Skip the auto shop - Our mechanics make house calls

Q: What Is a Voltage Drop Test?

asked by on

What is a voltage drop test?

A: Voltage drop is the amount of voltage lost ...

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.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: When weather gets hot, SUV randomly won't start

Hello. This sounds like an issue that would be difficult to diagnose online without inspecting the vehicle. I’m thinking that if you have replaced the ECM module and reprogrammed it, as well and the BCM module, fuse block, and mass...

Q: Can a Car Battery Overheat in Hot Weather?

People often only think a battery will get damaged in the winter. However, the summer heat can be even more damaging than the winter’s cold temperatures when it comes to car batteries. The summer heat is when most of the...

Q: How can I tell if my car switches are dying?

Different switches can start to fail in different ways. Something like a window switch, for example, won’t be as crisp as it used to be and start to feel rough and have glitches whenever you use it, but continue to...

Related articles

How Long Does a Distributor O Ring Last?
The distributor is part of the ignition system in your vehicle and its purpose is to route high voltage from the ignition coil to the spark plug. The spark plug then...
P0240 OBD-II Trouble Code: Turbocharger Boost Sensor B Circuit Range/Performance
P0240 code definition Turbocharger Boost Sensor B Circuit Range/Performance What the P0240 code means P0240 is an OBD-II generic code triggered when the Engine Control Module (ECM) detects the intake boost...
How Much Does a Mechanic Make in Vermont?
Automotive technician jobs in Vermont have an average mechanic salary of $37k, with some mechanics earning a salary of $53k.