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 Get a Louisiana Driver's Permit
s licensing program. The first step in this program is to obtain...
Veteran and Military Driver Laws and Benefits in Idaho
The state of Idaho offers a number of benefits and perks for those Americans who have either served in an Armed Forces branch in the...
How to Avoid Back Pain in a Car
If you have back problems, sitting in a car for an extended period of time can be excruciating. Even without back problems, you could experience discomfort and soreness from...


Related questions

Q: While driving, car will stall

You will need to have the charging system and battery system checked to see if the alternator is putting out enough volts and amperage to recharge the batteries. The alternator is the most common failure to correct the problem for...

Q: Interior lights flickering

The problem most likely stems from the main power relay box having shorts or loose connections, causing the interior control module to become erratic. It could also be that the battery is not being charged, or making good connections to...

Q: car shuts off

Hello, thank you for writing in. The vehicle's battery is typically not used for much when the vehicle is running. The battery is used mostly for starting the vehicle and completing the charging circuit. Due to the battery seemingly causing...