It seems like anytime someone mentions doing a voltage drop check a ton of questions arise. Believe me it is not that difficult. Remember you are only going to check the available voltage from one point in the system to another. Place the positive probe at the supply side and the negative at the other, read the voltage, it will read the amount of voltage lost between the two points. Anything over .10 is too high, most DVOM fluctuate at least .03 at zero. The first time you check a defective wire you will discover how simple it is. The readings are usually over 2 volts or more. If you suspect a battery cable end of causing problems check from the post to the cable and you will see what I mean if it is corroded. This is also a excellent tool the check for a bad battery cable to the starter, older Fords are a main concern here as they require 12+ volts at the starter and are prone to cable problems, but anytime on any model you think you are having a voltage problem this will work. Also if you are searching for a bad ground, more prevalent today than ever before, simply start at a known good ground and the farthest away ground you can find and work your way back.To check supply voltage ground the negative terminal and check the positive terminal of the item. To check for a voltage drop across the load do the same except the positive goes to the ground side. I have never found this test very useful as it is complicated to figure out the correct drop in the item. Don’t forget that many items are also ground side controlled, almost all 5 volt sensors are.