Voltage drop testing, what do we need it for? What are we looking for? Most of the articles I have seen concerning voltage drop testing are somewhat confusing, and very technical.They really don't get into the detail of it and show its practical use in diagnostics. Simply put, voltage drop testing is looking for the amount of voltage lost that can be measured in an active circuit across each part of the circuit, be it a wire, any connections from the wire ends to the component in the circuit, and to the source voltage. When you use this test you are looking for excess voltage lost any where in the circuit, that would point to high resistance. When you see a high amount of voltage, say, 1 to 2 volts, you have found the source of high resistance affecting the circuit.
Voltage needs a solid conductive path from battery to component and back again for things to work properly, and when the path gets blocked by a wall of resistance, voltage backs up at that point and thats what this test is looking for, the wall the voltage runs into.
Basic Voltage drop test, checking connection at battery post. Most often, you will see no voltage reading at all, indicating a clean, tight connection.
More samples here
From the negative post of the battery to the positive post, any resistive load put between them will cause voltage drop, and how much depends on the amount of resistance. We need to look at voltage drop to find resistance not engineered into the circuit. In the image above, the wire on each side of the component has resistance, the terminals on each end of the wire has resistance, the connection between the wire and the terminal has resistance, the connection of the terminal at the battery or the load component has resistance, and of course the load component itself has resistance, and all of these total a certain amount of resistance in the total circuit. Looking at all these with voltage drop tests will show if any excessive resistance exists across them. What you are looking for, are large amounts of voltage where none should exist, more than 0.1 volts across each connection.
Probably whats most confusing about Voltage drop testing, is the term itself. Isn't voltage supposed to drop across a circuit? Yes, you are actually measuring that amount, and if all votage is not dropped, you see that amount as a result of resistance at that point. Does that make sense? It really is hard to get a grip on. I think of it as voltage lost testing, since thats actually what you see, voltage lost across a resistance. Just remember, its the lost voltage you see, not whats used, on the meter.
We can measure the total resistance of a circuit from the negative and positive ends of the circuit, but only while the circuit is inactive and isolated from power and ground, but sometimes unloaded circuits will not indicate high resistance. We find out by doing voltage drop measurements to see unwanted resistance on a live or loaded circuit. Here are some examples of voltage drop testing.
You have probably seen a headlight on a car that seemed very dim, and most of you probably know, its usually a case of a bad ground connection and you would need to voltage drop the circuit to find the source of the high resistance.
96 Chev S-10 Pickup lighting Weirdness
Heated Backglass testing
Testing heated back windows is easy if you know voltage drop testing.
91 Dodge Spirit has a bad TPS Ground, drop testing sensors is easy.