You're working on an electrical problem. You suspect you've got a bad wire somewhere, but you aren't looking forward to resistance-testing half the wiring harness. Try a voltage drop test instead. What's a voltage drop test?
Well, what happens when you touch a multimeter's test probes to a battery's terminals? You read battery voltage, of course. But why? Because you are actually reading the potential difference between the battery's two terminals. That's what makes voltage, i.e. a differential, and that's what battery is, a "differential box". Now put both meter test probes on the same terminal. (Huh? Don't laugh). Now what do you get? You get 0 volts, as expected. But why, technically speaking? Because the battery is dead? No, because the two probes resting on the same terminal lack the necessary two points of reference from which to gauge a difference, a differential. No differential, no voltage. All voltage readings, no matter what kind, are in reality differential, or "voltage drop", readings. A volt drop test then is merely a voltage test from one point to another in the system, wherein the meter's reading is simply a direct indication of that difference. To ensure that we always read only that difference, the multimeter is connected across, that is, straddling, the switch, connector, or component.
What should the reading be? A connector should theoretically drop 0 volts. However, we can allow up to a very tiny 0.2 volt drop, as all connectors have a slight amount of resistance. However, too much resistance absorbs electrical pressure as if it were a light bulb. Is a connector supposed to use electricity like a light bulb, or merely carry it like a wire? You know the answer. Not use, but carry, naturally.
Let's volt drop test power to the ignition coil. One meter test probe to battery positive, the other to the ignition coil's black/white wire. Turn the key on. Say you get a 4-volt drop. Not good. Remember what we're straddling. We're straddling the entire ignition coil power circuit from battery to coil. What's in this power path? Main fuse, key switch, engine stop switch, and at least four or five connectors. Somewhere along this electrical path is a problem. To find the glitch, keep the positive meter probe at the battery, and move the negative probe closer to the other test probe onto the next switch or connector in the system (have the factory schematic in front of you). Note the reading. Move the test probe again, on to the next connector in the line, and keep moving until the reading decreases suddenly a large amount. That’s the tip-off point. Somewhere between here and the last point tested will be the bad spot in the circuit. It works, and it really is this simple.
The voltage drop test is one of the most powerful electrical troubleshooting techniques in your troubleshooting arsenal. Focusing on a conductor’s voltage instead of its resistance provides two benefits. First, it speeds up the job. Resistance tests require disconnecting the component, and even then the results are not always reliable. Second, the voltage drop test delivers more accurate values. It can even uncover resistances too small for an ohmmeter to measure. Just don’t forget that the key switch must be turned on for each test, or you will get some misleadingly large readings. Happy troubleshooting!