Friends, it's time to say goodbye to those faithful ol' testlights you've used for so many years. The ones that look like a clear-handled screwdriver with a long cord running from that handle. You'd clamp the end of the cord to a known ground-point, poke the pointed end of the "screwdriver" into a socket or connector, and a little light in the handle would light up to indicate power was present.
It's time to upgrade to a multi-meter. A test light is good for indicating the presence of voltage, but that's about all it offers. A multi-meter, on the other hand, can indicate how MUCH voltage is present; a multi-meter can test resistance in a wire, circuit or component; a multi-meter can test continuity through a wire, through a switch, or through a connector. A multi-meter beats a test light, hands-down.
Some folks are intimidated by the "complexity" of a multi-meter. Fancy, expensive ones have lots of dials and buttons and settings. Forget about fancy. Get a simple, durable digital multi-meter that will read voltage, resistance and continuity. Take 15 minutes to learn how to use those three capabilities:
To read voltage: Set the multi-meter to read, "DC volts." Go to a tractor, truck or vehicle and press the red lead from the voltmeter to the positive terminal of the battery. Touch the black lead to the negative terminal. The digital display will show between 11.5 and 13.5 volts, depending on how well-charged the battery is. That's the basic idea of testing voltage--put the black lead on a known ground-point, put the red lead against the wire you want to test, and the meter tells you how much voltage is present. Cool!
Reading resistance: Set the multi-meter to read, "resistance" which is measured in ohms. Ohms is the funny symbol that looks like the letter "O" with an opening on the bottom with little flared legs on either side of the opening. To test resistance the meter sends a faint voltage through the test leads and measures how much resistance there is to that flow. So, with the meter set to read resistance and the red and black lead not touching each other, notice the display shows something like "OL," reflecting that there is absolutely no flow of electricity between the two test leads. Now touch the ends of the red and black test lead together. The meter will show 0.01, maybe 0.1, or some other very small number, indicating the minimal resistance of electricity to flow through the wires and circuits of the multi-meter itself. Okay--now find an old light switch and, WITH THE SWITCH DISCONNECTED FROM ANY POWER SOURCE, measure resistance between the poles on the switch, first with the switch off, then with the switch on. The multi-meter will show extremely high resistance when the switch is "off", and extremely low resistance when it is "on." Think about it--you can do the same thing with a 30-foot-long wiring harness--touch the read test lead to one end of a wire, touch the black test lead to the other end of the wire, and as long as there's very little resistance indicated, you know the wire is, "good." If resistance is high, or shows "OL" on the display, there's a break somewhere in the circuit.
Even cooler, there's probably a "continuity" setting on any decent multi-meter. That setting tests resistance, but is wired into a little beeper or horn in the multi-meter so that when there is minimal resistance, you hear a continuous beep from the meter. Bottom line--when testing an unpowered switch, wiring circuit, or other component you just listen for the "beep" to indicate the wire or component has continuity (ie--electricity will flow through it without interruption).
Those are just the basic things you can do with a multi-meter. Figure on $50 to $100 for one that tells you volts, ohms (resistance), and registers continuity. (Mine cost $250 ten years ago but has functions I still haven't figured out how to use.) If the one you select reads milli-amps, that's cool, but you'll have to learn to use that feature with great discretion. Once you've bought a multi-meter, and if you've never used one before, spend 15 minutes playing with it to figure out how to read volts, ohms and continuity.
Once you build a little confidence and figure out all the different things a multi-meter can measure, you'll wonder why you ever wasted time with that old test light.