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In the Shop

RSS By: Dan Anderson, Farm Journal

As a farm machinery mechanic and writer, Dan brings a hands-on approach that only a pro can muster. Along with his In the Shop blog, Dan writes a column by the same name as well as the Shop Series for Farm Journal magazine. Always providing practical information, he is a master at tackling technical topics and making them easy for all of our readers to understand. He and his wife, Becky, live near Bouton, Iowa.

How To Use A Voltmeter

Mar 20, 2010
 If you've never used a voltmeter to test electrical circuits on 12-volt farm equipment, cars or trucks, but wanted to learn, here's how:

Voltmeters cost from $15 to $300. $50 will buy an economy-grade unit adequate to test for 12 volts on farm equipment. If you eventually graduate to testing sophisticated electronic circuits related to computerized systems, you'll need to upgrade to a more expensive voltmeter with at least 10 million ohms of internal resistance to protect those sensitive circuits.

Connect the two test leads that come with the voltmeter to appropriate sockets in the voltmeter. The red (positive) lead goes to the socket marked "+". The black (ground/common) lead goes in the socket marked "-", "ground", or with a little icon that looks like an inverted pyramid made of horizontal lines.

To build confidence and learn the basics of testing voltage, go to a 12-volt battery in a tractor, car or pickup truck. Hold the red test lead against the positive battery terminal. Hold the black test lead against the negative terminal. The digital display will show 11.5 to 12.5 volts on a fully charged battery. 

Now find a light bulb on the machine. Lights require a power circuit and a ground circuit. Older tractors have a single power wire going to lights, and use the metal frame of the light as the ground circuit. Newer lights will have a red or colored power wire and a black, ground wire. With the lights turned on, hold the red test lead against the light's positive terminal and the black test lead against the ground terminal. The digital voltmeter's display should read the same or slightly fewer volts than the battery.

And that's the basic concept of testing with a digital voltmeter. Start at the battery and test between power wires and ground wires/circuits. When in doubt, go back to the battery and prove the voltmeter is working correctly, then trace wires and work from the battery toward the component or circuit in question. When voltage disappears, you've found the short or faulty component that's causing the problem.

Yes, electrical diagnostics can be much more complicated than that, but a lot of electrical problems can be diagnosed using this basic understanding and strategy of voltmeter usage. 

My friend and co-worker Wade uses a less technical explanation when customers come in and ask for help diagnosing electrical problems. Wade leans against a tractor tire then asks, "Did you notice a puff of smoke come out of the wiring or any part of the electrical system?" If the customer says, "Yes," then Wade nods solemnly and says, "Well, there's your problem. You let the smoke out of the wiring. Them electrical systems won't work once you let the smoke out of the wires."

Then he smiles, winks and helps them figure out what's wrong with their "smoked" electrical system.
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