mikes84vette

i have taken the altenator, and battery to autozone for testing, both test fine. this is the deal. when i crank the car, at idle, volts read 10.2. when i tach the motor up to 3k, then the voltage will rise to 13.9 and stay. it will only show to be charging in the 800-3krpm range, after that, above or below that it will not show to be charging. when i turned the headlights the voltage dropped to 11.9 volts. i thought i had it fixed, but a few minutes ago i got in my car to run to the atm machine and there it was. WTF??? :mad

bill mcdonald

check for corroded contacts at the terminal on the back of the alt marked BAT.

I lost all voltage, after a bunch of crap, pulling and swapping parts, I found the wire finally broke and was all green,

mikes84vette

alright, let me ask this. the terminal marked BAT is it suppose to spin freely, or not at all? when i took the alt. off i had to use some pliers to keep it from turning to get the nut off. this is when i took it off to have it tested.

bill mcdonald

No it is not suppose to spin. mine turns a hair when I tightened it, and this was both brand new alts.

that could be adding to the problem....

Have you tried getting a volt meter on the battery, with the car off, and then running.

then try checking voltage with the car riunnign on the BAT terminal on the alternator.

I found I was getting 3 volts at the alternator.
I moved the voltmeter contact to the ALT bolt and found 18 V, then 6 etc... it was freaking out

Inside the ALT usually the stud which is turning is held in place by plastic. This is to keep it from grounding out on the housing, the pastic usually takes a beating from installs and uninstalls, and heat.

bill mcdonald

I suppose the regulator could be on it's way out too. I had an alt hold out for 1 week, it would throw on the low voltage light every one in a while. Then it died. usually it is just the regulator that dies.

I dont know for sure what alt the 84 has. Or if the regulator is mounted inside the housing.

mikes84vette

yeah it is close to the sam with voltmeter on alt and battery, as it shows on gauge incar

mikes84vette

that is basically what is happening.

jfb

The bolt on the alternator should remain stationary when you remove the nut holding the red wire solder lug. If it turns along with the nut, that can cause a resistance connection which will decrease the alternator output. Your symptoms are of an alternator not working right. It checked out ok because autozone doesn't run the alternator through its rpm range. Above 3k rpm, it doesn't have output because the slip rings for the field are slightly out of round and vibrate the brushes off the rings. Unless you have an unusually low idle rpm (should be 700 to 900), the alternator should be charging the battery if you don't have a heavy electrical load and the battery voltage should be 14-15 volts alternator cold. As the alternator warms up the battery voltage will fall to 13.5-14 volts. I recommend that you replace your alternator, or find someone who will let you try theirs to prove that yours is defective.

bill mcdonald

if you put the neg of the volt meter on the battery, and the pos of the volt meter on the stud on the back of the alternator while it is running, it will tell you if the alternator is charging the system, if it is ~12 volts or lower, I would say the alt is dying. if it is 12 plus, I would say you have a short/ bad connection in the wire somewhere.

I wonder what RPM auto zone tests at?

I had a bad alt, took it to pep boys, they tested it, they said it was good. It would not work on my car, I swapped it and the new one was good.
After that, I wonder what goes on durring that test....


[Modified by bill mcdonald, 6:25 PM 1/9/2002]

mikes84vette

does the test that they do on the alt. put a load on it? or does it just spin it at a certain rpm to check voltage output?

jfb

The alternator test puts a load on the alternator, but the rpms aren't run through the alternators normal operating range. Brushes that are at the end of their life cause their springs to be extended and don't apply as much force to hold the brushes on the slip rings and can contribute to the brushes lifting off the slip rings at high rpm.

mikes84vette

seems like what you are saying can be applied to what is happening now. because it will only work at certain rpms.. thanx for all you replies and advice.. :)

merrick

Put a full charge on your battery(about 13 volts). Then crank the car and at idle check voltage at the battery. If it is 14+ alternator is good and battery is weak( get a new one). If volts are about high 11s to 12s, your alternator is shot.