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.
- Early Spring 2014