drawing and can help work out what is causing a flat battery. With everything switched off move the circuit along the fusebox testing each fuse location and note any that show a value. If you have an alarm one of the locations should show the current it is using. By using this method you can locate a circuit which is using power and then by removing items from that circuit can work out what is using the power.

      This is a lot easier using the correct device and at about £6 it is worth having in your electrical tool box.
      Last edited: Jul 5, 2015
    5. cowasaki
      In vehicle voltage tester.

      These are very cheap devices on ebay (for about £3) which effectively add a temporary volt meter to the vehicle by plugging into the cigarette lighter socket. If you look at the section above on using the multimeter to check if the alternator is charging you can do this from within the cabin just by plugging it in. A handy little device to have in your tool box.


      Reserved for additional instruments for this tutorial.
      Last edited: Jul 5, 2015
    6. wooly123
      brilliant thankyou
    7. Coffeelandy
      Bloody nora MATE THIS IS AMAZING!!!