Dropping In On Electrical Faults

March 25, 2015
In its simplest terms, voltage drop testing is the measuring of the voltage differences that may be present at various points in a circuit. The truth is, every circuit has some voltage drop.

Those who know me know I talk to myself. Actually, I just think out loud (a lot). Sometimes in frustration I might ask questions out loud, even when no one’s in the room. Fortunately, I don’t always answer my own questions. With the intention of having fun and at the same time attempting to educate, in this article I took some literary freedoms and imagined “Jaime has a conversation with his split personality.”

Jaime, I frequently hear the term “Voltage Drop Testing.” What is it? In its simplest terms, voltage drop testing is the measuring of the voltage differences that may be present at various points in a circuit. The truth is, every circuit has some voltage drop. Even the best made wire (conductor) doesn’t have the same number of electrons available at its end as it has at its source. The difference is considered the “drop.”

That doesn’t sound very technical. Is that all there is to it?
Is anything we write about simple? Like the commercials say, “But wait! There’s more!” Remember high school physics? (No, I don’t mean the girl we liked in that class.) We were taught, among other things, that “Energy cannot be created nor destroyed — but it can be transformed into other types of energy.” In fact, that is exactly what we’re doing when we bring a vehicle to a stop. We are converting kinetic energy (vehicular motion) into thermal energy (heat) when braking friction is used to slow us down.

More physics – “An electron at rest” (will remain at rest)… There is one condition that must be present in order to have a voltage drop. That is, the circuit must
have been intended to be in operation. In other words, the circuit was completed between the battery positive and the battery negative terminals, of course, with control circuits and a load in between. If the circuit isn’t completed, electron flow will not occur. Electron flow can be measured in Volts (V or E) and Amperes (Amps, A or I) and is affected by resistance (R). Ohm’s Law is E=I x R. Here is a simple concept to remember: If there are not enough pathways for amperage to flow, the volts will decrease in numbers (voltage drop), and usually heat is produced as well.

TMI! Too Much Information! You’re hurting my brain!
This is the same reason power companies generate a lot more electricity than their consumers use. A certain percentage of what’s produced is lost in transmission; some is lost transforming it to what we use in our homes and businesses, but typically the vast majority of energy lost is in the form of heat (just like when braking). It isn’t that the energy is gone but it has been changed into another form, leaving less energy (power) available at the end of its line.

So, what does that have to do with voltage drop testing or with auto repair? Well, as we all know, today’s automobiles have a lot more electronic gizmos than the cars did even a
few years ago. Just like the power company’s electricity, the heat generated by the loss of electrons traveling from the car battery, through all the electronic devices and back to the battery again, is why there aren’t any cars made without air conditioning anymore! Don’t believe me? Then name one car that has A/C optional. OK, just kidding.

Really! Get serious! Why should we care about a little loss of voltage?
We generally don’t have to “care” about the voltage drop that is engineered into a circuit. Think about it — if every circuit has a certain amount of electron loss then the smart engineer will build a tolerance into the circuit allowing for it. It’s only when the amount of energy lost causes something to not operate as designed that we even concern ourselves with it.

Let’s take for example, a simple circuit used to illuminate a light bulb. There should be a supply voltage, a switch, the load (bulb) and a ground. If our supply voltage is as it was intended, and is measured the same at one conductor of the bulb (assuming the other conductor is grounded), the bulb should burn brightly. But, if there isn’t the same voltage measured at the bulb as what is supplied (a drop in voltage) and all other conditions are the same, will the bulb burn as brightly? No, not as brightly as it was designed to be. If our supply voltage is correct at its source but not at the bulb, all we have to do in order to make the bulb illuminate brightly is determine where the difference lies. Doing so is performing a voltage drop test.

Dim wit! Why else do we need voltage drop testing?
The typical result of an excessive voltage drop is an unusual or unexpected operation of the device intended to be controlled, that is in the circuit that shares the fault. Just like the dim illumination mentioned, any device can “act up” if its circuits have enough of a voltage drop. Not all circuits are as simple as the one in my previous example. Can you imagine what might happen to a multiplexed network whose gateway module experiences a severe drop in its power supply?

What about circuits that fail to operate correctly – some of the time? Some failures are intermittent. The usual methods of addressing intermittent electrical problems consume extraordinary amounts of time. Fortunately, today’s technicians have tools that weren’t available until recently, that help them find intermittent voltage drops more easily.

Isn’t it comical how the repair order that includes the word intermittent causes so many technicians to develop sudden fevers and sore throats, requiring them to go home for the rest of the day, possibly longer? Most of us would rather not chase a problem that only occurs under certain conditions but would prefer to use that time more productively. Therefore, a systematic approach and proper tooling are absolutely essential when finding the cause of intermittent voltage drop problems.

So smarty-pants, are you saying I’ve been diagnosing wrong?
No, not at all! Wait. How have you been diagnosing voltage drops? What I am saying is, just as automotive technology has kept on changing, we might need to change our diagnostic procedures in order to keep up. Today we have complex wiring systems that are more sensitive to influencing factors, therefore are more susceptible to intermittent operation. Because it is 

likely we will see more and more of these, we must learn how to find them as soon as possible so we can get back to “making the gravy.” Recognizing that some procedures require more time than others is our first step.

OK, simple or complex, what can cause a voltage drop?
Well, since you asked, lots of things can cause an undesirable loss of voltage in a circuit. Think of anything that might affect the conductivity of a circuit and you’ll know what to look for when the circuit voltage doesn’t equal supply voltage. If two terminals are not mated correctly it might not be able to carry a load. If the integrity of the wire insulation has been violated, the wire may have corroded (oxidized) resulting in the amperage flowing to be slowed (which then results in the production of heat - remember energy loss?) and in a voltage drop. Almost all of us have seen a loose or a corroded ground attachment. Those can also cause voltage drops. What about carbon in between a relay’s contacts? Yup, those cause a change in voltage as well. The list goes on.

What other tests should we do in conjunction with a voltage drop test?
Knowing how many volts are missing when compared to how many were available is only part of the information we need in order to be able to say there is something wrong. We must take into consideration what percentage of affect the measured voltage difference has on the circuit. For example, would a one-volt-drop have the same effect on a 1 amp circuit as it would on a 200 amp circuit? Think about that for a moment.

Now you’re making my brain REALLY hurt! Are we gonna use foul language?
No, silly! But you know every now and then, we let four-letter words slip out, especially this one: MATH! We sometimes have to understand it mathematically to correctly condemn a failed component. For example, shouldn’t we calculate how much of an impact a 1 volt drop has on a starter circuit before we condemn the starter? How much affect would it have on a Throttle Position Sensor (TPS)?

In addition, we should keep in mind what the customer’s complaint is, all the while calculating if the voltage drop we find is substantial enough to cause the complaint. Finding and fixing a voltage drop that doesn’t solve the complaint might be in the customer’s best interest but isn’t worthy of remuneration. In other words, the customer should not be expected to pay for the fixing of a leaking muffler when the vehicle came in because the engine doesn’t start (unless of course, the replacement muffler miraculously allows the engine to start).

The really important question is: How do we perform a voltage drop test?
I’m glad you asked that! I’ll answer with a question: Can we drive across the country from one coast to the other? Of course we can, but if aided with an accurate road map, could we do it more efficiently? So, the first step to diagnose a voltage drop is performed by obtaining and analyzing the wiring diagram of the affected circuit. Consider the wiring diagram your road map.

Because we know what causes a difference between supply voltage and circuit voltage, it becomes a process of elimination finding where the circuit fault has developed. Remember, the fault must be present while you measure the voltage at various points in the circuit! The saying, “You can’t fix
something that’s not broken,” is true! There are proven, reliable methods to use that when combined with your Digital Volt-Ohm Meter (DVOM) and with some relatively new tools on the market, you too can become an expert at determining where circuit faults are.But, how can I force an intermittent problem to be present when I test it? While serving in the U.S. Navy, we took the ship out weeks before a scheduled long deployment to stress her. We ran her hard just to see where her weak points were. We looked to see if anything might fail under the most adverse conditions – while we were still
close to our home port – so if anything “gave” we would be able to take care of it then, not while underway overseas somewhere. We put the ship under a heavy load.

Isn’t there enough stress in our lives?
To find the location of the fault in a circuit we stress it – we load the circuit. What that means is we infuse the maximum amperage that the circuit was designed to handle and measure the voltage at certain points. Exactly where those points are is determined by many factors including ease of access, which components are involved, whether there is a module in the circuit, etc.

What do you mean by “measure the voltage”?
Well, there two common methods used to measure voltage differences in a circuit. In the first method, we would leave our negative DVOM lead attached to the battery negative terminal while we used the positive DVOM lead to sample at various locations in the circuit, one of which would be the point in the circuit that is closest to our power supply. All readings would individually be subtracted from our supply voltage reading – any difference is a voltage drop.

In the second method, we would attach our positive DVOM lead to the point in the circuit that is closest to our power supply, then using the negative DVOM lead, sample the voltage at various locations of the circuit. The difference (the voltage drop) will be displayed on the DVOM screen.

Did you notice I used the phrase “point in the circuit that is closest to our power supply?” I did so because not all circuits use the battery as their source of voltage. Although most do, some are ignition fed, some might originate in a module, etc. We need to know how much our source voltage is in any case.

Also, did you notice there is no math required when using the latter method? The meter read the difference and displayed it. Did you also notice the latter method is a one-step process, unlike the first method? Whichever method is chosen should produce the same voltage drop reading. Remember, these tests should only be performed when the circuit is in use, or is intended to be, or one that is loaded.

Will voltage drop testing decrease my diagnostic time?
You are guaranteed that it will not! That is, not if you don’t use it. All too often there are components replaced needlessly when the problem was actually an excessive voltage drop. That isn’t cost effective or profitable. We should always take a few moments to verify circuit integrity before condemning any component. If we perform our diagnosis accurately, in the case of an excessive voltage drop, the component wouldn’t be replaced (wasted time) and our diagnostic time would be far less than it would be otherwise.

How much slower does a starter turn with a 1 volt drop in the battery cable? How much more heat is created, decreasing the life expectancy of that starter? What effect does that have on the battery itself? Can it affect the computer systems in the vehicle?

Jaime, it’s been fun chatting with you. Now that we better understand voltage drop testing, do we want to reminisce about that girl in high school?

About the Author

Jaime Lazarus

Jaime Lazarus retired in 2020 after 41 years in the transportation repair sector. Throughout his career, he filled such positions as “lube tech", mechanic, technician, shop-owner, inventor, automotive technologies instructor, and published author. Also known as “The Car Whisperer”, he was widely diversified in automotive diagnostics. Lazarus focused his career on emerging automotive technology, recognizing early on that the biggest challenge for automotive repair technicians is diagnosing electrical systems and electronic components. He was a four-time certified ASE Master Automotive Technician that had held the L-1 (Advanced Engine Performance) certification since the test's inception.   

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