Voltage drop testing is probably the best way to find an electrical fault. Because the voltage drops as it travels through the circuit, we can tell where it's being used simply by measuring the voltage at different points in the circuit. Think of a car that leaves the positive battery post with just enough gas to work the load and get to the negative battery post. By the time the car reaches the negative battery post, it runs out of gas and that's it. That's how voltage works; it starts at its highest level at the positive battery terminal and ends up at zero when it reaches the negative battery terminal.
Let’s say that somewhere in the circuit, we take a detour and go down a different road and decide not to take the road (circuit) all the way back to battery negative. We call this a short circuit. When this happens, we often blow fuses and sometimes melt or burn up parts. This is because of the unrestricted amp flow that occurs during a short circuit; we'll talk about that in the next section.
Or, let’s say there is an obstruction in the road (circuit) and we use more gas getting through the obstruction on our way back to battery negative. We'll still make it back to battery negative, but since we used so much fuel (voltage) getting through the resistance and the load, there's hardly any fuel left to do any work. This is what happens when we have increased resistance in a circuit. If that's the case with, say, a window motor, the window motor will still work; it will just work very slowly. Why? Because of the increased resistance we had to go through to get back to battery negative. We ended up using almost all our fuel (voltage) overcoming the resistance in the circuit. Less fuel (voltage) means there isn’t enough to run the window motor at full capacity.
If we did a voltage drop test following the path of the circuit, we could see where the fuel (voltage) was being used. We should see the biggest drop across the load of the circuit, which, in the case of a window motor, is the window motor. While the circuit is operational, we can check the voltage before and after the motor. Voltage drops need to be done on a live circuit; if they’re not active, you won't get any readings. Remember that voltage is electricity's potential energy; if it's not being used, it's just potential. Before the motor, we should see something close to the 12V we started with. We won't see the full 12V because we had to use some of the gas (voltage) to get through the wires, and probably switches, going to the motor itself. These things give us a little resistance that we need to account for when checking the voltage before the motor. After the motor, however, we should see some pretty low voltage, because we used all our gas (voltage) to run the motor.
This works for every electrical circuit according to Ohm's law. Once you know the circuit, you know where the gas (voltage) is going and you can measure it at different points in the circuit to get a good idea of how the circuit is operating.
One last example that might give you some practical insight: Say you have an electrical connector with some corrosion in it. You can't see the corrosion, but you suspect it’s there. You can take a voltage reading before and after the connector to check the voltage drop across the connector. You should see close to zero volts. If you see a higher voltage than expected, you've found increased resistance in the circuit.
Voltage drop testing is your best friend when it comes to diagnosing automotive circuits. Learn how to do it, and you'll be an electrical wiz in no time.