Alternator test and battery drain test?

[Vanagon-tom]

Would this test work to test for battery drain?

Disconnected the negative on the battery, hook a test light to the negative post on the battery and the negative battery cable. Take out one fuse at a time to see if the test light gets brighter. If the light gets brighter, you know what electrical system is shorting to ground. Will this work?

To see if its the altornator not charging, disconnect the negative to the battery, if the vehicle dies, then the altornator is bad, is this safe on the altornator or is there a better way to test the alternator? Is testing the bat while the engine is running with a volt-meter good enough?

Thomas

[UncleBob]

this is one of the age old tests from back when cars had no electronics. It worked back then and was an accuate way of testing the system.

It doesn't work on any car thats been made in the last 30 years though. You need a multimeter (at least) to test the voltage of the battery while the car is running, and test the amps when the car is shut off. Depending on the car, many have a GEM module now that stay energized for up to an hour before shutting themselves off, and will show a higher-than-normal draw until then. So it takes a bit of time to test for a draw

[curtis73]

agreed. Typically a bad alternator shows up as less voltage running than when the engine is off. Look for 12-13v off and 13-14.4v running. Anything less and you might have an alternator issue. A battery amperage test is best performed with a battery tester that can handle large loads to test the battery's reserve.

[UncleBob]

the amp test with the car off I'm refering to, is testing the amount of amps the system is drawing from the battery with everything off. Most multimeters have a 25amp ampmeter circuit, which is more than enough to safely do this test. Obviously, don't turn the key on with it hooked up or it will blow the fuse in the meter

[Vanagon-tom]

Originally Posted by UncleBob

the amp test with the car off I'm refering to, is testing the amount of amps the system is drawing from the battery with everything off. Most multimeters have a 25amp ampmeter circuit, which is more than enough to safely do this test. Obviously, don't turn the key on with it hooked up or it will blow the fuse in the meter

Do I disconnect negative on the battery, then hook the leads of the multimeter to the negative battery post and the negative main wire? How many amps should it be reading?

[UncleBob]

I don't pay much attention to which lead goes where, since I only use digital meters. If you get it backwards, it will read as a negative number instead of a positive number.

The general rule is anything over 60 miliamps (that would be 0.06 amps) is too much draw.

[Vanagon-tom]

Originally Posted by UncleBob

I don't pay much attention to which lead goes where, since I only use digital meters. If you get it backwards, it will read as a negative number instead of a positive number.

The general rule is anything over 60 miliamps (that would be 0.06 amps) is too much draw.

Ok, so if its over .o6 amps would the battery drain test work to find which electrical system is draining the bat?(test light and fuse removal test)

[UncleBob]

you don't use a test light. You just pull fuses and watch the multimeter until you find the one that causes the most significant draw.

Of course if you have fuse block inside the car, you will first have to pull the fuse for the courtesy lights, because when you open the door, that will create a draw. I prefer doing this AFTER I've hooked up the meter, to make sure there's no courtesy lights causing the draw I'm looking for. Sometimes the fuse that powers the courtesy lights also powers the trunk light or glove box light, etc. IE, hook up meter, note draw amount, then open door and pull the courtesy light fuse, then check draw and see if its the same.

Also keep in mind that not everything is fused (depending on the car) the alternator, for example, is often not fused, so if you run out of things to check, make sure you remove the wires going to the alternator, but if you do it with the meter/battery hooked up, be careful, because the big wire going to the alt is hot

[Vanagon-tom]

Originally Posted by UncleBob

you don't use a test light. You just pull fuses and watch the multimeter until you find the one that causes the most significant draw.

Of course if you have fuse block inside the car, you will first have to pull the fuse for the courtesy lights, because when you open the door, that will create a draw.

Also keep in mind that not everything is fused (depending on the car) the alternator, for example, is often not fused, so if you run out of things to check, make sure you remove the wires going to the alternator, but if you do it with the meter hooked up, be careful, because the big wire going to the alt is hot

BIG THANKS