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.
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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.
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.