Multimeter
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
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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 “banana pins” and these are my go to test leads.
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Set A – Get two heavy duty crocodile clips, two 50cm lengths of red and black “equipment” 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 – 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 – 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 – Get two 10cm lengths of red and black wire and attach male spade crimps at either end.
Set E – 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 – 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 – 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.
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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’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’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
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 – 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’ve checked the fuse and changing the bulb does not fix it…..
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’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’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.
