Use a Volt/Ohm Meter

March 21, 2014 09:22 PM
Dan Anderson close

Tis better to learn to use a volt/ohm meter than to curse a dead circuit, but many farmers are not adept at using volt/ohm meters and multimeters to test electrical circuits. More than once I’ve been on a service call, testing circuits on farm equipment with my own multimeter, and the customer says, "I’ve got one of those."

Volt/ohm meters and multimeters are actually quite easy to use. A basic, beginner’s level volt/ohm meter tests only voltage or ohms of resistance and costs $10 to $50. Multimeters offer more testing capabilities and are $40 to $500. Either way, here’s a pri­mer on how to use them for basic farm-style electrical circuit tests.

Test meters come with two test leads—a red wire and a black wire. Plug the black wire into the jack or input port labeled "common" or "ground." Plug the red wire into the jack marked "V" or "voltage."

Turn on the unit. You’ll see "0.00" in the digital display. If your unit has a needle-and-dial display instead of digital, retire it and get a digital test meter. You’ll be glad you did.

Test voltage.
To test electrical circuits on farm equipment, set the meter’s selector to "DC voltage" or the "V" with a straight line above the V. "AC voltage" or the "V" with a wavy line above it is for alternating current in 120-volt household-type circuits.

Open the hood of your pickup truck or any other vehicle with a 12-volt battery. Place the end of the red test lead on the positive battery terminal. Touch the end of the black lead to the negative terminal. The meter’s display should show around 12 volts, depending on the charge of the battery.

Reverse the leads (black lead to the positive terminal and red lead to the negative terminal) and the display simply reads "-12.0" in recognition that polarity is reversed.

So the basic principle of testing DC voltage is: Touch the red lead to any wire or connection attached to the positive side of the battery, then connect the black lead to a wire or connection on the negative side of the battery, and the meter reads the voltage through the wire or component.

If the meter reads "0," there’s a problem because no voltage is flowing through the wire or component. If it is less than 12 volts, it can get tricky deci­ding if there is a problem.

Many circuits on modern farm equipment operate on 4.5 volts, and return signals from potentiometers can vary 1.5 to 5.0 volts. Don’t assume low voltage means there’s a problem until you know what the correct voltage for that circuit should be. In general, starters, lights and main electrical system components operate on 12-volt electrical supplies.

Test ohms of resistance.
Set the meter to the "ohms" symbol, which sort of looks like an upside-down "U." The meter is now set to measure ohms of resistance through the test leads.

Touch the ends of the test leads together and the display will show a small number like "0.1," indicating there is minimal resistance to electrical flow through the test leads. Separate the test leads and the display reads "OL" or some designation indicating there is infinite, complete resistance between the test leads—because they aren’t in contact with each other.

This is helpful for wiring connectors that are notorious for looking good but have hidden internal corrosion. To see if a connector is functional, disconnect the connector and touch one test lead to a pin in the connector and the other test lead to the wire coming from that pin on the back side of the connector. If the meter shows more than a few ohms of resistance, the connector needs repair.

Many test meters have a "beep" function for testing resistance. The meter beeps when continuity (a fancy word for "no or very low resistance") is present between the test leads. This is handy when you’re trying to reach wires deep within a machine. Simply touch the leads to the wires in question and listen for the beep to confirm if continuity is present.

Cool Tool of the Month

FJ 074 F14205

Checking voltages and continuity without unplugging wiring harnesses is a challenge. "Piercing" wires with sharp-tipped probes is also a good way to draw blood from a fingertip. Instead, use the V-shaped cradle of the Ferret Claw to hold the wire while a needle-sharp, screw-in needle pierces the insulation. Once tightened, the Claw stays firmly in place for repeated testing but removes easily when done testing. Around $55 for a four-piece piercing kit.


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