A digital multimeter is one the most versatile and useful instruments in your auto shop. It is important to own a good model and understand how to use it properly. A digital multimeter is actually three devices in one. It is a voltmeter that measure electrical potential across a device in volts. It is an ammeter that measures the amount of electric current through a device. This is measured in amps. Finally, a digital multimeter is an ohmmeter that measures electrical resistance of a device. Electrical resistance is measured in ohms.
Today, modern digital multimeters are designed to be rugged and easy to operate. A good multimeter will have a rugged plastic case and large, easy to use selector knobs. The top part contains the digital read out screen. This is something you should thoroughly check out before you purchase one. Make sure the screen is large enough to read it and make sure you see the readout in sunlight. Chances are you will be using this instrument outside in direct sunlight.
Below the digital readout is a large knob called the function switch. The function switch allows you to change the modes the digital multimeter operates in. For example, you can easily change from voltmeter to ammeter to ohmmeter with the turn of the dial. Again make sure the function switch is large and easy to operate. Most function switches have approximately eight positions. Most have three V markings that measure voltage. They measure AC, DC and low voltage currents in the millivolt range. Next there will be two positions marked with A~ and A=. The A~ measures AC current in amps and the A= measures DC current in amps. The upside down horseshoe Ω measures resistance in ohms.
In order to measure voltage, first turn on the digital multimeter and let it go through its startup procedure. Generally the digital readout lights up and the unit goes through its self diagnostic checkout. Once that is completed you are ready to measure volts. Now turn the function switch to V= to measure DC volts. Now you will need to connect the red and black leads to the digital multimeter. Connect the red lead to the red input terminal labeled VΩ and connect the black lead to the terminal labeled COM for common terminal. Now you can measure volts by putting the red lead on the terminal with the higher potential and the black lead on the lower one.
To measure amps, the leads must be connected in a different fashion. First set the function switch to A= position. Connect the black lead to the COM terminal. Now you must connect the red lead to the terminal labeled 300mA. Now you are ready to connect the meter in series the device being measured by opening up the circuit and inserting the meter between the open points. The results will be in milliamps because you are using the 300mA terminal.
The third feature of a digital multimeter is its ability to measure to Ohms. Ohms is a measurement of resistance in an electrical circuit. First disconnect all wiring and power sources from the device being measured. Now turn the function switch to the Ω position and connect your leads. The red lead is connected to the terminal labeled VΩ and the black terminal connects to the COM terminal. The display will indicate OL. This is normal and means there is an overload. Now connect the leads across the device to measure the Ohms.
These are the basic functions of a digital multimeter. Remember to shut off your multimeter before storing it back in the your toolbox. You do not want a drained battery the next time you will need it. There are several good brands on the market today. Fluke digital multimeters are probably the most popular and you won’t go wrong with one.