Voltage drop testing is a fundamental test for diagnosing electrical circuit performance. This is the most important test that can be used for checking the integrity of electrical wiring and circuits in any low voltage DC circuit found on the automobile. In order to understand what this test is we must first understand the way voltage behaves in a circuit.
As a quick refresher: Gustav Robert Kirchhoff was a famous German physicist who made some fundamental discoveries concerning the way electricity behaves in a closed circuit. His second law in relation to electrical circuit function states that the sum of all drops in a closed circuit is equal to source voltage. Just to review, every time voltage pushes current through any resistance in a closed circuit, the resistance will cause a drop in voltage on the back side of the resistance.
Let’s consider a simple series circuit with two identical light bulbs and a switch. The circuit has 12 volts powering these lights when the switch is closed. Since both bulbs are the same they are going to have the same resistance, with the same resistance they will drop the same amount of voltage. All voltage must get used up in any circuit so both bulbs in this example will each drop 6 volts. The switch does not drop any voltage because it should not have any resistance. In any normal electrical circuit, only the load should have resistance. In reality everything has resistance, wires, switches, connectors, but the resistance of these things should be so low that we don’t need to factor them into the equation.
If we have a problem with unwanted resistance in the circuit, the way our loads, or bulbs, use up voltage will have to change if we are going to stay compliant to Kirchhoff’s law, which of course we must. If one of the bulb sockets was exposed to the elements it would be susceptible to corrosion build up and could cause us some problems. Corrosion in such a spot is of course very common on the automobile.
As the voltage tries to push the current through the corroded bulb socket it would struggle to do so and would be diminished by the extra effort. Voltage would drop across this unwanted resistance just as it does through the resistance of a normal load, and it would leave less for the loads to use. If the corrosion dropped 2 volts, that would leave 5 volts for each bulb. The lights would still light up but they would be dim compared to what they would normally be. Not only that, but the energy used up in the corrosion would not simply vanish, it would be converted to another form there, just as it gets converted to another form in the bulbs. The bulb socket where the corrosion is would become hot.
This is one of the consequences of electricity moving through resistance, whether the resistance is wanted or not. Heat is bad in an electrical circuit because it sets off a chain reaction the leads to the complete failure of the circuit. The heat produced by the unwanted resistance would cause even more resistance, which would lead to more heat, which would lead to more resistance and so on.
Getting back to where we began, a voltage drop test will allow us to find these spots of unwanted resistance in a circuit, and it will do so quickly and accurately. Spotting a melted bulb socket is easy to do with a visual inspection, but what if the problem is in the switch? This might not be so easy to find with a visual inspection. What if the problem is a bad ground? This is very common since the ground side of all circuits is the chassis of the vehicle. This means that nearly all ground connections are exposed to the elements in some way or another.
To perform a voltage drop test we need a halfway decent volt meter. Pretty much anything digital will be good enough for this test, you don’t need a Fluke meter to do this accurately. Set the meter to read volts DC and make sure the leads are in the proper slots on the meter. Start by placing your red lead on the most positive point in the circuit. This is might just mean putting the lead on the positive battery post. The black lead should be placed on the most negative point of the positive side of the circuit. This is probably going to be the point where the wiring goes into our load device. Next you must activate the circuit, turn it on, or at least attempt to do so. With current flowing through the circuit take your reading from the meter. Voltage only drops when flowing through a circuit. If the circuit is not activated voltage will not drop and this test will not work.
Being generous with our specification for maximum voltage drop on the positive side of a circuit, we don’t want to see more than .5 volts dropped across the entire length this side of the circuit. If we are dealing with electronic controls or signals, we would want to tighten that spec up quite a bit, but for regular 12 volt circuits on the automobile, .5 volts is the limit.
That’s the test for the positive side of the circuit, but the negative side of the circuit is important as well. To test this side of the circuit we are going to place our red lead at the most positive point in the negative side of the circuit. This would be the connection coming out of our load device. The black lead would be placed at the most negative point of the negative side of the circuit, the battery negative terminal. Now make sure the circuit is closed and take your reading. On the ground side we don’t want to see more than .2 volts being dropped. Once again this is fairly generous.
So if we do this test and we find we have 2.3 volts or something like that being dropped on the ground side of the circuit, how can we narrow it down some so we can pinpoint the problem? To do this we need a thorough understanding of the circuit we are testing, or we need a wiring diagram so that we might gain a thorough understanding in order to know which way to go next.
Working on the ground side of the circuit to try to narrow this down, we are going to leave one of our two leads right where it started, and we are going to take the second lead and work upstream or downstream to close in on this nasty unwanted resistance. So we might leave the red lead at the connection coming out of our load, and then take our black lead off of the negative battery post and move it upstream to wards to the red lead.
We need to look for the easiest places to access the circuit with our black lead. Since this is the ground side, there is probably a place the circuit comes out of the load and attaches to the frame, body, or engine. This would be a spot that might be accessible. Move the black lead upstream to this point and take another reading. Keep moving upstream in this manner until the voltage drop displayed by the meter goes away. By the time you have narrowed it down, chances are the problem will be obvious. You will see the corrosion, the damaged wire, the switch that is half melted. It’s almost as if you will literally find a smoking gun. A wiring diagram is a must so that you can locate all the best test points. Wiring diagrams are very critical.
This is how to measure resistance in a circuit. An ohmmeter is only good for measuring resistance in components or for looking at continuity. They are not good for measuring resistance in wiring and switches. The reason for this is a subject for another column.
Anyone who wants to master automotive electrical system diagnosis must master this skill. Try practicing on simple circuits that are easy to access. Things such as starter circuits or charging circuits are easy to get to and make a good place to practice, not to mention they must be commonly tested when looking for bad starters and alternators. Don’t be afraid to try something new. Electrical systems are not that difficult to repair if you know how to test them.