Skip the auto shop - Our mechanics make house calls

What Is a Voltage Drop Test?

voltage drop test

Here’s the problem – your engine is cranking slowly, or maybe not cranking at all, but the battery and starter are both working just fine. Or your alternator is charging normally, but it’s not keeping your battery charged. Obviously, YourMechanic will have to troubleshoot this electrical problem.

Often, this type of car electrical problem is due to too much resistance in the high amperage circuit. If the current isn’t getting through, the battery isn’t going to be able to hold a charge, and the starter isn’t going to be able to crank the engine. It doesn’t take a whole lot of resistance to create a problem. Sometimes, it doesn’t take much, and the problem might not be visible to the naked eye. That’s when a voltage drop test is done.

What is a voltage drop test?

This is a way to troubleshoot an electrical problem that doesn’t require disassembly, and will show in little time whether your connection is good. To do it, YourMechanic creates a load in the tested circuit, and use a DVM (digital volt meter) to measure the voltage drop while the connection is under load. The thing about voltage is that it will always follow the path of least resistance, so if there’s too much resistance on the connection or circuit, some of it will flow through the DVM and deliver a voltage reading.

With a good connection, there should be no drop, or at least very little (under 0.4 volts usually, and ideally under 0.1 volts). If the drop is more than a few tenths, then there’s too much resistance, the connection will have to be cleaned or repaired.

There can be other reasons why your car engine is not cranking – it’s not always voltage drop. A voltage drop test, though, can diagnose car electrical problems without the need for a lot of tear-down.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
Icon-warranty_badge-02

Skip the repair shop, our top-rated mechanics come to you.

At your home or office

Choose from 600+ repair, maintenance & diagnostic services. Our top-rated mechanics bring all parts & tools to your location.

Fair & transparent pricing

See labor & parts costs upfront, so you can book with confidence.

12-month, 12,000-mile warranty

Our services are backed by a 12-month, 12,000-mile warranty for your peace of mind.

Get A Quote

Need Help With Your Car?

Our certified mobile mechanics make house calls in over 2,000 U.S. cities. Fast, free online quotes for your car repair.

GET A QUOTE

More related articles

How to Transfer a Car Title in Idaho
In order to prove ownership of a car, you must have the title. However, when a car is sold, given away or inherited, the title needs...
Veteran and Military Driver Laws and Benefits in New Mexico
The state of New Mexico offers a number of benefits and perks for those Americans who have either served in an Armed Forces branch...
P2422 OBD-II Trouble Code: Evaporative Emissions Control System (EVAP) Vent Valve Stuck Closed
Diagnostic Trouble Code (DTC): P2422 P2422 code definition Evaporative Emissions Control System (EVAP)...


Related questions

Q: Battery cells keep dying

The problem is not with the vehicle but the batteries that are defective. I am seeing a lot of batteries on the market that only give 1 year warranty. I thing the other batteries that give more than a year...

Q: Changed battery twice in 2 weeks, I go to turn on and car is completely dead not even the lights work, what could it be? 2000 Acura TL

Hi there - a repeating "dead battery" scenario has a couple of possibilities. One possibility is a bad alternator that is not charging properly. With the engine running, the system voltage should be close to 14 volts if the alternator...

Q: Battery jump reverse connectors

Hey there, thanks for writing in about your 2013 Kia Fote Koup. Fuses are in place to protect wiring and components from failure. By connecting the cables in reverse direction you created a surge in amperage, and the flow of...