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.
This article originally appeared on YourMechanic.com as What Is a Voltage Drop Test? and was authored by Valerie Johnston.