Thread: Battery or Alternator bad?

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hay bucs hook up your volt meter to the battery read the voltage should be about 12.2 to 12.5 start the eng at about 2000 rpm should be 13.9 to 14.2 if the alt is working hope that helps ted

NAPA (and I assume the other parts chains) have an alternator checking machine. Take the alternator there and have it checked out. It sounds like you fried the alternator (diode) and its now draining the battery.

Do you have a voltmeter or ammeter on your dash? If so, what are you getting there?

Don

Great info.

Don- I have a voltmeter on my dash. Since I run a one wire alternator I thought I heard somewhere that that just takes the reading from the battery/alternator combined so it's hard to tell which is bad???
My understanding is when you run a 3 wire alternator you get a better reading of the alternator since the reading comes only from the alternator and not the battery. Anyone know?

Also, I know I can take the alternator into Auto Zone and they can test it, off the car. Is there anyway I can test it ON MY CAR with my voltmeter? Ted, I know you said to get battery read then start car and take another read.....Is there any way to take a reading direct from the alternator? I assume that is all Auto Zone would do. Right?

Sorry guy's. I really like learning this stuff!

one wire alternators are great.....


for forklifts & tractors. A 3 wire setup gives you a better system. The alternator charges when it starts spinning, not waiting until you "wing" the motor. Also, with a 3 wire, the regulator gets a voltage sense signal, and actually knows (about) what to charge, as opposed to the one wire, which charges at a predetermined rate.

Take your test meter & check the alternator, once you have the alt spinning enough to charge. It should be 13.8-14.2 volts. If so it's operating. Next step would be to have the unit checked at your local parts store. If they have a good test unit, it should be able to tell you if you have a bad diode.

While you're at it, have 'em load test the battery as well.

Originally Posted by fitzwell

Take your test meter & check the alternator, once you have the alt spinning enough to charge. It should be 13.8-14.2 volts.

How do I check the alternator? At the battery like mentioned above? or can I put my voltmeter direct to the alternator some how?

Whew!
What the other guys are saying:
1. Take your volt meter and measure the battery voltage with the engine OFF. Put the + lead on the positive battery terminal and the - lead on the negative terminal. Write down the voltage reading.
2. Start the engine and rev it slightly to get the alternator going (one wire alternators take rpm to start). Let the engine go back to idle. On the back of the alternator is a big terminal lug...the one you touched to make a spark and start this in the first place. Put your + voltmeter lead on this and the - voltmeter lead on the battery negative OR any other ground....like the intake manifold. Read the voltage. Write it down
3. Compare the two readings. When you charge a battery, the charging voltage must be greater than the battery voltage....battery voltage is roughly 12 VDC and the charging voltage out of the alternator should be in the 13.8 to 14 ish range. If the alternator is not putting out more than 12 volts, suspect the alternator.
4. Another way to test the alternator (poor man's test) is to start the engine, rev it to get the alternator started, and then disconnect the battery. If the engine runs on the alternator, then the alternator is putting out some energy...maybe not enough but if the alternator is completely dead, the engine will not run.

Originally Posted by bucs012

How do I check the alternator? At the battery like mentioned above? or can I put my voltmeter direct to the alternator some how?

Basically if the volts are right you don't have to worry about amps or anything else. Test at the battery. Volts at rest on battery as mentioned, 12.6-12.8. Start the car and with everything turned on that uses electricity, you want to see 1 volt more than battery voltage with the engine off.
I know a lot of people love those one wire delcos but I would wire in a regulator to run an old style Ford alternator personally.