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

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

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: Cranks but doesn't start

You have replaced lot's of things on your car. I'm not sure you replaced it all trying to fix a no start or if you replaced it all out of maintenance. When we replace this many parts at once we...

Q: Brake, Battery sensor, lights, and a crackling noises.

The issue sounds to be a ground return wire on the speakers is shorting out with a ground circuit in the vehicle causing the lights and sensors to register a warning light. The brake light could be coming on for...

Q: Car won't start

Hi there, thanks for writing in. It's highly unlikely that the computer is damaged, especially with only 48,000 miles on the vehicle. You may have an issue with your key or the ignition system that is causing some electrical gremlins...

Related articles

How Do Power Car Windows Increase Passenger Safety?
Power windows are responsible for approximately 2,000 emergency room visits every year. When a power window closes, it exerts enough force to bruise or break bones, crush fingers, or restrict an airway. Though...
What Causes Hoses to Leak?
While the largest part of your engine is mechanical, hydraulics plays a significant role. You’ll find fluids at work in a number of different areas. Your car's fluids include: Engine oil Transmission...
P2103 OBD-II Trouble Code: Throttle Actuator Control Motor Circuit High
P2103 means there is a fault with the throttle actuator control motor circuit, likely due to a defective electrical component or part.