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Why we do voltage drop checks
Posted to Technical Tips Forum on 7/28/2012 143 Replies

I've recently heard again from several technicians that they use test lights instead of a voltmeter. I am of the opinion that test lights do have their place and my test light is in a place where I haven't seen it for a few years now. I still have it I'm just not sure where. My DVOM is kept in the bottom drawer of my tool cart where I can grab it quickly and easily because it gets used often.

One of the top tests for charging system analysis is a voltage drop check using a DVOM. It is very effective at finding connection issues that otherwise could not be found with a test light or a resistance check. Most of the time a normally operating system will have very close to a 0 volt drop across terminals. Starting circuits are slightly higher due to the amperage that flows through when the starter is activated. By slightly higher I mean 0.1-0.2 volts.

Usually when something is broken it's broke. Drops of several volts on a broken connection are common. Up to battery voltage on a corroded battery terminal is also seen often. What we don't see very often is this: [1995 Mercury Mystique, BATT/Charging/Starting Photo] Yes, the meter is reading correctly. What you are looking at is an open circuit between the alternator output terminal and the battery positive. If the meter was connected between ground and the alternator output it reads over 36 volts!

This car could have easily had an alternator (or three) thrown at it and still have a no charge condition. Do the proper tests before throwing parts. It only takes a few minutes which is far less time than doing a second alternator R&R.

Andrew from Utah

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