Introduction to Voltage Drop Testing
Auto Service Tech
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What exactly is voltage drop testing? Voltage drop testing is a way to test dynamic, or working, resistance of an automotive electrical circuit. This is a more accurate way of testing than conventional measurements using an ohmmeter. But before I tell you how to do it, a few definitions are in order. Let's start with what makes up an electrical circuit.
First is the "load"
A load is any electrical component that does the "work" in the circuit.
Headlights are one example of a load.
The headlight bulbs are the "load" in this circuit.
Other example of automotive loads include:
Window motors, fuel pumps, horns, fuel injectors... any device that performs the function the circuit is designed to perform.
Lighting is the easiest to start with because any fault is easily visible.
Next it the "source".
The source is the supply of voltage to the circuit being tested. For the majority of the circuits on an automobile, it's the battery. When the source is connected to the load by wiring, we have a complete path and a basic electrical circuit.
Important point to remember!
All circuits start and end at the source! To make a circuit practical, however, we need to add some form of "control". A device that will open and close the circuit so we can turn it on or off. This can be some form of switch, switches can be manually operated, or electrically operated... like a relay or control module drive. Last, but not least, is some type of protection device. A fuse, circuit breaker, or fusible link that protects the wiring from an unintended short.
Now that we have a simple circuit, let's take a look at the elements affecting that circuit. First is "current", measures in amperage or "amps" This is a measure of electron flow through the complete circuit as it is working. Next is "Voltage". Measured in volts. This is the "push" or pressure needed to get the electrons to move. Last is " Resistance" measured in " ohms". This is a measure of the resistance to electron flow in a circuit.
A word on resistance. Most techs are used to measuring the static resistance of a circuit... that is, with no power flowing through the circuit path. But that doesn't show the complete picture. True resistance is best measured dynamically... with the circuit "on". Let me show you using a battery cable as an example.
Notice the static resistance measurement on a section of complete cable... this tells me the cable is intact... the path through it is complete. But...
Notice the measurement on a single strand of wire taken from the some cable.
But do you think this single wire do you power a starter motor? That is the inherent weakness of static resistance measurements. They only tell you if the path is complete... not how sound it is...
Let's play with our simple lighting circuit some more...
We'll start by measuring the voltage potential at the battery itself...
Make a note of that reading. That's "source voltage" and it is my baseline reading. Now let's measure the voltage potential right at the load on the power side... notice that the light is "on" and working... a requirement for voltage drop testing... I'm looking for this reading to be within 0.50 volts of my baseline reading for most circuits. More than that indicates a problem... more in a moment...
Last, let's measure voltage potential on the other side, or "ground" side, of the load.
No voltage? Exactly... that is "voltage drop". All potential voltage will be used by the load in series circuit.
If there is more than one the voltage will be shared proportionally among them. In automotive circuits, there is typically only ONE load in the circuit. And that forms the basis of "voltage drop" testing... any additional loads or resistance in the circuit path will "steal" potential voltage from the component I want to work. Let's see what happens when I add another source of resistance...
Did you notice the bulb is dimmer now? That's because than it did before...
Current is what makes an electrical component work. And it is affected by voltage and resistance.
An increase in voltage causes an INCREASE in current. And vice verse...
And an increase in resistance causes a DECREASE in current. A decrease in resistance will increase current flow... To find out what's wrong with this circuit we use "voltage drop" testing. Since voltage potential is proportional to the resistances in the circuit, I can test by checking this potential at the load. If the potential is not what it should be there is a problem...
Start by first measuring the source potential... right at the battery. Now with your negative meter lead still connected to the battery negative terminal measure voltage potential getting to the load on the power side.
Get as close to the load as you can... you must test the ENTIER circuit path ALWAYS have your negative meter lead attached to the battery for the same reason. Low, but within my 0.50v allowance. Now the ground side... just move the positive meter lead. A reading over 0.50v here is a "RED FLAG" that something is wrong on the ground side. There is something between my load and the battery ground terminal stealing voltage potential... to find it, start heading back to the battery using common sense testing points, like harness connectors or switches. You'll need a schematic for the circuit you are testing to locate these points.
As soon as your meter reading returns to normal, you have passed the "thief".
Now narrow it down by testing between your last two test points...
Notice the really dim light? The second bulb is our "thief"!
Now you try one... Here's the reading at the power side of the load... and now the ground side of the load...
Voltage potential to the ground side reading is perfect... but we don't live in a perfect world. This "perfect" reading means no current is flowing, and the load isn't working. Why? Only one possibility... the load itself! What if your ground side reading was this? If the ground side reading is the same as the power side reading, then the path through the load is intact... but the ground path is not... there is an "open" in the circuit on the ground side. Here are some guideline for acceptable "drop".
For most circuits: 0:50 volts for starter circuits: 1:00 volts for control module circuits: 0:10 volts don't let a minor variance fool you... the drop will be obvious if there is a problem! Voltage drop testing procedure: The circuit tested MUST be "on" when testing! The negative meter lead always attaches to the battery (source) negative post.
Then use the positive meter lead to measure the potential at the source, the positive side of the load, and then the ground side. You'll then know exactly what section of the circuit has a problem! I hope you enjoyed this presentation and that you'll try it out for yourself. You'll fix electrical faults faster and easier, once you fully understand this testing method.
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