Vehicle electrical system problems can be solved with voltage drop testing.
Problems in modern electrical systems can cause all sorts of complaints, from a taillight that doesn't work to an engine that runs poorly. All of us, whether in school or on the job, have learned basic electrical testing techniques; measuring voltage, resistance and maybe even current. Yet, still I meet techs of all skill levels that miss problems that could be discovered had they understood and used one of the most effective techniques there is — voltage drop measurement.
It all starts with the infamous Ohm's Law and the interrelationship between voltage, current and circuit resistance. If you don't remember, that relationship is expressed as:
Voltage = Current x Resistance
While plugging in the numbers has some use in diagnosis, the key points related to Ohm's Law that I want you to remember are these:
For a constant voltage, an increase in resistance will cause a decrease in current.
For a constant resistance, a decrease in voltage will cause a decrease in current.
Current is what makes an electrical component work. If the current received is less than what it should be, the component won't work as designed, if it works at all. Source voltage, usually supplied by the battery, is the "push" needed to overcome any resistance in the circuit, including that of the component itself. Resistance is just that — the restriction to current flow in a circuit or component.
For the majority of electrical circuits, the only significant resistance in a circuit is the resistance of the component doing the work, which I'll refer to as the load. Some loads are probably obvious to you, like light bulbs, ignition coils, and fuel pumps. But ANY component that performs work is a load, and this includes some that may not be as obvious, like relays, Hall effect sensors and control modules. Many components play more than one role, depending on their function in a given circuit. For more, check out the electrical features in the January and February 2009 editions of Motor Age.
One more point for you to remember:
All the available voltage will be used to overcome the resistance in a circuit.
If there is more than one source of resistance, voltage will be split proportionally between these sources of resistance.
If the battery and charging system are functioning properly, the source voltage remains a constant. It is this source voltage that overcomes the resistance of the load allowing current to flow. Source voltage will then "drop" across that load, having been used to overcome that resistance.
What causes problems in the real world are changes in total circuit resistance, and that affects the amount of push given to the load, even if that source of additional resistance is after the load in the circuit. This is an important idea to wrap your head around.
Just for sake of explanation, let's say a bad ground point is creating an additional resistance equal to the load in that circuit. Being equal, source voltage will be split between them equally. And while your test light may light on the power side of the load, it will not necessarily show the fact that the load is only getting half of the voltage it really needs to work properly. The circuit power side will even measure correctly if you use your DMM (Digital Multimeter) instead, but the reality remains that only a portion of the total source voltage is available for use by the load if a fault lies on the ground side of the circuit path.
These "unwanted" additional resistances usually are caused by poorly fitting connectors, loose grounds and corrosion in the circuit path. It can also be caused by something more dramatic, like a broken wire or failed control device. And trying to find them using conventional, static resistance measurements with your ohmmeter can be an exercise in futility.
How To Perform A Voltage Drop Test
If there is more than one source of resistance, then that voltage will be shared by all, with a portion of the total "dropping" as it passes each resistance. That is something you can measure very easily and that is the basis for voltage drop testing. This is a dynamic test, allowing you to accurately test the entire circuit under its normal operating conditions.
The procedure used to measure voltage drop is very simple. Here are the steps to follow:
- First, verify the health of the battery and charging system. If they aren't working properly, source voltage to the system you are diagnosing won't be correct to begin with. Remember, lower voltage against the same resistance results in lower current flow. If a fault is found, repair it first.
- Second, attach your negative meter lead as close to the battery negative post as possible. You must measure the entire circuit path to insure accurate results, and it all starts and ends at the battery (for the majority of circuits you will test). Your negative meter lead should always be at the battery negative post. Make a lead extension, about 20 feet long or so that will allow you to reach all the way back to the rear of the car if need be. Resist the urge to use a body ground as your reference.
- Third, measure the voltage at the battery positive post with your positive meter lead, as close to the post as you can. Record this measurement. This is the source voltage available to your load.
- Fourth, turn the circuit you are diagnosing on. The circuit must be working and current flowing in order to perform a voltage drop test.
- Fifth, keeping the negative meter lead where it is, use the positive meter lead to measure the voltage on the power side of the load, as close to the load as possible. Remember, you have to measure the entire path or you may miss the problem entirely. Record this measurement.
- Sixth, move the positive meter lead to the ground side of the load, as close to the load as possible and again measure voltage. Record this measurement as well.
Most technicians who have tried to use voltage drop as a testing method are comfortable with the steps. It is the result of the measurements taken that tends to throw them off.
When measuring voltage at the power side of the load, you should get a reading equal to the reading you recorded when you measured the voltage potential at the battery. If your reading is more than 0.50 volt less than the battery measurement, there is an unwanted resistance between the load and the positive post of the battery. To find the thief, start working back towards the battery, taking additional voltage measurements in the power side of the circuit path, until your meter reading is within this 0.50-volt tolerance. The problem will be located between this point and your last, out of tolerance, measuring point. Then you simply start heading back the other way, towards the load, until you have narrowed down the exact location of the problem.
If the reading at the power side of the load is normal, then check the ground side. If everything is working as it should, electrical theory tells us all the source voltage should have been used up by the load. Even so, no circuit is perfect and some voltage reading is normal. Any reading over 0.50 volt here means that the thief is located between the ground side of the load and the battery negative post. Continue moving along the ground circuit path back to the battery until your meter reading returns to normal. As before, you now have the problem area isolated between this test point and the one immediately preceding it.
Typically, these thieves will make a noticeable impact on your readings. Don't worry too much if your readings are out of range by just a bit. When narrowing down the problem area, start with easily accessible points; like harness connections, control devices and fuses. Circuits associated with a control module have less tolerance for thieves and starter circuits a little more tolerance. Experience will guide you in what is allowable and what is not, so start practicing today on known good circuits.
Once you understand what your meter is trying to tell you, you will be able to quickly isolate many electrical problems that you may have otherwise missed by taking just three initial measurements.
Pete Meier is an ASE CMAT, member of iATN, and full-time tech in Tampa, Fla. His experience reaches back more than 30 years, and his contributions to Motor Age reflect a wide variety of experience with almost every make and model. You can contact Meier directly at www.autoservicetech.com.
