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-   -   How to test an Alternator (http://www.twoguysgarage.com/forum/showthread.php?t=5395)

nascar#20 06-27-2004 06:56 PM

How to test an Alternator
 
I am wondering the best way to test for a faulty alternator.

gnewby 06-27-2004 07:46 PM

1st off I would give everything a good visual examination. Look for loose or missing alternator belt. Also look for wires that could have become disconnected or cut that leads to the alternator.

Then I would hook up a volt meter to the battery cables and check the voltage. After this I would start the engine and recheck the voltage. The voltage on a charged battery should read around 12.5 volts. While the engine is running turn on a few accesories and speed up the idle a little the voltage should read around 14.5 volts.

If the voltage does not increase I would suspect either a faulty alternator or regulator.

thecarguy 06-27-2004 08:37 PM

start the car. Pull off the negative battery cable, dont let it touch anything, just hold it in your hand. If the car dies go get an altenator.

KA9MOT 06-27-2004 08:57 PM

Quote:

Originally posted by thecarguy
start the car. Pull off the negative battery cable, dont let it touch anything, just hold it in your hand. If the car dies go get an altenator.
You are joking right?
And your trying to pass you ASE Certification?

Many of today's Alternators as well as other electronic components can be damaged by doing as carguy suggested. Also, an alternator may be making enough amperage to keep the car running but not enough to keep the battery charged. An example would be a 100 Amp Alternator that is only making 5 Amps. Enough to keep the car running but certainly not enough to charge the battery plus run all of the accessories.

KA9MOT 06-27-2004 09:03 PM

BTW, I posted earlier that Gnewby's answer to this question was the correct procedure and for some reason it is gone now. While you can not accurately check the output Amperage this technique will get you close.

Good job Gnewby. I think you should stick around awhile.

danny 06-27-2004 09:04 PM

point of interest
 
todays car implement computers and will die anyway if the power is cut from the computer.

KA9MOT 06-27-2004 09:08 PM

Re: point of interest
 
Quote:

Originally posted by danny
todays car implement computers and will die anyway if the power is cut from the computer.
Darn it !!! I forgot that part!!! Doh!!!!!

vrek 06-27-2004 09:28 PM

Or I got a even easier way, get in your car. Start It. Drive to Autozone. Walk up to conter. Ask "Yo, Dude, Can you Test my alternator so my trick ride will go faster with all the lights blinking?" or how ever you want to phrase it.
Sorry Im was just reading some posts on a civic Message board for laughs.

gnewby 06-27-2004 09:34 PM

Thanks for the complement Beefy.

KA9MOT 06-27-2004 09:41 PM

Quote:

Originally posted by vrek
Or I got a even easier way, get in your car. Start It. Drive to Autozone. Walk up to conter. Ask "Yo, Dude, Can you Test my alternator so my trick ride will go faster with all the lights blinking?" or how ever you want to phrase it.
Sorry Im was just reading some posts on a civic Message board for laughs.

All of this is assuming that the goofball at DA ZONE knows what he is doing.

gnewby 06-27-2004 09:43 PM

Vrek the guy you run into at Auto Zone or Pep boys may have been carrying out groceries the week before and not no anything about working on a vehicle.

He may not know the alternator fromt the radiator.

thecarguy 06-27-2004 09:47 PM

Was not listed the type of car we are working on here. An OBDII car will have a volt light on for a bad altenator or battery. Go ahead and change both. A new battery with an old altenator wont get full charges, a new altenator with an old battery will hurt the battery. In most of the materials I have seen, it is recomended to change them in pairs, however we all know that you dont HAVE to. A pre OBDII car will not be hurt by doing so.

I already have my electrical cert.

Autozone no longer does free testing or code reading. They ended up selling batteries, and o2 sensors that wernt needed.

gnewby 06-27-2004 11:21 PM

carguy although it may be recommended don't know of two many people or places that would condem or throw away a good battery cause the altenator is bad or replace a $100.00 Altenator cause the battery is bad.

Just like the motto if it ain't broke don't fix it.

Porcupine 06-28-2004 08:48 AM

As gnewby suggested.
check for static voltage first, then start the car and check for the dynamic voltage roundabout 13.8 to 14.5. It varies pending on your regulator. That says your alternator is either good or... not so good. Anything much below 12.0 or 12.5 is a minimal output.

You might have a perfectly good battery but due to a low output alternator, it just hasn't been charged up enough and leaves the impression of a bad battery.

While an alternator might be pumping out as it should, a battery might have a high resistance (open) or one bad cell and won't except the charge. Therefore, even though you charge the heck out of it, the next morning it's pooped out or seems weak.

Yesteryear we used to do the hydrometer test, Specific gravity per cell. Today they do the load test, because most batteries are sealed. (I still crack open the sealed caps on the cells, cuz I'm nosey that way :) )

But, as stated, check all wiring, grounds, broken or CORRODED connections (they cause a high resistance too), not to mention loose belts. A noisy alternator bearing can add to the RPM resistance and while still charging as it should is an indication the alternator may be due for replacement.

elbi_chorrea 07-01-2004 01:32 AM

Quote:

Originally posted by gnewby
Vrek the guy you run into at Auto Zone or Pep boys may have been carrying out groceries the week before and not no anything about working on a vehicle.

He may not know the alternator fromt the radiator.

Actually they run tests on the wholesystem, with some kind of computer device or something like that, they don't just look at the car and say: I think is a bad alternator!!, they run tests, you should get outside a bit more, there are other ways now, we are not in the stone age.


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