Originally the test meter used by electronics engineers and electricians throughout the world way a comparatively large analogue instrument that drew its power from the test piece. This meant that it was not very portable and was priced out of the reach of the do it yourself user. Things have now changed greatly and the multi meter is now a self contained and compact digital instrument that has been mass produced bringing the price down to a few pounds or dollars, well within the reach of most of us.
The most simple multi meters can measure both AC and DC voltage and are capable of current measurement in AC and DC currents up to 20 amps as well as voltage measurement up to 400 Volt. The measurement of resistance in ohms is usually included with a transistor tester as well. There is often the capacity to test diodes. Often it is possible to use the meter to test for continuity in a circuit. These multi meters tend to have an internal 9 volt battery power source which makes them self contained and portable.
This simple and compact meter is very useful for testing the basic electrical functions in the motor car, especially if, like mine there are no computers involved, yes it is a 17 year old Land Rover Defender. The last time that I used my multi meter on the Land Rover the wiring loom adjacent to the gear box had caught on the prop shaft and been ripped to pieces. This took out the rear lights and all the connections to the instrument panel and switches. The simple solution would be to have the wiring loom replaced, but the cost was prohibitive and far too much for a potentially do it yourself (myself) job. My multi meter was invaluable when used in conjunction with a wiring diagram that showed the colours of each wire. With it I was able to check that the correct wires were attached to the correct terminals and were live when the switch was turned on.
Such multi meters are easy to use but care needs to be taken when selecting the range by the large rotary switch on the front of the unit. In the instance above the selector was set for DC voltage up to 20 volts. The Land Rovers system like most cars is battery operated and therefore a Direct Current is produced. The only exception to this is the HID head lamps that I have just fitted where a ballast in the circuit produces an Alternating or AC current for the bulbs.
Used on domestic single phase electrical supplies the switch needs to be set for AC and usually up to 300v or the nearest setting above 240v. It is always worth checking that the leads to the probes are correctly connected to the meter and care should be taken not to short anything out with the probes. I find that for work on the Land Rover it is better to use a negative lead terminating with a crocodile clip attached to a really good earthing point.
For more information on multimeters go to www.lcr-meter.net