How to use a Multimeter

OR

UNDERSTANDING ELECTRICS.


To use a multi-meter correctly and actually make use of it to solve electrical problems it is in the first instance necessary to have a more than vague understanding of D.C. electrical systems. Having being an electrical and instrumentation engineer all my life I can assure anyone that an understanding of D.C. electrical systems is extremely easy, if explained in layman’s terms. I hope that in the following article I can do just that and my time will not have been wasted.

In the past I have found that explaining auto electrical systems to electrical people is fairly easy, trying to explain them to mechanical people requires a totally different approach and I will use that approach here.

We need to consider 3 elements of electrical systems
(a) Voltage Volts
(b) Current Amperes
(c) Resistance Ohms

Before we move on I would like you all to consider now if electricity was water in a hosepipe or some other pipe work. Again we would have 3 elements

(a) Pressure
(b) Flow
(c) Resistance (valve)

OK so let’s assume Voltage is the same as pressure in our hose pipe. If the valve at the end of the hose pipe was closed and we had a pressure gauge on the pipe we would see the pressure would be maximum. The pressure gauge is actually measuring the difference between the high pressure in the pipe and the pressure outside the pipe in the surrounding air, this is generally referred to as atmospheric but to us mortals we can assume it is 0. Equally if we had an electrical circuit with a supply from battery pos through a wire to a switch and from the switch to battery neg. Then we could open the switch (off) and measure the voltage (pressure) between the wire before the switch and the return wire to the battery neg. We again would see maximum voltage/pressure, exactly the same as the test with the hose pipe.

Moving on to (b) we can now open the valve on our hose pipe and we will immediately see full flow. If we put the switch on in our electrical circuit we would also see full flow of current. This would be unwise in our electrical circuit as this would be a direct short and the current flow would be enormous and would probably either blow the battery up or melt the wire. However impractical or not we can see that current is the same as the flow of water, and to measure the current we need a meter in the wire to measure the flow of electricity through it.

Now we need (c) resistance. We often call this the load and it is very important in electrical systems. As described above without a load/resistance we simply have a short circuit and incur maximum current flow that will result in damage. In our case we will use a light bulb as our load; this may typically have a resistance of 3 ohms. In our hose pipe we could squeeze the end of the pipe to partially block it which ought to demonstrate the same effect. Squeezing the pipe or putting a load into our electrical circuit causes a restriction/resistance to the current/flow. So our current will reduce from our maximum in the short circuit. If we increased this resistance/restriction then the flow/current would reduce. So a 6 ohm light bulb would provide twice the restriction to current and naturally the current would be halved.

There is a simple formula that describes all this called Ohm’s Law. It states:

V = I x R

Where V = Voltage, I = Current and R = Resistance.

So: 12 volts = 4 amps x 3 ohms

SAFETY

Before we go any further we need to understand the dangers posed when using such meters. I do not want to discuss here the use of test meters when measuring higher voltages found around our homes but will stick solely to the use of the meter in testing 12Volt D.C. circuits. Even at these extra low voltages serious injuries can occur if the meter is not used correctly.
On our vehicles using the meter on the voltage scale and the leads in the volts and common connection very little can go wrong. However the problems arise when we use the current/amps range. In this range the leads are put into a different socket on the meter. They are in the common and Amps sockets and this forms a short circuit through the meter as it will need to become part of the circuit being tested. This is fine when it is used correctly and has a load in the circuit, but the problem occurs when the operator has it in this configuration and tries to measure voltage. This can then cause a direct short across the battery causing, hopefully, the fuse inside the meter to blow. If the fuse has been fiddled with, higher value put in, shorted out with silver paper etc, then the meter will quickly become toast in your hand, hopefully with only damage to the meter and not the hand.
There is not a technician living that has not tried to measure voltage whilst the meter leads were in the amps sockets, indeed a friend of mine caused a complete refinery shutdown (at least 3 million pounds in lost production as a result) by simply having the leads in the amps sockets and trying to measure voltage on a programmable control system. The trick to not having this nightmare is to ALWAYS keep the leads in the voltage sockets and never mess with the internal fuse. Indeed I would go as far as to say that unless you are absolutely certain you know what you are doing and equally if there is no other way to test then do NOT use the amp function at all. 99% of all auto electrical problems can be sorted by simply measuring for voltage, with a little use of resistance to test for continuity.

TESTING

Here is how to perform a simple test on a circuit using the voltage range on the meter.
If something is not working, we generally have to find somewhere accessible where we can start to test. This is normally the fuse. Many people will remove the fuse and yes using the meter on the resistance range the fuse can be checked for continuity. However it can also be easily checked whilst still installed with the meter on the voltage range. Connect the black lead of the meter to the vehicle earth/body, then place the red lead on the feed side of the fuse (B). Should read 12 volts, if not then no feed to the fuse, switch not on or faulty? If 12 volts then measure the other side of the fuse (C). If 12 volts, then fuse is fine, if nothing then fuse is blown!! If this circuit supplies a light and there was 12 volts after the fuse then move to the light socket. On the wire feed to the light socket (D) again we should be able to measure 12 volts, if not then the wire between the fuse and the lamp is broken, if we have 12 volts then move on. Measure the other side of the light socket on the wire going to earth (E). If you have 0 volts then likely as not the bulb is blown, however if you measure 12 volts then the earth connection is faulty.


In understanding meter usage fully, it is essential to understand what makes a LOAD. The load is always the thing we are switching on and off, a light bulb, a fuel pump, a brake light, a fan, the ignition coil, or even our radio. And as can be seen in the diagram measuring before the load we get full voltage, measuring after the load we should see zero! That is assuming everything is working, the voltage disappears into the load. The term we use in electrical circles to describe that is we say we have dropped the voltage across the load.
We also need to understand how we measure current/amps. The easiest way to measure the amps in any circuit is to remove the wire from the load side of the fuse. Connect the red meter wire into the fuse where you disconnected the circuit wire from. Then connect the black wire to the circuit wire that was disconnected to form a circuit. This is then classed as having the meter in series with the circuit. Remember to put the meter wires into the correct connection points on the meter and please remember to return them to voltage at the end of testing. Please also remember to only test for amps if it is absolutely necessary, accidents can happen when a meter is used in this mode.

Finally a quick word on resistance and continuity. With the meter in this mode we can check for a good fuse, a broken wire or even a blown light bulb, be careful to always have the power switched of when using this mode, it is simply used for checking from one point to another to ensure wires are not broken.

Hope all this has helped just a little. It would be easy to explain electricity and meters for a month, sadly we have only a couple of pages but anything must be better than nothing. Happy fault finding!!!