What Is a Voltage Drop Test?

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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.


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Eladio

8 years of experience
127 reviews
Eladio
8 years of experience
Honda Civic - Brake Pads Replacement (Rear) - Greensboro, North Carolina
Eladio arrived at the scheduled time and was great to work with. He was very professional and I am very satisfied with his work. This was my first time using YourMechanic and I am very pleased with how easy and convenient this service has been from start to finish. Definitely would recommend.
Nissan Altima - Brake Pads Replacement (Front) - Greensboro, North Carolina
Eladio was on time, expedient, and very professional. Very pleased with provided service (brake shoes replacement).

Willie

22 years of experience
7 reviews
Willie
22 years of experience
Dodge Avenger - Brake Pads Replacement (Front) - Long Beach, California
Willie is an amazing mechanic and person. He was very professional, helpful, knowledgeable and personable. I really appreciated all of his help!!

Richard

15 years of experience
52 reviews
Richard
15 years of experience
Mercedes-Benz C300 - Brake Pads Replacement (Front, Rear) - Pleasanton, California
Richard is very knowledgeable , he was prompt on arrival, change out of pads on my 2014 C300, was well done, and clean up was great. I would highly recommend Richard.

Simon

25 years of experience
16 reviews
Simon
25 years of experience
Infiniti G37 - Brake Pads Replacement (Front, Rear) - Upper Marlboro, Maryland
Simons was GREAT! Very easy to talk to and very knowledgeable on car repair. I hope to use him again very soon.

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