While using a digital multimeter to diagnose electrical problems on customers’ equipment, I frequently hear the comment, “I’ve got one of those multimeter things, but I never learned how to use it.”
Here’s how to use a multimeter, a tool that measures electrical values, to test for voltage.
It doesn’t matter if it’s a $30 discount store special or a $1,000 Snap-on multimeter with 25 functions, a multimeter will come with two electrical leads. One is black. Plug it into the socket labeled “common” on the face of the multimeter. Plug the red electrical lead into the socket marked “VDC” in red, which translates as Direct Current (DC) voltage.
The black electrical lead is the ground wire for any testing. The red lead is used to test for 12 volts DC in powered wires or components.
The multimeter has a dial that can be turned to different settings. Identify the setting for DC voltage, often marked by a symbol of a couple dashes, indicating the unidirectional flow of direct current. A setting for testing AC voltage might be nearby, indicated by a wavy line representative of the alternating “waves” of AC voltage. Forget about the AC setting for now because mobile equipment generally runs on DC voltage.
With the multimeter turned on and the test leads plugged into their holes, find a car, truck, tractor or lawn mower with a 12-volt battery. Touch the black lead to the negative terminal and the red lead to the positive terminal. The meter’s display should read somewhere above 12 volts.
Now, with the black lead still on the negative terminal, visually follow the red positive battery cable to its first connection. Touch the red test lead to that connection and the meter should again read slightly more than 12 volts. You’ve just proved that voltage is flowing through that cable.
If an electrical system isn’t working properly — a light bulb is dead, for example — follow the same strategy: Touch the meter’s negative lead to the negative battery terminal (or bare metal on the machine’s frame, since the battery is “grounded” to the frame,) then test for 12 volts at the failed light bulb. If 12+ volts are present, replace the burned-out bulb. If voltage isn’t present, follow and test the wire that’s supposed to power that bulb back toward the battery until you find a connection where voltage reappears. Decipher why voltage doesn’t flow past that point on its way to the light, and you’re on the way to having used a multimeter to diagnose an electrical malfunction.
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