When voltage drop strikes

Feb. 1, 2019
Unwanted circuit resistance can "steal" from the primary load and impacting its function. Learn how to catch a thief!

In this article on voltage drop (VD), we will explore what it is, how to check for it and share some vehicles that I have come across with VD issues. Let’s start with an explanation of VD so you can better understand what we are dealing with. Remember that there are many vehicle problems with a component or system not working right that can be traced back to a voltage drop. Have you ever noticed a vehicle driving down the road with one headlight that is not as bright as the other? How about a vehicle that has high LTFT numbers? A blower motor not turning fast enough, or a rear window defroster that partially clears the window? Well, if you answered yes to any of them you have experienced VD.

A dynamic problem found with a dynamic test

You should know that voltage drop = electrical resistance that we measure in Ohms and check dynamically by performing a voltage drop test with our volt meter. There are many connections on a vehicle that may contribute to a VD issue such as loose, stripped or crushed connections and broken wire strands. We cannot expect damaged wires or loose and dirty connections to provide the proper current flow or voltage. If all connections are not intact and well connected, the result will be unwanted resistance, and that equals a voltage drop. A VD problem will prevent the proper flow of current causing a starter motor, bulb, blower motor, solenoid or any other electrical device from performing as designed. In other words, VD results in the poor performance of a load.

Let’s consider a vehicle’s headlight that is dim even after it has been replaced. What is the next step in getting that headlight to operate correctly? We know that many techs have a Power Probe or if not, they at least have jumper wires that they can use to test the circuit quickly. This quick test is just that, a quick test that can be utilized at the load to see if the headlight will illuminate correctly. Take the Power Probe/fused jumper wires and apply power to the B+ side of the circuit and see if there is any noticeable change to the brightness of the headlight. If it helped or not, never think you’re finished until you apply ground to the negative side of the headlight. If the headlight now illuminates to the level of the other headlight, you now know that there was a bad ground but what you don’t know is how much of a voltage drop there was.

Now let’s try using a VD check the correct way so we can measure the exact amount of VD. First, we will start with the DVOM. The DVOM, when it is set to read voltage, measures the voltage potential between the two leads. Keep this in mind as you take your measurements so you can learn how to speak the language of the meter. Connect the leads to the battery positive and negative post. You should read the battery's voltage potential on your meter. On a healthy, fully charged battery, that potential will be 12.6 volts.

Figure 1

Next, leave the positive lead at the battery post and take the negative lead of the meter and go to the positive side of the headlight. With the headlight "on," the meter will measure the voltage potential (difference, or drop) in the circuit. In other words, all of the available voltage (that is, all of the 12.6 volts you measured at the battery) should be going to the headlight is being checked to make sure that it did indeed make it to the battery, give or take a couple hundred millivolts. This is followed by taking the negative test lead and placing it back on the battery negative post followed by taking the positive meter lead and going to the negative side of the headlight. The meter is still on the DC voltage scale and will provide the exact reading of the drop.

In this example, let’s say your meter reads 00.90 on the meter's 40/60-volt scale. How much of a voltage drop is the meter measuring? That’s right — 900mV. Let’s take a look of a voltage drop test on a vehicle since a picture is worth a thousand words. In Figure 1, we have our Power Probe connected to the vehicles battery and then connected our meter’s positive lead to the Power Probe positive tip (note the rocker switch is depressed to the positive position as indicated by the red light) while the meter negative lead is connected to the starter positive post.

An important thing to remember when performing a voltage drop is to always make sure that the load is "on." There will be no voltage drop if no current is flowing. In this case, I have my tech up in the vehicle so he can crank the starter over while the meter captures the voltage drop. The complaint on this vehicle was the engine cranks over slowly intermittently. Can you see why? Yes that 00.96 equals 960 mV, almost 1 volt on the feed/B+ side. Now how about the ground side?

Figure 2

Well as you can see in Figure 2, the reading there was 320 mV for a total voltage drop of 1280 mV or 1.28 volts. As we know, the battery has 12.60 volts before we crank the engine over. The voltage usually drops about a volt or so making the available voltage level about 11.60 minus our 1.28 voltage drop, only leaving 9.8 volts available. Now add in mechanical resistance from a cold motor or thick oil and "Bingo!" — we have a starting problem. Does that make sense? If not, as we continue on, we will have more real world examples to help you grasp the concept. 

Essentially, a voltage drop test measures the reduction in voltage due to resistance (more than normal/excessive) in the circuit.  It is impossible to get a 0v voltage drop on a working/complete circuit. A reading of up to 200 mV is permissible on a non-computer circuit, while when dealing with a computer component, 100 mV or less is permissible. To measure the small amount more accurately, scale your meter down to the mV scale. A couple of things to also remember is that there are 1,000 mV in 1 V.  For example, 1.11V = 1,110 mV. When performing a voltage drop test on mV DC scale and the meter displays "OL," you definitely have VD and you need to take care of it!

Real-world examples of a thief at work

Our next example is checking a voltage drop on an engine with an open ground circuit (Figure 3) at the intake manifold. The volt meter leads are connected a little differently than what I described earlier when doing the starter circuit ground check. Here, the negative (black) meter lead is attached to the battery negative post and the positive (red) meter lead is connected to the engine’s intake manifold.

Figure 3

If you look at the battery negative post, you will notice that the cable is disconnected from the battery on purpose. The volt meter is reading 11.87 volts — equal to the battery’s voltage level.

If the two measurements are equal, there is 0v voltage drop. If our meter leads were placed in the same manner as our starter circuit test, the meter would read exactly that — 0v. Since everything in a circuit has some resistance, we should always measure a few hundred millivolts. A perfect reading like this indicates reveals the true reality. There is no current flow and no voltage drop, likely due to an open (or infinite resistance) between our two test leads.

A great use for voltage drop testing is testing for Parasitic Draw. Again, every component in an electrical circuit has some resistance and voltage will "drop" across that resistance, even if it's a small amount. But only if current is flowing, remember? And isn't that what we're looking for when chasing down a parasitic draw — something that is flowing current when it isn't supposed to? To use the voltage drop method (Figure 4), place the meter lead ends on the exposed metal blade tops with the meter set to mV. This method is not usable on all fuses but for this example we have fuses where the metal is exposed on the top. If the leads ends are connected correctly the meter will either read 0 or a mV reading as the one pictured. If the meter reading is bouncing all around (numbers not steady) you are not connected correctly to the metal of the fuse.

Figure 4

Important point to remember is that there is NO voltage drop if the load is NOT on. So, if the lead ends are connected properly you will either get an mV reading that indicates a voltage drop thus equaling a parasitic draw, or zero that means there is no draw.

Case studies

A Ford Explorer came in with a no start complaint that was traced down to a voltage drop. The vehicle owner had already replaced the battery and the starter motor before he had the vehicle towed in. While performing a voltage drop on the B+ side there was nothing abnormal, but it was another story on the ground side. The meter indicated a reading of 8-plus volts that was causing the no start condition on this Ford. We made a temporary ground cable so we could start the engine up and drive the vehicle into the bay. We attached both ends of the negative clamps from our jumper cables, placing one at the negative battery post and the other to engine block. With those connections, the engine cranked over and ran. Since the temporary cable worked, we knew that we had to check the negative cable. Take a look at this source of voltage drop! This major ground (Figure 5) is a common problem on many vehicles that have a battery that has an out gassing issue leading to cable corrosion. 

Figure 5

A 2005 VW Jetta came in with another recurring issue — the fuse box (Figure 6) that is on top of the battery is melted. This has been a problem that goes back at least to the 2002 model year on VWs. A common mistake that is made is just replacing the fuse box and not finding the root cause of the problem. Each of the terminal wire ends needs to be checked very carefully for a VD. There has been an issue with the wire insulation not being totally stripped back from the factory that causes amperage to flow through less strands. The current flow through a small area of wire strands causes heat buildup due to voltage drop. Since the circuit is not designed to flow the high demand of current through a small connection, this causes the voltage drop. The most common terminal that causes the box to melt is the black wire that goes to the alternator, followed by the cooling fan terminal. Before you change the fuse box, it is recommended that you open up the terminal ends and make sure the connections have the full contact with the wire. The fix for this vehicle was new terminal ends with full contact of the wire strands, heat shrink and a new fuse box.

Figure 6

Our next case study is on a 2000 Volvo S80 came into our shop about 12 years ago with the complaint of no start, no crank and shifter locked out. Even though this is a case study from awhile back, it has very important information that can help bring a voltage drop issue to life. I sometimes use this case study when I am teaching a class to show a more advanced voltage drop problem. I believe that the information will help you to understand what VD can do beside prevent a motor, bulb or load not to function as designed. The vehicle owner stated the vehicle was starting and running normal until the no start issue appeared. We started our diagnostic procedure as we usually do, by asking the customer when it happens, if any work was recently performed, followed by a visual inspection.

Our visual inspection at first did not uncover anything unusual so we moved on to scanning the vehicle systems by connecting our Autologic scan tool. At the time the Autologic blue box Volvo software provided very good information along with vehicle programming capabilities. The scan tool uncovered a problem with no communication, so we installed our BOB (DLC Breakout Box) and checked for power at Pin 16 along with ground at Pin 4. The results of our voltage checks were normal, so before getting ourselves in too deep, we checked for TSBs and information in Identifix and iATN. Neither information sources had any published information regarding our problem. Since we came up empty handed, we looked in Alldata and ProDemand to check the wiring diagrams and traced them out. We found that the system we were working on was a CAN (Controller Area Network) system that meant that there could be something on the BUS that was preventing scan data from being transferred to the scan tool.

Figure 7

Next, we connected a labscope leads to Pins 6, CAN High and Pin 14, CAN Low (high speed CAN) looking for the normal square wave but that’s not what the scope displayed on the screen. We proceeded to check the Pins 3 and 11 that are a Low speed CAN network that are the communication lines for the CEM (Central Electronic Module) aka body module. The results that we found was the same issue, no square wave communication. When you look at the wire diagram (Figure 7) you will notice that the Low and High speed lines both go through the CEM making it the best place to start.

Figure  8

Our next step was to locate the CEM in Alldata then seek out its physical location under the left dash. Looking at (Figure 8) you’ll notice the burn at the pin connections caused by a poor connection, aka high resistance, resulting in voltage drop. Another problem that we uncovered was a water leak caused by clogged body drains. The clogged drains allowed a path for the water to drip right down on to the CEM connection not exactly helping our poor connection or possibly causing it. Our next step was to repair this problem by cleaning the drains making sure that no more water would be able to leak down on the CEM. We followed that up by cleaning the connections and applying Stabilant 22A to enhance them before we reinstalled the new CEM. After the physical repair was complete, we had to program the module with the Volvo software, so the engine would start. With the engine now running the only thing left was to check the gear shifter issue. We proceeded to move the shifter in all the different gears to make sure they all worked then test drove the vehicle. After the test drive was complete, we ran another vehicle scan of all modules making sure that everything was back to normal. The Volvo was now over the VD issue and was ready to ship out.

I hope this article has shed some light on VD and has helped you better understand one of the biggest problems that we facing diagnosing on today’s electronic load vehicles.

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