How to Use a Voltmeter

08:10PM Mar 18, 2019
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A customer recently called to ask questions about the electrical system on his machine. I asked if he had a voltmeter. There was a long pause, then he said, “Yes, but I don’t know how to use it.”  

He had purchased a fairly high-tech voltmeter, but there were so many settings and options he never figured out how to use it. So here’s a primer for any voltmeter.

First, find the on/off switch. Some meters have a separate button or switch. Others turn on anytime the dial that changes settings is moved. Whatever the case, when it’s on numbers should show up on the digital display screen.

Second, look at all the settings and find a capital “V” with straight and dashed lines running horizontally above the “V.” That setting is for direct current (DC) volts, which is what you want for farm equipment. When you set the meter to read DC volts it will show “0.00” on the screen.

Third, go to a tractor, truck or other  machine with a 12-volt battery, and touch the red lead of the tester to the positive terminal. Then touch the black lead to the negative terminal. The display will read from 11.5 to 13 volts, depending on battery condition.

At its simplest, that’s how to use a voltmeter. Test for voltage anywhere on the machine by touching the red lead to a wire that should have voltage and the black lead to either the frame or a wire grounded to the battery’s negative terminal.

Yes, it can be more complicated. Yes, you can set the meter to read alternating current (AC), check resistance, read milliamps and all sorts of other things.

Of course, you can refer to the owner’s manual to learn how to use it. That’s my plan, to someday find my owner’s manual and learn how to use all the settings on my voltmeter. 

Mechanic Dan Anderson shares practical shop tricks and fixes each week at