What is a voltage drop test?
Voltage drop is the amount of voltage lost as it passes through a component or resistance in a circuit. We can relate a voltage drop to water in a pipe: if water is flowing in a pipe that is clogged, the water that is able to leak past it would be the voltage drop. All components in a vehicle, such as lights, are supposed to use up all the voltage they are provided. We can use a simple bulb as an example: there are two wires that light up a bulb, one from the battery to a switch and then to the bulb, while the other goes back to the battery ground side. When you turn the switch on, the bulb lights up, using the voltage you supplied it by turning the switch on. The wire going back to the battery ground side should not have any voltage. If it does, this is your voltage drop. Voltage drop could also be before the bulb, such as a very corroded wire.
Going back to the water example, if you have a clogged toilet, what happens when you flush it? It overflows. A really corroded wire where the electricity can't get through to light up the bulb is the same idea. You will need a meter and some automotive experience In order to perform a voltage drop test, voltage must be present in a circuit. The voltage drop is the difference between voltage at the source or battery and the voltage at the component you are testing.
One of the most common electrical issues in the automotive service industry today is related to voltage drop. Left unchecked, voltage drop causes countless unsolved electrical issues. It can also confuse you into replacing components in a vehicle that are not bad. The more connections and wiring a vehicle has, the more vulnerable the electrical system is to voltage drop.
To perform an electrical voltage drop test, use a digital voltmeter to find a poor or corroded ground connection. Turn the ignition key on KOEO (key on engine off). Use the service manual to find the key points to probe and set the meter to D/C volts. Connect the negative probe to the battery's negative terminal and back probe the connection at the proper ground wire with the positive probe to identify any corroded, faulty, or poor ground wires and connections. The typical voltage drop for this type of circuit is below or between 1 and 2 volts. Anything over 3 volts is considered unacceptable and further inspection of the circuit is required. Always consult the manufacturer’s specifications, because voltage tolerance varies again depending on the year and make of the vehicle you are driving. If you are not very knowledgable on car mechanics, I don't recommend you try doing this on your own. Instead, you should consult a professional mechanic from YourMechanic who can inspect any electrical issues you have in your car.
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