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

Post a question and get free advice from our certified mechanics.

ASK A QUESTION

More related articles

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...
The Traveler’s Guide to Driving in Malaysia
CraigBurrows / Shutterstock.com Malaysia is a popular destination for many tourists today. The country has amazing sights and attractions that you will want to explore....
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...


Related questions

Q: The car won't start unless it sits for over four hours

Hello there. A few possible faults could cause this type of intermittent no start condition. The most common would be a battery, starter, battery cables, fuel pump, fuel injector, or a wiring issue. This type of failure indicates a fault...

Q: Alternator problems. Lights and horn don't work.

Your car is showing all the classic signs of a bad battery. I would recommend having a mechanic inspect the battery. They will start with the battery and have it load tested. When batteries get old they may not take...

Q: It lost power and now won't start

It sounds like the battery is dead from the alternator not charging. This would account for the gradual loss of power and why the battery is dead and you have no lights. Have the battery charged and tested. Afterwards, start...